Species asynchrony and species richness stabilize grassland productivity under different rainfall addition regimes

Changes in global precipitation patterns are increasingly impacting the structure, function, and stability of terrestrial ecosystems, especially in water-limited grasslands. These changes lead to reduced ecosystem stability, as manifested by enhanced productivity fluctuations, shifts in biodiversity, and ecological uncertainty. Understanding how different rainfall addition treatments influence grassland ecosystem stability is, therefore, essential for predicting ecosystem responses under increasingly variable precipitation regimes. In this three-year study, we examined the effects of control (natural rainfall), phenology-based rainfall addition (applied during the green-up, tillering, and flowering stages, 40 mm/month from May to July), and full-season rainfall addition (40 mm /month from April to September) on aboveground net primary productivity (ANPP), species richness, species asynchrony, and the temporal stability of community ANPP in a semi-arid grassland in Inner Mongolia. Both rainfall addition treatments markedly increased ANPP (by 100.8 % and 145.8 % for phenology-based and full-season, respectively) and enhanced its temporal stability. Full-season rainfall addition increased productivity via dominant species growth, with no significant effect on species diversity. In contrast, phenology-based rainfall addition simultaneously boosted productivity and significantly increased species richness. Structural equation modeling revealed that community ANPP stability was mainly driven by species asynchrony and richness, with minimal contribution from dominant species stability. These findings highlight the pivotal roles of biodiversity and asynchronous species responses in buffering productivity fluctuations. From a management perspective, phenology-based rainfall addition offers a more sustainable approach to enhance ecosystem function by improving productivity while conserving biodiversity. These insights provide practical guidance for sustainable grassland management in the face of shifting precipitation patterns.