Species-specific responses in forage production and quality to reduced precipitation amount and frequency:...

Drought threatens global forage supply, especially in the vulnerable alpine grasslands of the Qinghai-Tibetan Plateau. However, the interactive effects of reduced precipitation amount and frequency on forage supply remain poorly understood. To address this, we focused on two dominant alpine forage species: the resource-acquisitive grass Elymus nutans and the resource-conservative forb Bistorta vivipara. Using a completely randomized pot experiment, we subjected each species to two precipitation amounts (medium and low) and three frequencies (high, medium and low), and assessed forage production, quality parameters, functional traits, and carbon (C)/nitrogen (N) metabolites. Reduced precipitation frequency did not amplify the effect of reduced precipitation amount on most indicators. For E. nutans, reduced precipitation decreased production (-22.79%), increased non-fiber carbohydrates (NFC, +27.15%), and decreased neutral detergent fiber content (NDF, -17.19%). The shifts in NFC and NDF were driven by higher soluble carbohydrates and lower starch contents, and linked to lower specific leaf area (SLA) and photosynthetic nitrogen-use efficiency (PNUE), and higher leaf dry matter content (LDMC) and C:N ratio. Reduced precipitation also led to low ammonium-N and crude protein content (CP, -6.8%). While for B. vivipara, reduced precipitation did not alter production, NFC and NDF; but increased CP content (+12.10%) through the accumulation of amino acids and structural protein associated with reduced C:N ratio. This study highlights the critical role of functional traits and metabolic regulation in mediating species-specific responses of forage supply to reduced precipitation. Thus, integrating trait-based insights and establishing strategically mixed-species artificial grasslands could provide a valuable theoretical framework for optimizing management.