Mapping the global research landscape of sap flow studies in forest ecosystems: a bibliometric analysis (2000–2024)

Mapping the global research landscape of sap flow studies in forest ecosystems: a bibliometric analysis (2000–2024) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Systematic Review Mapping the global research landscape of sap flow studies in forest ecosystems: a bibliometric analysis (2000–2024) Sharat Kothari, Sumantra Basu, Ilham Bano, Amitha Panwar, Veenu Mahecha This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8739987/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Sap flow techniques are widely applied in forest research to examine tree water use and forest–atmosphere interactions across diverse climatic and ecological settings. Over the past two decades, the volume of sap flow–based studies has increased substantially, spanning multiple disciplines, regions, and research contexts. Despite this growth, a quantitative overview of how sap flow research has evolved in terms of publication trends, thematic focus, and collaboration patterns remains limited. This study presents a bibliometric analysis of global sap flow research in forest ecosystems published between 2000 and 2024, based on 1,989 peer-reviewed journal articles and reviews indexed in the Scopus database. Using bibliometric performance indicators and science-mapping techniques implemented through the bibliometrix package and VOSviewer, we analysed temporal publication dynamics, citation structures, leading journals and authors, international collaboration networks, keyword co-occurrence patterns, and thematic evolution. The results show a steady increase in publication output after 2010, accompanied by expanding international collaboration and diversification of research themes. Keywords related to established sap flow measurement approaches, particularly thermal dissipation and heat-pulse methods, remain consistently prominent throughout the study period. At the same time, increasing co-occurrence of sap flow with terms associated with drought, soil moisture, atmospheric demand, and climate variability reflects a broadening research emphasis in recent years. Co-citation and thematic analyses reveal a well-defined research structure linking methodological foundations with ecohydrological and climate-oriented research context. These findings provide an objective overview of research trends and emerging thematic directions. Tree transpiration forest ecohydrology xylem water transport canopy conductance bibliometric analysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Forests play a critical role in regulating terrestrial water fluxes through transpiration, thereby influencing catchment hydrology, regional climate, and ecosystem carbon dynamics [ 1 ]. Tree-level water use governs how precipitation is partitioned between evapotranspiration, runoff, and soil storage, making it a central process in forest hydrology and ecohydrology [ 2 ]. Accurate quantification of tree transpiration is therefore essential for understanding forest responses to drought, warming, and land-use change, as well as for parameterising hydrological and land-surface models [ 3 , 4 ]. Among the available approaches for measuring transpiration, sap flow techniques have become one of the most widely applied methods for estimating whole-tree water use. Since the late twentieth century, thermal-based methods such as thermal dissipation probes, heat pulse velocity, and heat balance techniques have enabled continuous, non-destructive monitoring of xylem water transport under field conditions [ 5 , 6 ]. These methods provide a practical bridge between leaf-level gas exchange measurements and ecosystem-scale flux approaches, allowing transpiration to be quantified across temporal scales ranging from diurnal cycles to seasonal and interannual variability [ 7 – 9 ]. Early sap flow research was largely oriented toward methodological development, calibration, and the challenge of scaling point measurements to biologically meaningful estimates of whole-tree and stand-level transpiration. Initial efforts focused on sensor design and on addressing radial and azimuthal variability in sap flux density, which were recognised early as major sources of uncertainty when extrapolating sap flow measurements beyond individual probes [ 10 ]. Subsequent methodological syntheses and comparative studies revisited these issues, refining correction approaches and scaling frameworks while explicitly situating sap flow techniques within broader ecohydrological applications [ 6 , 11 ]. Comparative reviews further demonstrated that sap flow measurements provide a robust basis for evaluating tree water use across species and forest types, supporting their application across contrasting biomes and climatic conditions [ 8 , 12 ]. As sap flow measurement techniques became more standardised, their application increasingly extended beyond methodological validation toward process-oriented questions in plant physiology and forest hydrology. Sap flow data began to be analysed alongside environmental drivers such as soil moisture, vapour pressure deficit, and radiation, enabling investigation of stomatal regulation, hydraulic constraints, and interspecific variation in water-use strategies [ 13 , 14 ]. Rather than serving solely as a means of quantifying transpiration, sap flow measurements were progressively interpreted as indicators of tree hydraulic functioning and physiological response to water limitation, contributing to a broader understanding of drought processes in forest ecosystems [ 15 , 16 ]. Over the past two decades, sap flow measurements have become increasingly integrated within ecohydrological research frameworks, particularly as tools for linking tree-level transpiration with ecosystem-scale water fluxes. Sap flow observations are frequently analysed alongside micrometeorological data, eddy covariance measurements, and catchment water balance approaches to assess forest evapotranspiration and land–atmosphere exchange [ 7 , 8 ]. In parallel, sap flow data have been increasingly used to inform studies of forest responses to drought and heat stress, where changes in transpiration dynamics are interpreted in the context of hydraulic limitation, carbon–water interactions, and elevated mortality risk [ 15 , 17 , 18 ]. Together, these developments underscore the growing relevance of sap flow measurements to ecohydrological research and to broader efforts aimed at understanding forest vulnerability under ongoing climatic change. Modern studies integrate sap flow data with soil moisture dynamics, canopy conductance, and atmospheric demand, providing critical insights into water-use efficiency and drought resilience. Despite the rapid expansion and diversification of sap flow research, the field has developed largely through cumulative contributions across multiple disciplines, biomes, and methodological traditions. As a consequence, the literature has grown both extensive and heterogeneous, making it increasingly challenging to track how research priorities, intellectual influences, and collaborative structures have evolved over time. Narrative reviews have played an important role in synthesising sap flow measurement approaches and their physiological interpretation [ 11 , 19 ]. However, such reviews are necessarily selective in scope and emphasis, and are not designed to capture broader structural patterns in publication activity, thematic development, or research networks across the field. Bibliometric analysis offers a complementary, quantitative approach for examining the evolution of scientific fields. By analysing publication metadata, citation relationships, and keyword networks, bibliometric methods can identify dominant research themes, influential contributions, and emerging directions without altering or reinterpreting primary results [ 20 ]. Such analyses are particularly valuable for mature measurement-based fields, where methodological continuity coexists with thematic diversification. In forest science, bibliometric approaches have been increasingly applied to topics such as forest hydrology, drought impacts, and climate adaptation, providing objective perspectives on research trajectories and knowledge gaps. Within sap flow research, however, a comprehensive bibliometric synthesis that explicitly situates the field at the interface of tree physiology, forest hydrology, and ecohydrology remains lacking. Given the central role of sap flow measurements in contemporary forest water-use studies, such an assessment is timely. Understanding how methodological foundations, thematic emphases, and international collaborations have evolved can help clarify the current structure of the field and inform future integrative research efforts. The present study addresses this gap by conducting a global bibliometric analysis of sap flow research published between 2000 and 2024, based on records retrieved from the Scopus database. The specific objectives were to (i) examine temporal trends in publication output and citation activity, (ii) identify leading journals, authors, institutions, and countries, (iii) characterise the thematic structure and evolution of sap flow research, and (iv) analyse patterns of international collaboration. By mapping the intellectual and social landscape of sap flow studies, this analysis aims to provide a structured overview of how the field has developed from methodological origins toward a central role in forest ecohydrology and climate-related research. This study focuses on mapping the structure and evolution of the sap flow literature using bibliometric indicators and does not involve qualitative evaluation of experimental designs or measurement methodologies 2. Materials and Methods 2.1 Data Source and Search Strategy The bibliometric analysis was based on records retrieved from the Scopus database, selected for its broad coverage of peer-reviewed journals in forest science, plant physiology, and environmental research. Data retrieval was conducted in November 2025 using a structured Boolean query designed to capture literature on sap flow and tree water use within forest and woody-plant contexts, while explicitly excluding agricultural and herbaceous systems. The final search string applied was: TITLE-ABS-KEY ((“sap flow” OR “sap flux” OR “sap velocity” OR “xylem flow” OR “tree transpiration” OR “whole-tree transpiration” OR “canopy transpiration” OR “tree water use” OR “tree water requirement” OR “transpirational water use” OR “tree water consumption” OR “sap transport”) AND (tree* OR forest* OR “woody plant*” OR woodland* OR plantation* OR “boreal forest” OR “temperate forest” OR “tropical forest” OR “subtropical forest” OR “dry forest” OR “arid forest” OR rainforest* OR savanna*) AND NOT (crop OR agricultur* OR herb* OR grass OR shrub OR maize OR rice OR wheat OR soybean OR vineyard OR orchard OR horticultur*)) This query was intentionally conservative to ensure thematic relevance to forest ecohydrology and tree physiology. 2.2 Screening, Inclusion, and Refinement Criteria The initial search returned 3,369 documents, which were progressively refined through a series of inclusion and exclusion steps to ensure relevance and analytical consistency. The PRISMA flow diagram used for screening is given in Fig. 1 . Figure 1 . PRISMA-style flow diagram illustrating the identification, screening, and selection of publications included in the bibliometric analysis of sap flow research. Records were retrieved from the Scopus database and sequentially filtered by publication year (2000–2024), subject area, document type, keyword relevance, source type, and language, resulting in a final dataset of 1,989 journal articles and review papers. Records were first limited to the period 2000–2024, yielding 2,922 documents. Subject area filtering retained records indexed under Agricultural and Biological Sciences , Environmental Science , Biochemistry, Genetics and Molecular Biology , Earth and Planetary Sciences , and Multidisciplinary Sciences . Records primarily indexed under chemistry or unrelated technical domains were excluded, as were articles with keywords not directly related to sap flow or tree transpiration. The document type was restricted to research articles and review papers, and only journal publications were retained as the source type. Article of English language and final published articles were selected for the study. After refinement and removal of incomplete records, the final dataset comprised 1,989 documents, published across 333 journals, representing 25 years of sap flow research. 2.3 Data Export and Pre-processing Full bibliographic metadata including the authorship, affiliations, titles, abstracts, keywords, cited references, publication year, and source information were exported from Scopus in CSV format. Prior to analysis, records were checked for duplication and inconsistencies [ 21 ]. 