Plant communities on nitrogen‐rich soil are less sensitive to soil moisture than plant communities on nitrogen‐poor soil

Plant species composition and diversity are known to change across local gradients of light, moisture and nutrients, but ecologists still have a relatively limited understanding of how communities respond to multiple limiting resources. We used a trait‐based approach to investigate how the functional composition and diversity of forest understorey plant communities change along gradients in light, soil moisture and nitrogen availability. We used a total of seven leaf, root and whole‐plant traits for 55–78 species, and estimated the effects of the three resources on the mean and dispersion of these traits in understorey plant communities across 50 forest sites. Soil moisture and nitrogen availability (C/N ratio) both influenced plant community traits, but light availability (canopy openness) did not. Generally, increases in moisture and nitrogen both resulted in shifts towards more acquisitive resource use strategies, including greater leaf area, specific leaf area and maximum plant height, and lower leaf dry matter content, root dry matter content and rooting depth. Functional diversity of most traits also increased with increasing soil moisture and nitrogen. Although most traits varied with soil moisture on nitrogen‐poor sites, moisture did not influence of the distribution of any traits on nitrogen‐rich sites. Synthesis. Independent co‐limitation of soil moisture and nitrogen appeared to influence the functional composition and diversity of understorey vegetation in our study area. The co‐occurrence of species with resource acquisitive and conservative strategies on nitrogen‐rich sites may make plant communities relatively resistant to changes to soil moisture. These results suggest that altered precipitation regimes under climate change could lead to greater changes in the composition and diversity of plant communities on nutrient‐poor soils than on nutrient‐rich soils.

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