Aridity Threshold Alters Trait-mediated Regulation of Grassland Productivity at a Global Scale

Understanding how grassland productivity responds to increasing aridity is critical under climate change, yet whether consistent thresholds exist and how underlying mechanisms shift remain unclear. Here, we conducted a global meta-analysis to examine the response of aboveground net primary productivity (ANPP) to the aridity index (AI) and the role of plant functional traits. ANPP showed a significant nonlinear relationship with AI, with a threshold at AI ≈ 0.51. Below this threshold, ANPP increased strongly with AI, whereas this relationship weakened above it. This threshold coincided with a shift in trait-productivity relationships. Before the threshold, ANPP was positively associated with traits linked to resource acquisition and conservation (LCN and SLA). After the threshold, key traits (LCN and SRL) showed reduced or reversed effects, indicating a reorganization of functional controls. Structural equation modeling further showed that the relative importance of direct climatic and trait-mediated pathways differed across the threshold. These results identify a distinct aridity threshold associated with changes in grassland productivity responses and their functional controls, emphasizing the importance of incorporating nonlinear climatic responses and trait-mediated pathways into predictions under future climate change.