Suppression of an invasive pine by a native shrub following a megafire

11 Seedling density of the Chilean wineberry Aristotelia chilensis negatively correlates with the 12 seedlings’ abundance of an invasive pine Pinus radiata, particularly in post-fire areas. This pattern 13 emerged following a megafire in Chile’s Coastal Maulino Forest, a biodiversity hotspot facing 14 increasing fire threats. This pattern, coupled with a high proportion of plots lacking pine seedlings, 15 suggests that A. chilensis may play a role in limiting P . radiata invasion. The negative relationship 16 was strongest in areas with moderate fire severity, likely reflecting differences in shade tolerance. 17 A. chilensis, a light-demanding species with some degree of shade tolerance, can persist in partially 18 shaded environments. In contrast, P . radiata, a more strictly light-demanding species, struggles to 19 establish under significant shade. In h igh-severity fires, however , we found no significant 20 relationship between these species , likely due to detrimental effects on both species, including 21 potential microbiome dependence for A. chilensis. As A. chilensis shows successful establishment 22 at low fire severity, enhancing its post-fire recruitment, particularly in moderately burned areas, 23 could be a valuable strategy for mitigating P . radiata invasion and restoring fire -affected 24 Mediterranean ecosystems. 25

26 Invasion resistance, Fire severity, Post-fire establishment, Soil microbiome 27 28 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900

29 Wildfires pose a significant threat to biodiversity, disrupting ecosystem functions and threatening 30 sensitive habitats worldwide. Their increased frequency and intensity are attributed to various 31 factors, including climate change and land -use modifications (McLauchlan et al. 2020) . The 32 Coastal Maulino Forest, a biodiversity hotspot in central Chile (Myers et al. 2000), is facing more 33 frequent and intense wildfires in last decades, driven by factors such as rising temperatures , a 34 megadrought and the forestry plantations of non-native species (González et al. 2018, 2023), some 35 of which could become invasive after fire disturbances . Primarily, the invasive species is Pinus 36 radiata (Pinaceae) which covers approximately 60% of the country’s 2.5 million hectares of forest 37 plantations (Bustamante and Simonetti 2005, Gonzá lez et al. 2018) . The devastating 2017 “Las 38 Máquinas” megafire burned over 200,000 ha of the Coastal Maulino Forest , a stark reminder of 39 the vulnerability of this ecosystem (Valencia et al. 2018) . Despite ongoing active and passive 40 restoration efforts in south -central Chile (Morales et al. 2021, Souza -Alonso et al. 2022) , 41 challenges persist, including the rapid arrival of post-fire pine regeneration that hinders restoration 42 success (Gómez et al. 2019, González et al. 2020, 2023). This highlights the need for conservation 43 and restoration practices tailored to this unique ecosystem. 44 Invasive species often display rapid resource utili sation, potentially outcompeting native 45 species and promoting more frequent fire events. This can create a positive invasion-fire feedback 46 loop (Contreras et al. 2011, Taylor et al. 2017). P . radiata is a light-demanding and shade-intolerant 47 species known for its aggressive post-fire regeneration through serotinous cones, which release 48 large amounts of viable wind -dispersed seeds after fire events (Franzese and Raffaele 2017) . 49 Studies have shown a higher probability of fire ignition in areas dominated by P . radiata 50 plantations compared to native forests in south -central Chile (Contreras et al. 2011, Gómez -51 González et al. 2019). 52 Previous research suggests limited success in controlling P . radiata invasion through 53 overall native species diversity (Gómez et al. 2019, González et al. 2020) . However, recent field 54 studies provide evidence that the native wineberry species Aristotelia chilensis (Elaeocarpaceae) 55 efficiently recolonises burnt areas even where P . radiata is present (Promis et al. 2019, Becerra et 56 al. 2022) . A. chilensis is a fast -growing, light -demanding, fleshy -fruiting bird -dispersed tree 57 species with a semi -dioecious leaf habit. Th ese traits allow it to not only colonise clearings but 58 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 also persist after plantations replace native forest s because it can exhibit some shade tolerance 59 (Guerra et al. 2010, Salgado -Luarte and Gianoli 2012). This rapid establishment and fast growth 60 of A. chilensis would align with the concept of the “ pre-emptive resource effect” – a mechanism 61 where early colonising native species can out compete invasive plants by monopoli sing essential 62 resources (Byun et al. 2013, Byun and Lee 2017, Delavaux et al. 2023) . Additionally, studies 63 suggest that P . radiata, being a shade-intolerant species, might struggle to establish into a darker 64 understory dominated by A. chilensis and other na tive species (Gómez et al. 2019, Becerra and 65 Simonetti 2020). The efficient colonisation and fast growth of A. chilensis suggest that it has the 66 potential to act as a native plant competitor against P . radiata invasion in fire-affected ecosystems. 67 Building upon competition-based biotic resistance (Elton 1958) and the theory of limiting 68 similarity, where native species can limit invasive plant establishment due to niche overlap, we 69 hypothesised that P . radiata abundance would negatively correlate with increasing A. chilensis 70 abundance. Specifically, we tested the relationship between the abundance of A. chilensis and P . 71 radiata in plots affected by varying fire severity levels caused by the Las Máquinas mega -fire in 72 the Maulino Coastal Forest. Additionally, we explored whether fire severity modulates this 73 relationship. Moderate - or low -severity fires that increase light penetration while retaining 74 understory vegetation could favour A. chilensis establishment, potentially strengthening its 75 competitive effect on P . radiata (i.e., a negative relationship). In contrast, high-severity fires that 76 create harsher conditions and potential soil disruption (i.e., depleting the soil microbiome) could 77 hinder the establishment of both A. chilensis and P . radiata, obscuring any competitive effects. By 78 elucidating these dynamics, we aim to provide valuable data to guide and enhance conservation 79 and restoration efforts in fire-affected areas across the central Mediterranean region of Chile. 80