2.4 Bibliometric Analysis Framework The bibliometric analysis was conducted using a structured and sequential workflow that combined complementary analytical components to characterise the development and structure of sap flow research. First, a performance analysis was undertaken to examine temporal trends in scientific output and citation patterns, as well as the productivity of authors, institutions, countries, and journals. This was followed by science-mapping analyses aimed at exploring the intellectual, conceptual, and social dimensions of the literature, including co-authorship networks at the author, institutional, and country levels, co-citation relationships among authors and sources, and patterns of keyword co-occurrence. Finally, thematic and conceptual analyses were performed to assess the organisation and evolution of research themes, drawing on thematic mapping, thematic evolution analysis, bibliographic coupling, and factorial analysis based on high-frequency keywords. All analyses were descriptive in nature and were applied to identify structural patterns within the literature rather than to evaluate research quality or infer causal relationships. Quantitative analyses were conducted using the Bibliometrix package in R and its graphical interface Biblioshiny. Network visualisation and clustering were supported by VOSviewer. This study is based exclusively on bibliometric indicators derived from publication metadata, citations, and keyword networks, and does not involve qualitative evaluation of experimental designs, sap flow measurement methodologies, or empirical results reported in individual studies 3. Results 3.1 Dataset Characteristics The final bibliometric dataset comprised 1,989 documents published between 2000 and 2024, indexed across 333 peer-reviewed journals. In total, the documents cited 9,916 references, with an average of 40.7 citations per document. The Summary characteristics of the bibliometric dataset is given in Table 1 . The dataset spans a 25-year period (2000–2024), capturing the expansion and diversification of sap flow research within forest-related literature. Table 1 Summary characteristics of the bibliometric dataset Parameter Value Timespan 2000–2024 Number of sources (journals) 333 Number of documents 1,989 Annual growth rate of publications 4.11% Number of authors 4,641 Authors of single-authored documents 0 International co-authorship rate 39.82% Average number of co-authors per document 10.5 Number of author keywords (DE) 4,729 Number of references cited 9,916 Average age of documents (years) 10.3 Average citations per document 40.71 3.2 Temporal Trends in Publication Output and Citations Annual scientific production shows a sustained increase over the study period. In the early 2000s, publication output remained below 40 papers per year. From approximately 2010 onward, the number of publications increased steadily, exceeding 150 papers per year after 2018 (Fig. 2 ). Citation patterns mirror this growth trajectory. Earlier publications (2000–2005) display higher mean citations per article (Fig. 2 ), reflecting their longer citation windows and foundational role. More recent publications show lower average citation counts, consistent with their recency, but contribute to a continuous rise in cumulative annual citations. 3.3 Core Journals and Source Distribution Sap flow research is distributed across a broad range of journals, yet clear patterns of source concentration are evident (Table 2 ). A small group of journals accounts for a substantial proportion of total publications and citation impact within the field. Agricultural and Forest Meteorology emerges as the most productive source, contributing 193 articles, followed closely by Tree Physiology with 169 publications. These two journals also exhibit the highest local H-index values (60 and 56, respectively), highlighting their central role in shaping sap flow research. In terms of citation influence, Tree Physiology and Agricultural and Forest Meteorology record the highest total citations and consistently high mean citations per article, underscoring their importance as primary outlets for frequently cited and widely used studies. Other journals, including Forest Ecology and Management , Ecohydrology , and Trees – Structure and Function , also make substantial contributions. Several journals with lower publication volumes nevertheless show high citation intensity. Notably, Plant, Cell & Environment exhibits the highest mean citations per article among the top sources. Table 2 Top 10 journals contributing to sap flow research (2000–2024) Journal Articles (n) Local H-index Total citations Mean citations per article Agricultural and Forest Meteorology 193 60 11,097 57.5 Tree Physiology 169 56 11,095 65.7 Ecohydrology 96 30 2,716 28.3 Forest Ecology and Management 89 42 4,252 47.8 Trees – Structure and Function 88 28 2,991 34.0 Forests 79 17 868 11.0 Hydrological Processes 70 30 2,228 31.8 Journal of Hydrology 62 28 2,303 37.1 Plant, Cell & Environment 46 31 4,066 88.4 Agricultural Water Management 35 21 1,306 37.3 3.4 Influential Documents and Citation Structure Citation analysis identifies a small set of highly cited documents that represent highly cited and frequently referenced works within the sap flow literature. The most globally cited works are methodological, and synthesis papers published in the late 1980s and 1990s, particularly those introducing and consolidating thermal dissipation and heat-pulse approaches (Supplementary Fig. 1). Local citation analysis, restricted to references cited within the dataset, shows a similar pattern, with foundational methodological papers remaining among the most frequently cited (Supplementary Fig. 2). In addition, several review and synthesis articles published in the 2000s and early 2010s appear prominently, reflecting their role in standardising interpretation and application of sap flow data. 3.5 Authorship Patterns and Productivity Authorship productivity in sap flow research exhibits a strongly right-skewed distribution. A large proportion of authors contributed to a single publication, whereas a relatively small number of researchers accounted for a substantial share of total output. The observed author productivity closely conforms to Lotka’s inverse square law, with single-paper authors comprising 67.6% of contributors compared with a theoretical expectation of 62.1%, and two- and three-paper authors showing similarly close agreement with expected values. Higher productivity classes (≥ 10 publications) occur at progressively lower frequencies, indicating a heavy-tailed distribution typical of established research fields (Supplementary Fig. 3). Author-level analysis indicates a marked concentration of research output among a relatively small group of contributors (Fig. 3 A). Based on total publication counts, Zhao P emerges as the most productive author (58 articles), followed by Steppe K (48), Wang Y (46), Otsuki K (42), Nadezhdina N (42), and Čermák J (41). Adjustment for co-authorship through fractionalised article counts produces a broadly consistent ranking, suggesting that high productivity among these authors reflects sustained individual contributions rather than reliance on large collaborative teams. Patterns of citation influence, assessed using the local h-index within the analysed dataset, largely mirror trends in publication output (Fig. 3 B). Several highly productive authors also exhibit high local h-index values, including Otsuki K and Čermák J (h-index = 26), Nadezhdina N and Steppe K (h-index = 25), and Oren R (h-index = 22). Total citation counts further indicate substantial and sustained uptake of their work within the sap flow literature. Figure 3 highlights a core group of authors who combine high publication output with strong citation impact, alongside variability in citation influence among authors with similar productivity levels. Panel (A) shows the most productive authors ranked by total number of publications, with symbol size proportional to fractionalised authorship, reflecting relative contribution after accounting for co-authorship. Panel (B) presents author-level citation impact based on the local h-index calculated within the analysed dataset, with symbol size proportional to total citations 3.6 Institutional Contributions Analysis of institutional affiliations highlights a concentrated distribution of research output among a limited number of organisations (Supplementary Fig. 4). The most productive affiliations are dominated by universities and research institutions with strong programmes in forest science, ecohydrology, and plant physiology. Mendel University in Brno and Kyushu University had over 180 publications within the analysed dataset. Beijing Forestry University and the Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems also show high publication output. Several European and East Asian institutions also feature prominently among the most productive affiliations. Georg-August-Universität Göttingen, Universität Gent, and Technische Universität München represent major European contributors, while Northwest A&F University, the University of Chinese Academy of Sciences, and Beijing Forestry University illustrate the strong presence of Chinese institutions. The University of California also appears among the leading affiliations, indicating sustained contributions from North American research centres. Analysis of funding acknowledgements indicates that sap flow research has been supported by a diverse set of national and international funding agencies. The most frequently acknowledged funder was the National Natural Science Foundation of China (261 publications), followed by the U.S. National Science Foundation (142) and the Japan Society for the Promotion of Science (68). Major European funding bodies, including the Deutsche Forschungsgemeinschaft (66), European Commission (63), and Horizon 2020 Framework Programme (24), also contributed substantially. 3.7 Country-Level Scientific Production and Collaboration Country-level analysis shows a strong concentration of sap flow research output within a limited number of nations. China, the United States, Germany, Australia, and Japan together account for a substantial majority of the total publications over the study period. Among these, China emerges as the leading contributor in cumulative publication output followed by the United States, with Germany, Australia, and Japan forming a second tier of high-output countries. The temporal trajectories presented in Fig. 4 reveal clear differences in the timing and pace of national contributions. Japan, the United States, Germany, and Australia display steady publication activity from the early 2000s onward, indicating their role as early leaders in the development of sap flow research. In contrast, China shows negligible output in the early years but a rapid and sustained increase after approximately 2010, followed by a sharp acceleration after 2015. By the early 2020s, China surpasses all other countries in annual publication counts, reflecting its growing dominance in the field. International collaboration networks (Supplementary Fig. 5) indicate dense linkages among North America, Europe, East Asia, and Australia. The strongest bilateral collaborations occur between China–USA, Belgium–Czech Republic, and Japan–Australia. Participation by countries from South America, Africa, and South Asia increases after 2015, although with lower overall connectivity. 3.8 Keyword Frequency and Co-occurrence Structure Keyword frequency analysis (Fig. 5 ) confirms the dominance of core terms related to measurement and process interpretation, including sap flow , transpiration , and tree water use . Environmental and physiological terms such as drought , soil moisture , vapor pressure deficit , and canopy conductance show increasing frequency in later years. Figure 5 . Keyword frequency analysis of sap flow research based on author keywords in the Scopus database (2000–2024). Keyword co-occurrence network analysis given in Fig. 6 identifies three major conceptual clusters: a methodological cluster centred on sap flow measurement techniques, an ecohydrological cluster linking transpiration with soil–atmosphere processes, and a climate- and stress-oriented cluster associated with drought and environmental variability. The multiple correspondence analysis (Supplementary Fig. 6) further clarifies the conceptual structure of sap flow research by grouping keywords into three broad clusters. One cluster is associated with ecohydrological processes and environmental drivers, characterised by terms such as soil moisture, water supply, climate change, vapor pressure deficit, and leaf area index, reflecting studies focused on soil–plant–atmosphere interactions. A second cluster centres on plant physiological and anatomical mechanisms, including xylem, hydraulic conductivity, stem, and water uptake. The third cluster is linked to stress physiology and species-level adaptation, grouping keywords such as drought, drought stress, coniferous tree, evergreen tree, and water-use efficiency. The factorial plane (Fig. 7 ) explains over 80% of the total variance, indicating a robust representation of conceptual relationships. Environmental and hydrological terms are positioned predominantly along one axis, while physiological and structural terms occupy the opposite side. Core terms such as sap flux, transpiration, and soil moisture occupy intermediate positions. 