Ecological restoration enhances biomass through a shift from R- to SR-strategy and functional richness in degraded alpine meadows on the Qinghai-Tibetan Plateau

Ecological restoration of severely degraded alpine meadows often involves introducing pioneer herbaceous species, with the goal of promoting succession toward a more stable, sedge-dominated community. Understanding the mechanisms by which plant CSR strategies and functional diversity co-drive biomass accumulation during alpine meadow restoration is critical for guiding effective ecological management, yet these linkages remain poorly understood in high-altitude ecosystems. To address this gap, we established a 23-year restoration chronosequence on the Qinghai-Tibet Plateau using a space-for-time substitution approach, encompassing nine restoration stages. We measured key plant functional traits, quantified community-level CSR strategies and functional diversity via the Trait Probability Density framework, and assessed above-ground, below-ground and total biomass along with soil properties. Structural equation modeling was employed to disentangle the direct and indirect drivers of biomass enhancement. Plant CSR strategies shifted from ruderal-dominated toward stress-tolerant mixed strategies along the restoration gradient, with SR-strategy species dominating later stages rather than pure S-strategy species as traditionally expected. This shift regulated biomass accumulation primarily through functional richness, whereas functional divergence declined, reflecting trait convergence under persistent environmental filtering. The pathways governing biomass compartments diverged markedly: soil factors directly promoted above-ground biomass and total biomass, independent of plant strategies or functional diversity, whereas below-ground biomass accumulation was driven primarily by recovery time, acting indirectly through time-mediated increases in CSR-strategy diversity and functional richness. The model explained 83-87 % of biomass variance, with recovery time showing the strongest total effect. These findings highlight that restoration in alpine meadows follows distinct above- and below-ground pathways: above-ground production responds directly to soil improvement, while below-ground accumulation depends more on time-mediated reshaping of plant strategies and traits. This study provides a mechanistic, trait-based framework for guiding species selection and monitoring in the restoration of high-altitude grasslands.