Climate and plant species richness shape productivity allocation under extreme drought across Eurasian steppes

Extreme drought events are becoming more frequent under climate warming, imposing deleterious effects on plant diversity and ecosystem functioning. Yet how such events influence the allocation of net primary productivity (NPP)—a key indicator of plant adaptation and carbon cycling—remains poorly understood. Using a coordinated extreme drought experiment across six grassland sites spanning Northern China’s Eurasian steppes, we investigated how extreme drought, implemented as control, chronic drought (66% precipitation reduction, May–August), or intense drought (complete rainfall exclusion, June–July), affects NPP partitioning along broad climatic gradients. We found that both drought treatments disproportionately reduced aboveground NPP (ANPP) compared to belowground NPP (BNPP), increasing the fraction of BNPP to NPP (fBNPP). The magnitude of this shift was modulated by aridity, interannual precipitation variability, and soil moisture changes, with the largest increases in fBNPP occurring in regions of intermediate aridity and low precipitation variability. Additionally, drought reduced plant species richness, which in turn exacerbated ANPP declines and further contributed to the shift in fBNPP. These findings highlight the joint regulatory roles of climatic context and biodiversity in mediating NPP allocation under extreme drought. Therefore, our study provides ecological insights into how extreme drought reshapes productivity allocation in grassland ecosystems.