3.9 Thematic Structure and Evolution The thematic mapping analysis identifies transpiration, soil moisture, and forestry as motor themes characterised by both high centrality and high density, indicating their strong internal development and central positioning within the sap flow research landscape. In contrast, sap flow, evapotranspiration, and drought are located within the basic theme quadrant, reflecting their broad relevance across the field while also suggesting continued conceptual expansion and integration with related research areas (Supplementary Fig. 7). Thematic evolution analysis further reveals a clear temporal structuring of research emphasis over the study period. An initial foundation phase from 2000 to 2010 was primarily oriented toward measurement techniques and species-specific investigations. This was followed by an integration phase between 2011 and 2020, during which sap flow studies increasingly incorporated hydrological and physiological drivers. In the most recent period (2021–2024), a synthesis phase is evident, marked by the emergence of climate-related, stress-response, and resilience-oriented themes. 3.10 Intellectual and Social Structure Co-citation analysis identifies distinct but interconnected clusters representing methodological pioneers, ecohydrological integrators, and climate-focused researchers. Source co-citation highlights Tree Physiology and Agricultural and Forest Meteorology as central publication hubs, with increasing linkage to interdisciplinary journals in ecology and climate science (Supplementary Fig. 8). The author co-citation network reveals three major, interconnected research clusters reflecting the intellectual structure of sap flow research (Supplementary Fig. 9). The first cluster groups methodological and technical contributions centred on authors such as Čermák, Granier, Clearwater, and Ford, whose work are frequently co-cited in relation to the development, calibration, and scaling of sap flow techniques. A second, central cluster is characterised by authors including Burgess, Chen, Baldocchi, and Cochard, representing studies that integrate sap flow measurements with physiological, micrometeorological, and ecohydrological processes at canopy and ecosystem scales. The third cluster comprises authors such as Allen, Anderegg, Adams, and Bartlett, and is associated with research on drought stress, hydraulic failure, and forest vulnerability under climate extremes. 4. Discussion 4.1 Consolidation of Sap Flow Research within Forest Ecohydrology Literature The bibliometric patterns observed in this study indicate that sap flow research has evolved from a relatively specialised methodological topic into a well-established research domain within the forest ecohydrology literature. The sustained increase in publication output after 2010, together with the continued citation prominence of early methodological contributions, points to a phase of consolidation rather than methodological replacement. From a bibliometric standpoint, this pattern reflects the long-term stability and continued relevance of sap flow techniques within forest-related research. The persistent citation influence of foundational studies associated with thermal dissipation and heat-pulse approaches further supports this interpretation. Rather than being displaced by alternative measurement techniques, these approaches continue to be frequently cited and co-occur across multiple thematic clusters, suggesting their role as enduring reference frameworks within the literature [ 10 ]. Such continuity is consistent with broader trends in forest hydrology research, where established measurement approaches are increasingly embedded within more complex analytical and modelling contexts [ 22 ]. Collectively, the bibliometric evidence positions sap flow research as a stable and widely integrated component of contemporary forest ecohydrology scholarship. 4.2 Shifts in Thematic Emphasis from Measurement to Ecohydrological Contexts Thematic mapping and keyword co-occurrence analyses indicate a gradual shift in thematic emphasis within the sap flow literature over time. Early phases of research are characterised by strong associations with measurement-focused terms related to sensor calibration, sapwood variability, and scaling challenges. In contrast, publications from later periods show increasing co-occurrence of sap flow with keywords related to environmental drivers such as soil moisture, vapour pressure deficit, and atmospheric demand. These patterns reflect a broadening of the thematic contexts in which sap flow measurements are discussed, as indicated by bibliometric indicators rather than direct evaluation of study content. The increasing prominence of keywords associated with drought, climate change, and forest resilience further highlights the expanding scope of sap flow research within the literature. These themes are frequently linked to studies addressing ecological stress and forest responses to environmental variability [ 15 , 18 ]. Importantly, the bibliometric patterns do not suggest a decline in physiologically oriented research. Instead, they indicate increasing integration of sap flow measurements within broader ecohydrological and climate-related research frameworks, where physiological perspectives continue to coexist alongside environmental applications [ 13 ]. 4.3 Intellectual Structure and Disciplinary Integration The co-citation networks identified in this study reveal a coherent intellectual structure comprising interconnected clusters associated with methodological development, ecohydrological integration, and climate-related applications. The continued centrality of frequently co-cited methodological contributors within both author and source co-citation networks suggests that contemporary sap flow research remains anchored in shared technical frameworks and measurement approaches. Such anchoring likely contributes to consistency in terminology and analytical perspectives across studies conducted in different forest types and climatic settings, a recurring concern within forest hydrology research [ 7 ]. At the same time, increasing co-citation of authors associated with carbon–water coupling, drought-induced forest responses, and ecosystem-scale fluxes indicates that sap flow research has become increasingly embedded within interdisciplinary research contexts extending beyond traditional forest physiology [ 22 , 23 ]. From a bibliometric perspective, this pattern suggests that sap flow studies occupy a bridging position within the literature, linking tree-level measurement approaches with broader ecohydrological and climate-oriented research domains. 4.4 Geographic Expansion and Collaborative Dynamics One of the most prominent structural trends identified in this analysis is the rapid geographic expansion of sap flow research, particularly the growth in contributions from East Asia. Temporal publication trends show that Japan, the United States, and several European countries played leading roles during the early development of the literature, whereas China emerged as the dominant contributor after 2010. Collaboration network analysis further indicates that this expansion has occurred within a strongly international research environment, characterised by dense co-authorship linkages among major research centres in Europe, East Asia, North America, and Australia. From a bibliometric perspective, these patterns likely reflect broader developments in forest research infrastructure, funding priorities, and increasing attention to forest–water interactions under changing climatic conditions. The persistence of European institutions as key methodological reference points, alongside the rapid growth of Asian research networks, suggests complementary rather than competing research trajectories. Such geographic diversification, coupled with sustained international collaboration, is indicative of a globally integrated research field rather than regional fragmentation. 4.5 Implications for Forest Hydrology and Tree Physiology Research Taken together, the bibliometric results suggest that sap flow research occupies a stable and central position within the forest hydrology literature. Its continued prominence appears to be associated less with methodological novelty and more with its frequent application across a wide range of forest research contexts. The strong association of sap flow studies with drought- and climate-related keywords in recent years highlights their widespread use in research addressing forest responses to environmental variability. At the same time, the bibliometric structure of the literature highlights potential constraints. The concentration of highly cited publications within a limited number of journals and research groups may reinforce dominant methodological conventions, potentially limiting the visibility of alternative approaches or underrepresented ecosystems. Although this study does not assess research quality or bias directly, the observed publication patterns suggest opportunities for broader representation of under-studied biomes, such as dry tropical forests and montane systems, as well as stronger integration with emerging datasets related to soil moisture and remote sensing [ 11 , 19 ]. From a literature-mapping perspective, these areas represent potential directions for future expansion of sap flow research. 4.6 Methodological Considerations and Limitations The findings of this bibliometric analysis should be interpreted in light of limitations inherent to database coverage and indexing practices. Reliance on the Scopus database ensures broad and consistent coverage of peer-reviewed literature but may underrepresent regionally focused journals or non-English publications. In addition, bibliometric indicators capture patterns of publication activity, citation relationships, and thematic associations rather than empirical validity or methodological accuracy. Accordingly, the results presented here describe structural and conceptual trends within the sap flow literature, rather than evaluating the experimental performance or reliability of individual sap flow measurement techniques. 5. Conclusions This bibliometric analysis provides a structured overview of the development of sap flow research in forest ecosystems over the period 2000–2024. The results document a sustained expansion of the literature, characterised by increasing publication output, growing international participation, and progressive diversification of research themes. Despite this growth, the intellectual structure of the field remains anchored in a shared set of methodological references, as reflected by the persistent citation prominence of established sap flow measurement approaches. From a bibliometric perspective, this pattern indicates consolidation rather than fragmentation within the literature. The analysis highlights a temporal shift in thematic emphasis, with early research primarily focused on measurement-related topics and later studies increasingly associated with ecohydrological, drought-related, and climate-oriented research contexts. This shift, as evidenced by keyword co-occurrence and thematic evolution analyses, reflects a broadening of the research landscape in which sap flow measurements are discussed and applied, rather than a departure from established methodological foundations. Geographically, sap flow research has become increasingly global. While institutions in Europe, North America, and Japan contributed prominently during the early development of the field, contributions from China have expanded rapidly since 2010. Patterns of international collaboration indicate strong cross-regional connectivity, suggesting that the growth of the literature has been accompanied by increasing integration rather than regional isolation. Taken together, the bibliometric patterns identified in this study characterise sap flow research as a mature and internationally connected research domain within forest ecohydrology. The value of this analysis lies in its ability to map research trends, thematic orientations, and collaborative structures, thereby providing an objective framework for understanding how the sap flow literature has evolved and where future research opportunities may emerge. Declarations Acknowledgements The authors acknowledge the Indian Council of Forestry Research and Education (ICFRE) for institutional support under the All India Coordinated Research