Understanding the degree to which the species diversity-productivity relationship (SDPR) is applicable to natural ecosystems–beyond modeling and experimental contexts - is of vital importance for comprehending the consequences of global biodiversity loss on terrestrial ecosystems. Two essential features of natural forests that have not received adequate attention in the SDPR are seasonality and species evenness. Here, we monitor the intra-and inter-annual growths of 6,515 trees in a subtropical seasonal (temperature-and rainfall-seasonal) forest over a six-year period. We investigate whether evenness affects forest productivity independently or interacting with richness and how the underlying mechanisms shift with seasonality and soil properties, employing structural equation modeling. Our findings reveal a consistent decline in species diversity, functional diversity and forest productivity from the wet-warm season to the dry-cold season, with community traits shifting from acquisition to conservative strategies. Species richness increases but evenness decreases forest productivity–uneven communities are more productive, and the attenuation effect of evenness on productivity varies slightly across different seasons. Species richness and evenness jointly affect productivity through community-weighted means (CWM) of functional traits in the wet-warm season and through both functional diversity and CWM of functional traits in the dry-cold season, indicating that the mass ratio effect is predominant during the wet-warm season, whereas both niche complementarity and mass ratio effects jointly drive productivity in the dry-cold season. Soil water availability directly affects forest productivity in the wet-warm season and indirectly through CWM of functional traits in the dry-cold season. Our study, the first to elucidate the seasonal dynamics of the SDPR in subtropical forests. Our results highlight species evenness as a key component of species diversity regulating seasonal productivity dynamics in heterogeneous, species-rich natural forests. To enhance forest resilience under climate change (e.g., drought), management should prioritize maintaining moderate evenness, while strategically planting drought-tolerant species and acquisitive species of subtropical forest ecosystems.
Xu et al. (Wed,) studied this question.