Competitive effects are significantly affected by soil water condition in the first year and by neighbor richness in the second year: A two-year simulated experiment

Abstract Background and aims Understanding how plant-plant interactions vary with biotic and abiotic factors is vital to protect degraded grassland communities, especially under global climate changes. However, how soil water condition and species diversity interactively affect competition between the dominant species and its neighbors over time remains unclear. Method Using a dominant species in semi-arid Inner Mongolia Steppe, Leymus chinensis, as the target species and ten other species as neighbors, we carried out a two-year microcosm experiment to evaluate how soil water condition (SWC) and neighbor richness (NR) affected the competitive effect of neighbors (CRCI) on L. chinensis directly or indirectly. Results (1) In the first experiment year, SWC affected CRCI directly and indirectly via regulating community-weighted mean (CWM) of height absolute distance, supporting competition-trait similarity hypothesis. (2) In the second experimental year, NR showed significant effect on CRCI. Moreover, functional dispersion (FDis) of neighbors mediated the response of CRCI to the changes in neighbor richness, supporting vacant niche hypothesis. In addition, there was a negative association between CWM of height hierarchical distance and CRCI, and a positive association between CWM of specific leaf area hierarchical distance and CRCI in the second year, supporting competition-trait hierarchy hypothesis. In general, the relative importance of NR on CRCI increased over time. Conclusions These findings provide strong empirical evidence of how plant-plant interactions respond to changes of biotic and abiotic factors in semi-arid Inner Mongolia Steppe over time, and novel insights for mechanisms by which biodiversity and climate changes affect plant-plant interactions.