Project (AICRP-19). The authors also express their sincere thanks to NPC N. Bala and Dr. Parmanand Kumar for their support and coordination during this work. The support and encouragement provided by the Director; Arid Forest Research Institute (AFRI) are gratefully acknowledged. Funding This research was supported by Compensatory Afforestation Fund Management and Planning Authority (CAMPA) under AICRP-19, titled “Assessing water requirement of different tree species and its impact on subsoil moisture.” Clinical trial number Clinical trial number: Not applicable. Ethics Approval and Consent to Participate This study is based exclusively on the analysis of published bibliographic data retrieved from the Scopus database and does not involve human participants, animals, or field experimentation. Ethical approval and informed consent are therefore not applicable. Consent for Publication Not applicable. Availability of Data and Materials All data analysed in this study were obtained from the Scopus database under standard access conditions. The bibliographic dataset can be regenerated by re-running the search query and filters described in the Materials and Methods section. Processed bibliometric outputs are available from the corresponding author upon reasonable request. Authors’ Contributions SK: Conceptualised the study and designed the bibliometric framework. SB: Manuscript drafting and data visualization IB: Manuscript drafting AP: conducted data retrieval, analysis, and visualisation. VM: interpreted the results and led manuscript drafting. All authors contributed to manuscript revision and approved the final version. Competing Interests / Conflict of Interest The authors declare that they have no competing interests. Generative AI statement The authors declare that ChatGPT was used during the preparation of this manuscript to assist with language editing and improvement of readability. Following the use of this tool, the authors critically reviewed, revised, and approved the content and take full responsibility for the accuracy, integrity, and originality of the work. Supplementary Material The supplementary material includes additional figures and tables supporting the bibliometric analyses, including extended author, institution, and journal-level metrics. References Bonan GB, Forests, Change C. Forcings, Feedbacks, and the Climate Benefits of Forests. Science. 2008;320:1444–9. https://doi.org/10.1126/science.1155121 . Calder IR. Water-resource and land-use issues. Iwmi; 1998. Fisher JB, Whittaker RJ, Malhi Y. ET come home: potential evapotranspiration in geographical ecology. Glob Ecol Biogeogr. 2011;20:1–18. https://doi.org/10.1111/j.1466-8238.2010.00578.x . Lawrence DM, Fisher RA, Koven CD, Oleson KW, Swenson SC, Bonan G, et al. The Community Land Model version 5: Description of new features, benchmarking, and impact of forcing uncertainty. J Adv Model Earth Syst Wiley Online Libr. 2019;11:4245–87. Granier A. Evaluation of transpiration in a Douglas-fir stand by means of sap flow measurements. Tree Physiol. 1987;3:309–20. Vandegehuchte MW, Steppe K. Corrigendum to: Sap-flux density measurement methods: working principles and applicability. Functional Plant Biology. CSIRO Publishing. 2013;40:1088–1088. Wilson KB, Hanson PJ, Mulholland PJ, Baldocchi DD, Wullschleger SD. A comparison of methods for determining forest evapotranspiration and its components: sap-flow, soil water budget, eddy covariance and catchment water balance. Agricultural and forest Meteorology. Volume 106. Elsevier; 2001. pp. 153–68. Oishi AC, Oren R, Stoy PC. Estimating components of forest evapotranspiration: a footprint approach for scaling sap flux measurements. agricultural and forest meteorology. Elsevier. 2008;148:1719–32. Poyatos R, Granda V, Molowny-Horas R, Mencuccini M, Steppe K, Martínez-Vilalta J. SAPFLUXNET: towards a global database of sap flow measurements [Internet]. Tree physiology. Oxford University Press; 2016 [cited 2025 Dec 31]. pp. 1449–55. https://academic.oup.com/treephys/article-abstract/36/12/1449/2571314 . Accessed 31 Dec 2025. Čermák J, Kučera J, Nadezhdina N. Sap flow measurements with some thermodynamic methods, flow integration within trees and scaling up from sample trees to entire forest stands. Trees. 2004;18:529–46. https://doi.org/10.1007/s00468-004-0339-6 . Steppe K, De Pauw DJ, Doody TM, Teskey RO. A comparison of sap flux density using thermal dissipation, heat pulse velocity and heat field deformation methods. Agricultural For Meteorol Elsevier. 2010;150:1046–56. Ford CR, Hubbard RM, Kloeppel BD, Vose JM. A comparison of sap flux-based evapotranspiration estimates with catchment-scale water balance. Agricultural For Meteorol Elsevier. 2007;145:176–85. Meinzer FC, Clearwater MJ, Goldstein G. Water transport in trees: current perspectives, new insights and some controversies. Environmental and experimental botany. Elsevier. 2001;45:239–62. Oren R, Pataki DE. Transpiration in response to variation in microclimate and soil moisture in southeastern deciduous forests. Oecologia Springer. 2001;127:549–59. McDowell N, Pockman WT, Allen CD, Breshears DD, Cobb N, Kolb T, et al. Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytol. 2008;178:719–39. https://doi.org/10.1111/j.1469-8137.2008.02436.x . Choat B, Jansen S, Brodribb TJ, Cochard H, Delzon S, Bhaskar R, et al. Global convergence in the vulnerability of forests to drought. Nat Nat Publishing Group UK Lond. 2012;491:752–5. Allen CD, Breshears DD, McDowell NG. On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. Ecosphere. 2015;6:1–55. https://doi.org/10.1890/ES15-00203.1 . Anderegg WRL, Hicke JA, Fisher RA, Allen CD, Aukema J, Bentz B, et al. Tree mortality from drought, insects, and their interactions in a changing climate. New Phytol. 2015;208:674–83. https://doi.org/10.1111/nph.13477 . Lu P, Urban L, Zhao P. Granier’s thermal dissipation probe (TDP) method for measuring sap flow in trees: theory and practice. ACTA BOTANICA SINICA-ENGLISH EDITION-. Sci PRESS. 2004;46:631–46. Aria M, Cuccurullo C. bibliometrix: An R-tool for comprehensive science mapping analysis. J informetrics Elsevier. 2017;11:959–75. Mahanta DK, Bhoi TK, Kothari S. Mapping the advancements in forest soil arthropod research: A bibliometric analysis from 1960 to 2024. Soil Adv Elsevier. 2025;3:100050. Baldocchi DD. How eddy covariance flux measurements have contributed to our understanding of Global Change Biology . Glob Change Biol. 2020;26:242–60. https://doi.org/10.1111/gcb.14807 . Allen CD, Macalady AK, Chenchouni H, Bachelet D, McDowell N, Vennetier M, et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest ecology and management. Elsevier. 2010;259:660–84. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8739987","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Systematic Review","associatedPublications":[],"authors":[{"id":594161073,"identity":"699eb926-a834-40f4-8ab1-1095d0b1dacf","order_by":0,"name":"Sharat Kothari","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYBACCQYGNjBlAOZWADEzcwMBLcxIWg6cAWlhJEoLA0TLwTYQSUCLZHv/sQc/91jYm7Offfj547zaaP52oJYfFdtwapHmOcxu2PNMInFnT7qxxMFtx3NnHGZsYOw5cxunFjmJZDYJngMSCQYH0hiAWo7lNgC1MDO24dci+eeAhL3B+WfMPw7OOZY7n5AWaaAWaaAtjBtupLFJHGyoyd1ASItkz2EzaZkDEokbbjxjszhz7EDuRqCWg/j8InG88ZnkmwN1QIelMd+oqKnLnXf+8MEHPypwa0EHh8HkAaLVA0EdKYpHwSgYBaNghAAAYF9bipk4n4sAAAAASUVORK5CYII=","orcid":"","institution":"ICFRE-Arid Forest Research Institute","correspondingAuthor":true,"prefix":"","firstName":"Sharat","middleName":"","lastName":"Kothari","suffix":""},{"id":594161074,"identity":"8134bc45-8c39-4c06-9594-987e730fc5a3","order_by":1,"name":"Sumantra Basu","email":"","orcid":"","institution":"ICFRE-Forest Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Sumantra","middleName":"","lastName":"Basu","suffix":""},{"id":594161076,"identity":"9f5d690f-0b84-468c-bd79-2b193a37d065","order_by":2,"name":"Ilham Bano","email":"","orcid":"","institution":"ICFRE-Arid Forest Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Ilham","middleName":"","lastName":"Bano","suffix":""},{"id":594161077,"identity":"ea24e118-4839-4f8c-ba41-105e1eddfe72","order_by":3,"name":"Amitha Panwar","email":"","orcid":"","institution":"ICFRE-Arid Forest Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Amitha","middleName":"","lastName":"Panwar","suffix":""},{"id":594161079,"identity":"ed611891-183d-4807-a6f8-c177e5de92e1","order_by":4,"name":"Veenu Mahecha","email":"","orcid":"","institution":"ICFRE-Arid Forest Research Institute","correspondingAuthor":false,"prefix":"","firstName":"Veenu","middleName":"","lastName":"Mahecha","suffix":""}],"badges":[],"createdAt":"2026-01-30 09:54:03","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8739987/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8739987/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103219315,"identity":"035b21f6-896e-4b9f-81e2-285ca4a0e00e","added_by":"auto","created_at":"2026-02-23 09:59:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":342459,"visible":true,"origin":"","legend":"\u003cp\u003ePRISMA-style flow diagram illustrating the identification, screening, and selection of publications included in the bibliometric analysis of sap flow research. Records were retrieved from the Scopus database and sequentially filtered by publication year (2000–2024), subject area, document type, keyword relevance, source type, and language, resulting in a final dataset of 1,989 journal articles and review papers\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/91725fff256640792c4025c2.png"},{"id":103219316,"identity":"f6e62344-7509-4617-9315-1e158a7bd649","added_by":"auto","created_at":"2026-02-23 09:59:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":124073,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal trends in annual publication output and mean citations per article in sap flow research from 2000 to 2024. The solid line (left axis) represents the number of publications per year, while the dashed line (right axis) shows mean citations per article.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/72394680be2a35fb8f255359.png"},{"id":103219318,"identity":"233bb75b-777a-4a2e-9b20-e4139c0014d4","added_by":"auto","created_at":"2026-02-23 09:59:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":162628,"visible":true,"origin":"","legend":"\u003cp\u003eAuthor productivity and citation impact in sap flow research (2000–2024).\u003cbr\u003e\nPanel (A) shows the most productive authors ranked by total number of publications, with symbol size proportional to fractionalised authorship, reflecting relative contribution after accounting for co-authorship. Panel (B) presents author-level citation impact based on the local h-index calculated within the analysed dataset, with symbol size proportional to total citations\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/51d42e5e715bd8eeaaaa5392.png"},{"id":103505811,"identity":"840a8708-1e8c-49ef-a3ea-accff784abcc","added_by":"auto","created_at":"2026-02-26 13:33:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":176566,"visible":true,"origin":"","legend":"\u003cp\u003eGlobal distribution and temporal evolution of sap flow research output.\u003cbr\u003e\n(A) Total cumulative publications by the ten most productive countries during 2000–2024.\u003cbr\u003e\n(B) Temporal growth in cumulative publications for the five leading countries.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/0480a1470f54ea021ff16929.png"},{"id":103219321,"identity":"5174c7c6-3d15-409e-aa23-d9dc14b182c1","added_by":"auto","created_at":"2026-02-23 09:59:52","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":215738,"visible":true,"origin":"","legend":"\u003cp\u003eKeyword frequency analysis of sap flow research based on author keywords in the Scopus database (2000–2024).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/f1f6277a61629edc321f19f8.png"},{"id":103505679,"identity":"13cebc66-e6f0-4f3c-93d1-f39ec37d66d7","added_by":"auto","created_at":"2026-02-26 13:32:34","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":416801,"visible":true,"origin":"","legend":"\u003cp\u003eKeyword co-occurrence network in sap flow research based on author keywords from the Scopus database (2000–2024). Nodes represent keywords, node size reflects frequency of occurrence, and link thickness indicates the strength of co-occurrence relationships, highlighting major thematic linkages within the literature.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/9b9dab851ee34dfe7a7288cc.png"},{"id":103505761,"identity":"159c841d-1bda-4ad1-84ac-6d1c406c5512","added_by":"auto","created_at":"2026-02-26 13:32:56","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":163117,"visible":true,"origin":"","legend":"\u003cp\u003eTwo-dimensional factorial map from multiple correspondence analysis (MCA) depicting the conceptual structure of sap flow research. Keywords are positioned according to their contributions to the first two dimensions.