Functional trait outperforms plant diversity in governing biomass production and allocation in semiarid grasslands undergoing grazing exclusion

Grazing exclusion is a key strategy for restoring degraded grasslands and enhancing their ecosystem services. Plant biomass production and allocation are critical indicators of restoration success, yet the mechanistic pathways by which plant functional traits and diversity regulate these dynamics under grazing exclusion remain unresolved. To address this, we measured aboveground (AGB) and belowground biomass (BGB) and roottoshoot ratio (RSR) across a 27–year chronosequence in semiarid grasslands of Songnen Plain, Northeast China, and investigated how plant functional traits and diversity (species and functional) regulate these processes. Results revealed temporally decoupled biomass peaks, with AGB peaking at 19 years (491.14 g m⁻²) and BGB peaking earlier at 15 years (1046.50 g m⁻²). Concurrently, RSR declined initially before stabilizing after 19 years, and plant diversity generally exhibited an opposite trend to biomass production. Principal component analysis identified key plant functional strategies under grazing exclusion, with PC1 partitioning resource–acquisitive traits (e.g., leaf area, LA) from conservative traits (e.g., leaf nitrogen content, LNC) and PC2 representing a defense axis dominated by traits such as leaf dry matter content (LDMC). Critically, correlation and variation partitioning analyses demonstrated that functional traits, rather than diversity, emerged as the dominant driver of biomass production and allocation Structural equation modeling further revealed that grazing exclusion directly impacted AGB and allocation, and indirectly regulated them via species richness. Notably, grazing exclusion modulated biomasses dynamics by altering functional traits (LA and LDMC) and LALNC tradeoff. Additionally, LDMCinduced shifts in functional diversity (Rao's Q) contributed to biomass allocation adjustments. Our findings establish that biomass dynamics under grazing exclusion exhibit strong temporal patterns primarily driven by plant functional traits, delineating key mechanistic pathways. This framework provides a robust basis for assessing restoration success and strategically guides optimal management of semiarid grasslands.