81 Study Site 82 The study was conducted at El Porvenir (35°42’ S, 72°22’ W), located at the northern edge of the 83 Coastal Maulino Forest in central -south Chile (Gómez et al. 2022) . El Porvenir is a fragment of 84 native mesic forest type, surrounded by large stands of planted P . radiata and Eucalyptus globulus 85 (Myrtaceae). The dominant tree species include Nothofagus glauca (Nothofagaceae), N. 86 alessandrii, N. obliqua, Cryptocarya alba (Lauraceae), Aextoxicon punctatum (Aextoxicaceae), 87 Gevuina avellana (Proteaceae), and A. chilensis. Study area ha s a Mediterranean climate with a 88 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 mean annual precipitation of 918 mm and a mean annual temperature of 12.7 °C (Becerra and 89 Simonetti 2020). 90 In January 2017, the Las Máquinas megafire affected El Porvenir, which experienced fire 91 severity ranging from low to high (see Valencia et al. 2018, Gómez et al. 2022). Following the fire, 92 several species exhibited regeneration at different levels, with high seedling recruitment for the 93 invasive P . radiata and the native A. chilensis (Gómez et al. 2022). 94 Plot establishment and seedling survey 95 To assess the potential role of A. chilensis in limiting P . radiata invasion, we established twenty-96 three 625 m 2 plots across El Porvenir. These plots were randomly distributed within areas 97 experiencing low (n = 8), moderate (n = 10), and high (n = 5) fire severity (see Gómez et al. 2022). 98 Seedling surveys were conducted at 8 and 24 months following the Las Máquinas mega -fire 99 (hereafter 2017 and 2019). To estimate the density of A. chilensis and P . radiata seedlings, three 1 100 m2 sub-plots were randomly located within each plot to search for regenerating A.chilensis and P . 101 radiata seedlings under 60 cm in height. The average number of seedlings per 3 m2 sampled area 102 was 21.98 ± 20.34 for A. chilensis and 7.46 ± 12.78 for P . radiata. To confirm that the seedlings 103 originated from seeds and not resprouts , we collected at least three random plant samples per 104 species from each sub-plot for root system examination. 105 Data analysis 106 We performed a negative binomial Generali sed Linear Mixed-effects Model ( NB GLMM) to 107 analyse the relationship between the abundance of A. chilensis and P . radiata seedlings. This 108 statistical method is suitable for counting data with overdispersion, a common characteristic of 109 ecological data. Here, we accoun t for the potential influence of sampling time at each plot by 110 including time since the fire as a random factor nested within fire severity. This nested structure 111 considers the variation in fire severity across the landscape while acknowledging the potenti al 112 influence of sampling time within each fire severity category (see above). Additionally, we 113 conducted separate NB GLMM analyses for each fire severity level, including time sampling as a 114 random factor. 115