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/bf04db1a78c6620419c40000.png"},{"id":108682477,"identity":"8c472a7e-1d17-4a33-ba16-476519e6e076","added_by":"auto","created_at":"2026-05-07 09:28:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1917217,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/62626204-d944-421f-af8c-86273e731ee8.pdf"},{"id":103219322,"identity":"7efc5608-e652-4d46-8fe0-401579104a42","added_by":"auto","created_at":"2026-02-23 09:59:52","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1105785,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryfigures.docx","url":"https://assets-eu.researchsquare.com/files/rs-8739987/v1/a0a256508bee9224187c9fe5.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Mapping the global research landscape of sap flow studies in forest ecosystems: a bibliometric analysis (2000–2024)","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eForests play a critical role in regulating terrestrial water fluxes through transpiration, thereby influencing catchment hydrology, regional climate, and ecosystem carbon dynamics [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Tree-level water use governs how precipitation is partitioned between evapotranspiration, runoff, and soil storage, making it a central process in forest hydrology and ecohydrology [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Accurate quantification of tree transpiration is therefore essential for understanding forest responses to drought, warming, and land-use change, as well as for parameterising hydrological and land-surface models [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAmong the available approaches for measuring transpiration, sap flow techniques have become one of the most widely applied methods for estimating whole-tree water use. Since the late twentieth century, thermal-based methods such as thermal dissipation probes, heat pulse velocity, and heat balance techniques have enabled continuous, non-destructive monitoring of xylem water transport under field conditions [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. These methods provide a practical bridge between leaf-level gas exchange measurements and ecosystem-scale flux approaches, allowing transpiration to be quantified across temporal scales ranging from diurnal cycles to seasonal and interannual variability [\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eEarly sap flow research was largely oriented toward methodological development, calibration, and the challenge of scaling point measurements to biologically meaningful estimates of whole-tree and stand-level transpiration. Initial efforts focused on sensor design and on addressing radial and azimuthal variability in sap flux density, which were recognised early as major sources of uncertainty when extrapolating sap flow measurements beyond individual probes [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Subsequent methodological syntheses and comparative studies revisited these issues, refining correction approaches and scaling frameworks while explicitly situating sap flow techniques within broader ecohydrological applications [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Comparative reviews further demonstrated that sap flow measurements provide a robust basis for evaluating tree water use across species and forest types, supporting their application across contrasting biomes and climatic conditions [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs sap flow measurement techniques became more standardised, their application increasingly extended beyond methodological validation toward process-oriented questions in plant physiology and forest hydrology. Sap flow data began to be analysed alongside environmental drivers such as soil moisture, vapour pressure deficit, and radiation, enabling investigation of stomatal regulation, hydraulic constraints, and interspecific variation in water-use strategies [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Rather than serving solely as a means of quantifying transpiration, sap flow measurements were progressively interpreted as indicators of tree hydraulic functioning and physiological response to water limitation, contributing to a broader understanding of drought processes in forest ecosystems [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOver the past two decades, sap flow measurements have become increasingly integrated within ecohydrological research frameworks, particularly as tools for linking tree-level transpiration with ecosystem-scale water fluxes. Sap flow observations are frequently analysed alongside micrometeorological data, eddy covariance measurements, and catchment water balance approaches to assess forest evapotranspiration and land\u0026ndash;atmosphere exchange [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In parallel, sap flow data have been increasingly used to inform studies of forest responses to drought and heat stress, where changes in transpiration dynamics are interpreted in the context of hydraulic limitation, carbon\u0026ndash;water interactions, and elevated mortality risk [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Together, these developments underscore the growing relevance of sap flow measurements to ecohydrological research and to broader efforts aimed at understanding forest vulnerability under ongoing climatic change. Modern studies integrate sap flow data with soil moisture dynamics, canopy conductance, and atmospheric demand, providing critical insights into water-use efficiency and drought resilience.\u003c/p\u003e \u003cp\u003eDespite the rapid expansion and diversification of sap flow research, the field has developed largely through cumulative contributions across multiple disciplines, biomes, and methodological traditions. As a consequence, the literature has grown both extensive and heterogeneous, making it increasingly challenging to track how research priorities, intellectual influences, and collaborative structures have evolved over time. Narrative reviews have played an important role in synthesising sap flow measurement approaches and their physiological interpretation [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. However, such reviews are necessarily selective in scope and emphasis, and are not designed to capture broader structural patterns in publication activity, thematic development, or research networks across the field.\u003c/p\u003e \u003cp\u003eBibliometric analysis offers a complementary, quantitative approach for examining the evolution of scientific fields. By analysing publication metadata, citation relationships, and keyword networks, bibliometric methods can identify dominant research themes, influential contributions, and emerging directions without altering or reinterpreting primary results [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Such analyses are particularly valuable for mature measurement-based fields, where methodological continuity coexists with thematic diversification. In forest science, bibliometric approaches have been increasingly applied to topics such as forest hydrology, drought impacts, and climate adaptation, providing objective perspectives on research trajectories and knowledge gaps.\u003c/p\u003e \u003cp\u003eWithin sap flow research, however, a comprehensive bibliometric synthesis that explicitly situates the field at the interface of tree physiology, forest hydrology, and ecohydrology remains lacking. Given the central role of sap flow measurements in contemporary forest water-use studies, such an assessment is timely. Understanding how methodological foundations, thematic emphases, and international collaborations have evolved can help clarify the current structure of the field and inform future integrative research efforts.\u003c/p\u003e \u003cp\u003eThe present study addresses this gap by conducting a global bibliometric analysis of sap flow research published between 2000 and 2024, based on records retrieved from the Scopus database. The specific objectives were to (i) examine temporal trends in publication output and citation activity, (ii) identify leading journals, authors, institutions, and countries, (iii) characterise the thematic structure and evolution of sap flow research, and (iv) analyse patterns of international collaboration. By mapping the intellectual and social landscape of sap flow studies, this analysis aims to provide a structured overview of how the field has developed from methodological origins toward a central role in forest ecohydrology and climate-related research. This study focuses on mapping the structure and evolution of the sap flow literature using bibliometric indicators and does not involve qualitative evaluation of experimental designs or measurement methodologies\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Data Source and Search Strategy\u003c/h2\u003e \u003cp\u003eThe bibliometric analysis was based on records retrieved from the Scopus database, selected for its broad coverage of peer-reviewed journals in forest science, plant physiology, and environmental research. Data retrieval was conducted in November 2025 using a structured Boolean query designed to capture literature on sap flow and tree water use within forest and woody-plant contexts, while explicitly excluding agricultural and herbaceous systems.\u003c/p\u003e \u003cp\u003eThe final search string applied was:\u003c/p\u003e \u003cp\u003eTITLE-ABS-KEY ((\u0026ldquo;sap flow\u0026rdquo; OR \u0026ldquo;sap flux\u0026rdquo; OR \u0026ldquo;sap velocity\u0026rdquo; OR \u0026ldquo;xylem flow\u0026rdquo; OR \u0026ldquo;tree transpiration\u0026rdquo; OR \u0026ldquo;whole-tree transpiration\u0026rdquo; OR \u0026ldquo;canopy transpiration\u0026rdquo; OR \u0026ldquo;tree water use\u0026rdquo; OR \u0026ldquo;tree water requirement\u0026rdquo; OR \u0026ldquo;transpirational water use\u0026rdquo; OR \u0026ldquo;tree water consumption\u0026rdquo; OR \u0026ldquo;sap transport\u0026rdquo;) AND (tree* OR forest* OR \u0026ldquo;woody plant*\u0026rdquo; OR woodland* OR plantation* OR \u0026ldquo;boreal forest\u0026rdquo; OR \u0026ldquo;temperate forest\u0026rdquo; OR \u0026ldquo;tropical forest\u0026rdquo; OR \u0026ldquo;subtropical forest\u0026rdquo; OR \u0026ldquo;dry forest\u0026rdquo; OR \u0026ldquo;arid forest\u0026rdquo; OR rainforest* OR savanna*) AND NOT (crop OR agricultur* OR herb* OR grass OR shrub OR maize OR rice OR wheat OR soybean OR vineyard OR orchard OR horticultur*))\u003c/p\u003e \u003cp\u003eThis query was intentionally conservative to ensure thematic relevance to forest ecohydrology and tree physiology.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Screening, Inclusion, and Refinement Criteria\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe initial search returned 3,369 documents, which were progressively refined through a series of inclusion and exclusion steps to ensure relevance and analytical consistency. The PRISMA flow diagram used for screening is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. PRISMA-style flow diagram illustrating the identification, screening, and selection of publications included in the bibliometric analysis of sap flow research. Records were retrieved from the Scopus database and sequentially filtered by publication year (2000\u0026ndash;2024), subject area, document type, keyword relevance, source type, and language, resulting in a final dataset of 1,989 journal articles and review papers.\u003c/p\u003e \u003cp\u003eRecords were first limited to the period 2000\u0026ndash;2024, yielding 2,922 documents. Subject area filtering retained records indexed under \u003cem\u003eAgricultural and Biological Sciences\u003c/em\u003e, \u003cem\u003eEnvironmental Science\u003c/em\u003e, \u003cem\u003eBiochemistry, Genetics and Molecular Biology\u003c/em\u003e, \u003cem\u003eEarth and Planetary Sciences\u003c/em\u003e, and \u003cem\u003eMultidisciplinary Sciences\u003c/em\u003e. Records primarily indexed under chemistry or unrelated technical domains were excluded, as were articles with keywords not directly related to sap flow or tree transpiration. The document type was restricted to research articles and review papers, and only journal publications were retained as the source type. Article of English language and final published articles were selected for the study. After refinement and removal of incomplete records, the final dataset comprised 1,989 documents, published across 333 journals, representing 25 years of sap flow research.