Negative Responses of Understory Plant Communities to Nitrogen and Phosphorus Enrichments in Forests: Tests From a Field Experiment and a Meta‐Analysis

Abstract

Anthropogenic nitrogen (N) and phosphorus (P) inputs have significant effects on plant community composition and diversity in forest ecosystems. Compared to canopy layers, understory communities are more sensitive to exogenous nutrient inputs. Although some experimental evidence exists on the effects of N addition on understory communities, knowledge about the impacts of P addition and its interaction with N, as well as the underlying mechanisms, remains limited. Here, we conducted two nutrient manipulation experiments in tropical primary and secondary montane rainforests, and synthesized 33 peer‐reviewed studies to evaluate the impacts of N and P enrichments on species richness, density, and cover of understory plants in forests. Integrated evidence from field experiments and meta‐analyses consistently demonstrated that nutrient enrichment with N, P, or both significantly reduced the species richness, density, and cover of understory plant communities. The negative impacts intensified with experimental duration and addition rate. Mechanistically, alteration in soil conditions including excessive accumulations of N and P as well as N‐induced soil acidification mainly caused the loss of understory plant species. Our study underscores the negative influences of anthropogenic nutrient inputs on understory community diversity and coverage, thereby altering forest structure and functioning in the future.

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