and Discussion 116 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 Our analysis revealed a negative relationship between A. chilensis and P . radiata abundance across 117 the study site (ꭕ2(1,46) = 8.0707, p < 0.01; Fig. 1). Thus, areas with higher numbers of A. chilensis 118 seedlings have fewer P . radiata seedlings, potentially indicating a suppressive effect of native 119 species on invasive tree establishment. 120 Furthermore, our results suggest that the strength of this negative relationship varied 121 depending on fire severity. A significantly negative relationship between A. chilensis and P . radiata 122 was found in areas with moderate fire severity (ꭕ2(1,20) = 16.385, p < 0.01; Fig. 2). In contrast, these 123 two species had no significant relationship in plots with high or low fire severity (Fig. 2). This 124 pattern hints that fire severity might play a role in mediating the interaction between A. chilensis 125 and P . radiata. Wildfire severity plays a crucial role in shaping post-fire succession and ecosystem 126 dynamics. Understanding the severity-specific effects of fires is essential for developing effective 127 forest restoration and conservation management strategies. 128 The observed negative correlation between the abundance of A. chilensis and P . radiata 129 suggests that the former’s presence, as a component of the pre-fire native flora, can be considered 130 a predictor variable influencing P . radiata establishment in post-fire areas. Given that A. chilensis 131 was already present in these ecosystems before the fires, its abundance at the time of the fire event 132 likely influenced the available resources and habitat conditions for P . radiata establishment. 133 Several mechanisms could explain this phenomenon, including the priority effect by pre-empting 134 resources and habitat filtering (Byun et al. 2013, Byun and Lee 2017). First, A. chilensis is a fast-135 growing, light-demanding species. In areas with a higher abundance of A. chilensis, competition 136 for light, water, or nutrients could be hindering the successful establishment of P . radiata seedlings. 137 Future studies that quantify resource availability and seedling performance concerning A. chilensis 138 density could provide stronger evidence for this hypothesis. Second, fire can have profound and 139 different effects on plant community assembly depending on its severity (McLauchlan et al. 2020). 140 The environmental conditions created by moderate fire severity may be more favourable for the 141 establishment of native compared to invasive species. Specifically, these fires create a more open 142 canopy with increased light availability in the understory, typically forming a patchy mosaic of 143 burned and unburned areas rather than eliminating the entire canopy. While A. chilensis is a light-144 demanding species with some degree of shade tolerance (Guerra et al. 2010, Salgado -Luarte and 145 Gianoli 2012) , P . radiata is a strictly shade -intolerant species (Gómez et al. 2019) . Thus, this 146 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 variation in light availability could favour A. chilensis over P . radiata establishment in suitable 147 microsites within the burned landscape. In low-severity fires with more remaining canopy cover, 148 P . radiata pine showed very low establishment (only two plots with 9 and 15 seedlings), likely due 149 to limited light availability for germination and seedling growth. In contrast, A. chilensis, which 150 can exhibit some shade tolerance, could persist and thrive, with an average of 16 seedlings per plot 151 and up to 67 in one case (Fig. 2). High-severity fires present a vastly different scenario, where 152 most or all vegetation is fire-consumed and heat sterilises the soil, eliminating vital microbes and 153 disrupting biogeochemical processes. These harsh conditions are detrimental to the establishment 154 of both species, resulting in the lack of relationship observed in Fig. 2. In this line, A. chilensis’s 155 lower establishment suggests a dependence on healthy soil microbes (Escobedo et al. 156 unpublished), which are eliminated by high -severity fires. P . radiata, meanwhile, sometimes 157 showed higher abundance in these areas, potentially benefiting from the open and A. chilensis-free 158 conditions since its establishment and survival are less reliant upon microbe communities 159 (Escobedo et al. unpublished). This resilience disparity highlights the threat of high-severity fires 160 to the Coastal Maulino forest. 161 Our findings suggest that native A. chilensis might play a role in limiting P. radiata 162 invasion, potentially through competition and/or habitat filtering. However, this beneficial effect 163 might be compromised in high -severity fire areas, where A. chilensis establishment is hampered 164 due to its dependence on a healthy soil microbiome. High-severity fires that disrupt soil microbial 165 communities pose a significan t threat to native plant communities and their ability to resist 166 invasion. Promoting the establishment of native species like A. chilensis, particularly in areas with 167 moderate fire severity, could be a valuable strategy for mitigating tree invasion and fostering the 168 recovery of fire-affected Mediterranean ecosystems like the Maulino forest in central Chile. 169

170 PG was supported by the Global Botanic Garden Fund number 2022/022 (Botanic Gardens 171 Conservation International, BGCI). ISAR was supported by ANID-FONDECYT grant 172 11240628. 173 Data Availability 174 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 The data supporting this study’s findings will be available in a repository upon manuscript 175 acceptance. 176

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Line indicates a statistically significant 298 negative relationship (p < 0.05) based on a negative binomial GLMM. 299 300 301 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900 Figure 2. Model-predicted relationships between Aristotelia chilensis and Pinus radiata seedling 302 abundance across fire severity levels (a, low; b, medium; c, high) for two sampling times (2017 303 and 2019). The solid line in the middle panel (b, moderate-severity fire area) indicates a 304 statistically significant negative relationship (p < 0.05) based on a negative binomial GLMM. 305 Relationships were not statistically significant in high- or low-fired-severity areas. 306 307 Author-formatted, not peer-reviewed document posted on 30/08/2024. DOI:  https://doi.org/10.3897/arphapreprints.e135900