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Data Export and Pre-processing\u003c/h2\u003e \u003cp\u003eFull bibliographic metadata including the authorship, affiliations, titles, abstracts, keywords, cited references, publication year, and source information were exported from Scopus in CSV format. Prior to analysis, records were checked for duplication and inconsistencies [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Bibliometric Analysis Framework\u003c/h2\u003e \u003cp\u003eThe bibliometric analysis was conducted using a structured and sequential workflow that combined complementary analytical components to characterise the development and structure of sap flow research. First, a performance analysis was undertaken to examine temporal trends in scientific output and citation patterns, as well as the productivity of authors, institutions, countries, and journals. This was followed by science-mapping analyses aimed at exploring the intellectual, conceptual, and social dimensions of the literature, including co-authorship networks at the author, institutional, and country levels, co-citation relationships among authors and sources, and patterns of keyword co-occurrence. Finally, thematic and conceptual analyses were performed to assess the organisation and evolution of research themes, drawing on thematic mapping, thematic evolution analysis, bibliographic coupling, and factorial analysis based on high-frequency keywords. All analyses were descriptive in nature and were applied to identify structural patterns within the literature rather than to evaluate research quality or infer causal relationships. Quantitative analyses were conducted using the Bibliometrix package in R and its graphical interface Biblioshiny. Network visualisation and clustering were supported by VOSviewer. This study is based exclusively on bibliometric indicators derived from publication metadata, citations, and keyword networks, and does not involve qualitative evaluation of experimental designs, sap flow measurement methodologies, or empirical results reported in individual studies\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Dataset Characteristics\u003c/h2\u003e \u003cp\u003eThe final bibliometric dataset comprised 1,989 documents published between 2000 and 2024, indexed across 333 peer-reviewed journals. In total, the documents cited 9,916 references, with an average of 40.7 citations per document. The Summary characteristics of the bibliometric dataset is given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The dataset spans a 25-year period (2000\u0026ndash;2024), capturing the expansion and diversification of sap flow research within forest-related literature.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary characteristics of the bibliometric dataset\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eParameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTimespan\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2000\u0026ndash;2024\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of sources (journals)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e333\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of documents\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1,989\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnnual growth rate of publications\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.11%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of authors\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4,641\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAuthors of single-authored documents\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInternational co-authorship rate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e39.82%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage number of co-authors per document\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of author keywords (DE)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4,729\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of references cited\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9,916\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage age of documents (years)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAverage citations per document\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e40.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Temporal Trends in Publication Output and Citations\u003c/h2\u003e \u003cp\u003eAnnual scientific production shows a sustained increase over the study period. In the early 2000s, publication output remained below 40 papers per year. From approximately 2010 onward, the number of publications increased steadily, exceeding 150 papers per year after 2018 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCitation patterns mirror this growth trajectory. Earlier publications (2000\u0026ndash;2005) display higher mean citations per article (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), reflecting their longer citation windows and foundational role. More recent publications show lower average citation counts, consistent with their recency, but contribute to a continuous rise in cumulative annual citations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Core Journals and Source Distribution\u003c/h2\u003e \u003cp\u003eSap flow research is distributed across a broad range of journals, yet clear patterns of source concentration are evident (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A small group of journals accounts for a substantial proportion of total publications and citation impact within the field. \u003cem\u003eAgricultural and Forest Meteorology\u003c/em\u003e emerges as the most productive source, contributing 193 articles, followed closely by \u003cem\u003eTree Physiology\u003c/em\u003e with 169 publications. These two journals also exhibit the highest local H-index values (60 and 56, respectively), highlighting their central role in shaping sap flow research. In terms of citation influence, \u003cem\u003eTree Physiology\u003c/em\u003e and \u003cem\u003eAgricultural and Forest Meteorology\u003c/em\u003e record the highest total citations and consistently high mean citations per article, underscoring their importance as primary outlets for frequently cited and widely used studies. Other journals, including \u003cem\u003eForest Ecology and Management\u003c/em\u003e, \u003cem\u003eEcohydrology\u003c/em\u003e, and \u003cem\u003eTrees \u0026ndash; Structure and Function\u003c/em\u003e, also make substantial contributions. Several journals with lower publication volumes nevertheless show high citation intensity. Notably, \u003cem\u003ePlant, Cell \u0026amp; Environment\u003c/em\u003e exhibits the highest mean citations per article among the top sources.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTop 10 journals contributing to sap flow research (2000\u0026ndash;2024)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eJournal\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eArticles (n)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLocal H-index\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTotal citations\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMean citations per article\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAgricultural and Forest Meteorology\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e193\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11,097\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e57.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTree Physiology\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e169\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11,095\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e65.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEcohydrology\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2,716\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e28.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eForest Ecology and Management\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4,252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e47.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTrees \u0026ndash; Structure and Function\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2,991\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e34.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eForests\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e868\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e11.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHydrological Processes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2,228\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e31.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eJournal of Hydrology\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2,303\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlant, Cell \u0026amp; Environment\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4,066\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e88.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAgricultural Water Management\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,306\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Influential Documents and Citation Structure\u003c/h2\u003e \u003cp\u003eCitation analysis identifies a small set of highly cited documents that represent highly cited and frequently referenced works within the sap flow literature. The most globally cited works are methodological, and synthesis papers published in the late 1980s and 1990s, particularly those introducing and consolidating thermal dissipation and heat-pulse approaches (Supplementary Fig.\u0026nbsp;1).\u003c/p\u003e \u003cp\u003eLocal citation analysis, restricted to references cited within the dataset, shows a similar pattern, with foundational methodological papers remaining among the most frequently cited (Supplementary Fig.\u0026nbsp;2). In addition, several review and synthesis articles published in the 2000s and early 2010s appear prominently, reflecting their role in standardising interpretation and application of sap flow data.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Authorship Patterns and Productivity\u003c/h2\u003e \u003cp\u003eAuthorship productivity in sap flow research exhibits a strongly right-skewed distribution. A large proportion of authors contributed to a single publication, whereas a relatively small number of researchers accounted for a substantial share of total output. The observed author productivity closely conforms to Lotka\u0026rsquo;s inverse square law, with single-paper authors comprising 67.6% of contributors compared with a theoretical expectation of 62.1%, and two- and three-paper authors showing similarly close agreement with expected values. Higher productivity classes (\u0026ge;\u0026thinsp;10 publications) occur at progressively lower frequencies, indicating a heavy-tailed distribution typical of established research fields (Supplementary Fig.\u0026nbsp;3).\u003c/p\u003e \u003cp\u003eAuthor-level analysis indicates a marked concentration of research output among a relatively small group of contributors (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003eA). Based on total publication counts, Zhao P emerges as the most productive author (58 articles), followed by Steppe K (48), Wang Y (46), Otsuki K (42), Nadezhdina N (42), and Čerm\u0026aacute;k J (41). Adjustment for co-authorship through fractionalised article counts produces a broadly consistent ranking, suggesting that high productivity among these authors reflects sustained individual contributions rather than reliance on large collaborative teams.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePatterns of citation influence, assessed using the local h-index within the analysed dataset, largely mirror trends in publication output (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003eB). Several highly productive authors also exhibit high local h-index values, including Otsuki K and Čerm\u0026aacute;k J (h-index\u0026thinsp;=\u0026thinsp;26), Nadezhdina N and Steppe K (h-index\u0026thinsp;=\u0026thinsp;25), and Oren R (h-index\u0026thinsp;=\u0026thinsp;22). Total citation counts further indicate substantial and sustained uptake of their work within the sap flow literature. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e3\u003c/span\u003e highlights a core group of authors who combine high publication output with strong citation impact, alongside variability in citation influence among authors with similar productivity levels.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePanel (A) shows the most productive authors ranked by total number of publications, with symbol size proportional to fractionalised authorship, reflecting relative contribution after accounting for co-authorship. Panel (B) presents author-level citation impact based on the local h-index calculated within the analysed dataset, with symbol size proportional to total citations\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Institutional Contributions\u003c/h2\u003e \u003cp\u003eAnalysis of institutional affiliations highlights a concentrated distribution of research output among a limited number of organisations (Supplementary Fig.