Trait-Mediated Competition for Light Underpins Plant Diversity Loss Under Eutrophication

Eutrophication is a major driver of plant diversity loss, yet the underlying mechanisms remain poorly understood. In particular, the role of eutrophication-induced light limitation in regulating plant diversity in natural communities has rarely been examined directly. Here we show that experimental light addition to the understory of a natural alpine grassland consistently restored lost diversity under different nutrient enrichment regimes. Under nitrogen enrichment, light addition recovered diversity primarily by promoting species gains, whereas under phosphorus enrichment, it primarily reduced species losses. When both nitrogen and phosphorus were enriched, light addition simultaneously increased species gains and reduced losses. These effects were primarily driven by shifts in the colonization and extinction of species with resource-acquisitive strategies (i.e., those with high specific leaf area and low leaf dry matter content), emphasizing the critical role of trait-mediated competition for light in biodiversity loss. Our findings point to light competition as a key driver of eutrophication-induced plant diversity loss, suggesting that managing light availability could help mitigate these losses in natural ecosystems.