\u0026nbsp;4). The most productive affiliations are dominated by universities and research institutions with strong programmes in forest science, ecohydrology, and plant physiology. Mendel University in Brno and Kyushu University had over 180 publications within the analysed dataset. Beijing Forestry University and the Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems also show high publication output. Several European and East Asian institutions also feature prominently among the most productive affiliations. Georg-August-Universit\u0026auml;t G\u0026ouml;ttingen, Universit\u0026auml;t Gent, and Technische Universit\u0026auml;t M\u0026uuml;nchen represent major European contributors, while Northwest A\u0026amp;F University, the University of Chinese Academy of Sciences, and Beijing Forestry University illustrate the strong presence of Chinese institutions. The University of California also appears among the leading affiliations, indicating sustained contributions from North American research centres.\u003c/p\u003e \u003cp\u003eAnalysis of funding acknowledgements indicates that sap flow research has been supported by a diverse set of national and international funding agencies. The most frequently acknowledged funder was the National Natural Science Foundation of China (261 publications), followed by the U.S. National Science Foundation (142) and the Japan Society for the Promotion of Science (68). Major European funding bodies, including the Deutsche Forschungsgemeinschaft (66), European Commission (63), and Horizon 2020 Framework Programme (24), also contributed substantially.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Country-Level Scientific Production and Collaboration\u003c/h2\u003e \u003cp\u003eCountry-level analysis shows a strong concentration of sap flow research output within a limited number of nations. China, the United States, Germany, Australia, and Japan together account for a substantial majority of the total publications over the study period. Among these, China emerges as the leading contributor in cumulative publication output followed by the United States, with Germany, Australia, and Japan forming a second tier of high-output countries.\u003c/p\u003e \u003cp\u003eThe temporal trajectories presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003e reveal clear differences in the timing and pace of national contributions. Japan, the United States, Germany, and Australia display steady publication activity from the early 2000s onward, indicating their role as early leaders in the development of sap flow research. In contrast, China shows negligible output in the early years but a rapid and sustained increase after approximately 2010, followed by a sharp acceleration after 2015. By the early 2020s, China surpasses all other countries in annual publication counts, reflecting its growing dominance in the field.\u003c/p\u003e\u003cp\u003eInternational collaboration networks (Supplementary Fig. 5) indicate dense linkages among North America, Europe, East Asia, and Australia. The strongest bilateral collaborations occur between China\u0026ndash;USA, Belgium\u0026ndash;Czech Republic, and Japan\u0026ndash;Australia. Participation by countries from South America, Africa, and South Asia increases after 2015, although with lower overall connectivity.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8 Keyword Frequency and Co-occurrence Structure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKeyword frequency analysis (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) confirms the dominance of core terms related to measurement and process interpretation, including \u003cem\u003esap flow\u003c/em\u003e, \u003cem\u003etranspiration\u003c/em\u003e, and \u003cem\u003etree water use\u003c/em\u003e. Environmental and physiological terms such as \u003cem\u003edrought\u003c/em\u003e, \u003cem\u003esoil moisture\u003c/em\u003e, \u003cem\u003evapor pressure deficit\u003c/em\u003e, and \u003cem\u003ecanopy conductance\u003c/em\u003e show increasing frequency in later years.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. Keyword frequency analysis of sap flow research based on author keywords in the Scopus database (2000\u0026ndash;2024).\u003c/p\u003e\n\u003cp\u003eKeyword co-occurrence network analysis given in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e identifies three major conceptual clusters:\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003ea methodological cluster centred on sap flow measurement techniques,\u003c/li\u003e\n \u003cli\u003ean ecohydrological cluster linking transpiration with soil\u0026ndash;atmosphere processes, and\u0026nbsp;\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003ea climate- and stress-oriented cluster associated with drought and environmental variability.\u003cbr\u003e\u003c/span\u003e\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003eThe multiple correspondence analysis (Supplementary Fig. 6) further clarifies the conceptual structure of sap flow research by grouping keywords into three broad clusters. One cluster is associated with ecohydrological processes and environmental drivers, characterised by terms such as soil moisture, water supply, climate change, vapor pressure deficit, and leaf area index, reflecting studies focused on soil\u0026ndash;plant\u0026ndash;atmosphere interactions. A second cluster centres on plant physiological and anatomical mechanisms, including xylem, hydraulic conductivity, stem, and water uptake. The third cluster is linked to stress physiology and species-level adaptation, grouping keywords such as drought, drought stress, coniferous tree, evergreen tree, and water-use efficiency.\u003c/p\u003e\n\u003cp\u003eThe factorial plane (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e) explains over 80% of the total variance, indicating a robust representation of conceptual relationships. Environmental and hydrological terms are positioned predominantly along one axis, while physiological and structural terms occupy the opposite side. Core terms such as sap flux, transpiration, and soil moisture occupy intermediate positions.\u003c/p\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.9 Thematic Structure and Evolution\u003c/h2\u003e\n \u003cp\u003eThe thematic mapping analysis identifies transpiration, soil moisture, and forestry as motor themes characterised by both high centrality and high density, indicating their strong internal development and central positioning within the sap flow research landscape. In contrast, sap flow, evapotranspiration, and drought are located within the basic theme quadrant, reflecting their broad relevance across the field while also suggesting continued conceptual expansion and integration with related research areas (Supplementary Fig.\u0026nbsp;7).\u003c/p\u003e\n \u003cp\u003eThematic evolution analysis further reveals a clear temporal structuring of research emphasis over the study period. An initial foundation phase from 2000 to 2010 was primarily oriented toward measurement techniques and species-specific investigations. This was followed by an integration phase between 2011 and 2020, during which sap flow studies increasingly incorporated hydrological and physiological drivers. In the most recent period (2021\u0026ndash;2024), a synthesis phase is evident, marked by the emergence of climate-related, stress-response, and resilience-oriented themes.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003ch2\u003e3.10 Intellectual and Social Structure\u003c/h2\u003e\n \u003cp\u003eCo-citation analysis identifies distinct but interconnected clusters representing methodological pioneers, ecohydrological integrators, and climate-focused researchers. Source co-citation highlights \u003cem\u003eTree Physiology\u003c/em\u003e and \u003cem\u003eAgricultural and Forest Meteorology\u003c/em\u003e as central publication hubs, with increasing linkage to interdisciplinary journals in ecology and climate science (Supplementary Fig. 8).\u003c/p\u003e\n \u003cp\u003eThe author co-citation network reveals three major, interconnected research clusters reflecting the intellectual structure of sap flow research (Supplementary Fig.\u0026nbsp;9). The first cluster groups methodological and technical contributions centred on authors such as Čerm\u0026aacute;k, Granier, Clearwater, and Ford, whose work are frequently co-cited in relation to the development, calibration, and scaling of sap flow techniques. A second, central cluster is characterised by authors including Burgess, Chen, Baldocchi, and Cochard, representing studies that integrate sap flow measurements with physiological, micrometeorological, and ecohydrological processes at canopy and ecosystem scales. The third cluster comprises authors such as Allen, Anderegg, Adams, and Bartlett, and is associated with research on drought stress, hydraulic failure, and forest vulnerability under climate extremes.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Consolidation of Sap Flow Research within Forest Ecohydrology Literature\u003c/h2\u003e \u003cp\u003eThe bibliometric patterns observed in this study indicate that sap flow research has evolved from a relatively specialised methodological topic into a well-established research domain within the forest ecohydrology literature. The sustained increase in publication output after 2010, together with the continued citation prominence of early methodological contributions, points to a phase of consolidation rather than methodological replacement. From a bibliometric standpoint, this pattern reflects the long-term stability and continued relevance of sap flow techniques within forest-related research.\u003c/p\u003e \u003cp\u003eThe persistent citation influence of foundational studies associated with thermal dissipation and heat-pulse approaches further supports this interpretation. Rather than being displaced by alternative measurement techniques, these approaches continue to be frequently cited and co-occur across multiple thematic clusters, suggesting their role as enduring reference frameworks within the literature [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Such continuity is consistent with broader trends in forest hydrology research, where established measurement approaches are increasingly embedded within more complex analytical and modelling contexts [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Collectively, the bibliometric evidence positions sap flow research as a stable and widely integrated component of contemporary forest ecohydrology scholarship.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Shifts in Thematic Emphasis from Measurement to Ecohydrological Contexts\u003c/h2\u003e \u003cp\u003eThematic mapping and keyword co-occurrence analyses indicate a gradual shift in thematic emphasis within the sap flow literature over time. Early phases of research are characterised by strong associations with measurement-focused terms related to sensor calibration, sapwood variability, and scaling challenges. In contrast, publications from later periods show increasing co-occurrence of sap flow with keywords related to environmental drivers such as soil moisture, vapour pressure deficit, and atmospheric demand. These patterns reflect a broadening of the thematic contexts in which sap flow measurements are discussed, as indicated by bibliometric indicators rather than direct evaluation of study content.\u003c/p\u003e \u003cp\u003eThe increasing prominence of keywords associated with drought, climate change, and forest resilience further highlights the expanding scope of sap flow research within the literature. These themes are frequently linked to studies addressing ecological stress and forest responses to environmental variability [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Importantly, the bibliometric patterns do not suggest a decline in physiologically oriented research. Instead, they indicate increasing integration of sap flow measurements within broader ecohydrological and climate-related research frameworks, where physiological perspectives continue to coexist alongside environmental applications [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Intellectual Structure and Disciplinary Integration\u003c/h2\u003e \u003cp\u003eThe co-citation networks identified in this study reveal a coherent intellectual structure comprising interconnected clusters associated with methodological development, ecohydrological integration, and climate-related applications. The continued centrality of frequently co-cited methodological contributors within both author and source co-citation networks suggests that contemporary sap flow research remains anchored in shared technical frameworks and measurement approaches. Such anchoring likely contributes to consistency in terminology and analytical perspectives across studies conducted in different forest types and climatic settings, a recurring concern within forest hydrology research [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt the same time, increasing co-citation of authors associated with carbon\u0026ndash;water coupling, drought-induced forest responses, and ecosystem-scale fluxes indicates that sap flow research has become increasingly embedded within interdisciplinary research contexts extending beyond traditional forest physiology [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. From a bibliometric perspective, this pattern suggests that sap flow studies occupy a bridging position within the literature, linking tree-level measurement approaches with broader ecohydrological and climate-oriented research domains.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Geographic Expansion and Collaborative Dynamics\u003c/h2\u003e \u003cp\u003eOne of the most prominent structural trends identified in this analysis is the rapid geographic expansion of sap flow research, particularly the growth in contributions from East Asia. Temporal publication trends show that Japan, the United States, and several European countries played leading roles during the early development of the literature, whereas China emerged as the dominant contributor after 2010. Collaboration network analysis further indicates that this expansion has occurred within a strongly international research environment, characterised by dense co-authorship linkages among major research centres in Europe, East Asia, North America, and Australia.\u003c/p\u003e \u003cp\u003eFrom a bibliometric perspective, these patterns likely reflect broader developments in forest research infrastructure, funding priorities, and increasing attention to forest\u0026ndash;water interactions under changing climatic conditions. The persistence of European institutions as key methodological reference points, alongside the rapid growth of Asian research networks, suggests complementary rather than competing research trajectories. Such geographic diversification, coupled with sustained international collaboration, is indicative of a globally integrated research field rather than regional fragmentation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e4.5 Implications for Forest Hydrology and Tree Physiology Research\u003c/h2\u003e \u003cp\u003eTaken together, the bibliometric results suggest that sap flow research occupies a stable and central position within the forest hydrology literature. Its continued prominence appears to be associated less with methodological novelty and more with its frequent application across a wide range of forest research contexts. The strong association of sap flow studies with drought- and climate-related keywords in recent years highlights their widespread use in research addressing forest responses to environmental variability.\u003c/p\u003e \u003cp\u003eAt the same time, the bibliometric structure of the literature highlights potential constraints. The concentration of highly cited publications within a limited number of journals and research groups may reinforce dominant methodological conventions, potentially limiting the visibility of alternative approaches or underrepresented ecosystems. Although this study does not assess research quality or bias directly, the observed publication patterns suggest opportunities for broader representation of under-studied biomes, such as dry tropical forests and montane systems, as well as stronger integration with emerging datasets related to soil moisture and remote sensing [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. From a literature-mapping perspective, these areas represent potential directions for future expansion of sap flow research.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec23\" class=\"Section2\"\u003e \u003ch2\u003e4.6 Methodological Considerations and Limitations\u003c/h2\u003e \u003cp\u003eThe findings of this bibliometric analysis should be interpreted in light of limitations inherent to database coverage and indexing practices. Reliance on the Scopus database ensures broad and consistent coverage of peer-reviewed literature but may underrepresent regionally focused journals or non-English publications. In addition, bibliometric indicators capture patterns of publication activity, citation relationships, and thematic associations rather than empirical validity or methodological accuracy. Accordingly, the results presented here describe structural and conceptual trends within the sap flow literature, rather than evaluating the experimental performance or reliability of individual sap flow measurement techniques.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis bibliometric analysis provides a structured overview of the development of sap flow research in forest ecosystems over the period 2000\u0026ndash;2024. The results document a sustained expansion of the literature, characterised by increasing publication output, growing international participation, and progressive diversification of research themes. Despite this growth, the intellectual structure of the field remains anchored in a shared set of methodological references, as reflected by the persistent citation prominence of established sap flow measurement approaches. From a bibliometric perspective, this pattern indicates consolidation rather than fragmentation within the literature.\u003c/p\u003e \u003cp\u003eThe analysis highlights a temporal shift in thematic emphasis, with early research primarily focused on measurement-related topics and later studies increasingly associated with ecohydrological, drought-related, and climate-oriented research contexts. This shift, as evidenced by keyword co-occurrence and thematic evolution analyses, reflects a broadening of the research landscape in which sap flow measurements are discussed and applied, rather than a departure from established methodological foundations.\u003c/p\u003e \u003cp\u003eGeographically, sap flow research has become increasingly global. While institutions in Europe, North America, and Japan contributed prominently during the early development of the field, contributions from China have expanded rapidly since 2010. Patterns of international collaboration indicate strong cross-regional connectivity, suggesting that the growth of the literature has been accompanied by increasing integration rather than regional isolation.\u003c/p\u003e \u003cp\u003eTaken together, the bibliometric patterns identified in this study characterise sap flow research as a mature and internationally connected research domain within forest ecohydrology. The value of this analysis lies in its ability to map research trends, thematic orientations, and collaborative structures, thereby providing an objective framework for understanding how the sap flow literature has evolved and where future research opportunities may emerge.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the Indian Council of Forestry Research and Education (ICFRE) for institutional support under the All India Coordinated Research Project (AICRP-19). The authors also express their sincere thanks to NPC N. Bala and Dr. Parmanand Kumar for their support and coordination during this work. The support and encouragement provided by the Director; Arid Forest Research Institute (AFRI) are gratefully acknowledged.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by Compensatory Afforestation Fund Management and Planning Authority (CAMPA) under AICRP-19, titled \u0026ldquo;Assessing water requirement of different tree species and its impact on subsoil moisture.\u0026rdquo;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical trial number\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eClinical trial number: Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Approval and Consent to Participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is based exclusively on the analysis of published bibliographic data retrieved from the Scopus database and does not involve human participants, animals, or field experimentation. Ethical approval and informed consent are therefore not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for Publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of Data and Materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data analysed in this study were obtained from the Scopus database under standard access conditions. The bibliographic dataset can be regenerated by re-running the search query and filters described in the Materials and Methods section. Processed bibliometric outputs are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSK: Conceptualised the study and designed the bibliometric framework.\u003c/p\u003e\n\u003cp\u003eSB: Manuscript drafting and data visualization\u003c/p\u003e\n\u003cp\u003eIB: Manuscript drafting\u003cbr\u003e\u0026nbsp;AP: conducted data retrieval, analysis, and visualisation.\u003cbr\u003e\u0026nbsp;VM: interpreted the results and led manuscript drafting.\u003cbr\u003e\u0026nbsp;All authors contributed to manuscript revision and approved the final version.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests / Conflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGenerative AI statement\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declare that ChatGPT was used during the preparation of this manuscript to assist with language editing and improvement of readability. Following the use of this tool, the authors critically reviewed, revised, and approved the content and take full responsibility for the accuracy, integrity, and originality of the work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary Material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe supplementary material includes additional figures and tables supporting the bibliometric analyses, including extended author, institution, and journal-level metrics.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBonan GB, Forests, Change C. Forcings, Feedbacks, and the Climate Benefits of Forests. 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Elsevier. 2010;259:660\u0026ndash;84.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Tree transpiration, forest ecohydrology, xylem water transport, canopy conductance, bibliometric analysis","lastPublishedDoi":"10.21203/rs.3.rs-8739987/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8739987/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSap flow techniques are widely applied in forest research to examine tree water use and forest\u0026ndash;atmosphere interactions across diverse climatic and ecological settings. Over the past two decades, the volume of sap flow\u0026ndash;based studies has increased substantially, spanning multiple disciplines, regions, and research contexts. Despite this growth, a quantitative overview of how sap flow research has evolved in terms of publication trends, thematic focus, and collaboration patterns remains limited.\u003c/p\u003e \u003cp\u003eThis study presents a bibliometric analysis of global sap flow research in forest ecosystems published between 2000 and 2024, based on 1,989 peer-reviewed journal articles and reviews indexed in the Scopus database. Using bibliometric performance indicators and science-mapping techniques implemented through the \u003cem\u003ebibliometrix\u003c/em\u003e package and VOSviewer, we analysed temporal publication dynamics, citation structures, leading journals and authors, international collaboration networks, keyword co-occurrence patterns, and thematic evolution.\u003c/p\u003e \u003cp\u003eThe results show a steady increase in publication output after 2010, accompanied by expanding international collaboration and diversification of research themes. Keywords related to established sap flow measurement approaches, particularly thermal dissipation and heat-pulse methods, remain consistently prominent throughout the study period. At the same time, increasing co-occurrence of sap flow with terms associated with drought, soil moisture, atmospheric demand, and climate variability reflects a broadening research emphasis in recent years. Co-citation and thematic analyses reveal a well-defined research structure linking methodological foundations with ecohydrological and climate-oriented research context. These findings provide an objective overview of research trends and emerging thematic directions.\u003c/p\u003e","manuscriptTitle":"Mapping the global research landscape of sap flow studies in forest ecosystems: a bibliometric analysis (2000–2024)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-23 09:59:43","doi":"10.21203/rs.3.rs-8739987/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6e6ce8cd-015f-4688-96e1-1bf91b533213","owner":[],"postedDate":"February 23rd, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Rejected","date":"2026-05-07T09:19:59+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-07T09:26:34+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-23 09:59:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8739987","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8739987","identity":"rs-8739987","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}