Grazing exclusion does not affect the water use efficiency of alpine steppe plants in central Tibet

Grazing exclusion is a key strategy for rehabilitating degraded grasslands, but its effect on plant water use efficiency (WUE) is not well understood. Given that carbon isotope discrimination (δ 13 C) is a well-established proxy for WUE, this study investigated interspecific variations in δ 13 C across five dominant alpine steppe plant species ( Astragalus arnoldii , Stipa purpurea , Leontopodium nanum , Kobresia pygmaea , Carex moorcroftii ) in response to varying durations of grazing exclusion (6, 11, and 12 years) in central Tibet. Results showed that δ 13 C remained stable over time across species, with significant interspecific differences. S. purpurea and A. arnoldii had higher δ 13 C values than L. nanum , while K. pygmaea and C. moorcroftii had intermediate values. Redundancy analysis (RDA) indicated that WUE in S. purpurea , C. moorcroftii , and A. arnoldii positively correlated with soil moisture content, whereas L. nanum and K. pygmaea showed negative correlations. These findings suggest that WUE is largely determined by species taxonomy rather than grazing exclusion. Divergent water use strategies may facilitate species coexistence through niche differentiation. Preserving taxonomic diversity through grazing exclusion can enhance long-term restoration success by maintaining functional redundancy and adaptive potential in alpine steppe ecosystems. Considering plant WUE alongside other ecological factors and management practices is essential for optimizing grassland restoration outcomes. • Stable δ 13 C values in five dominant alpine steppe species remain constant across grazing exclusion durations, showing significant interspecific variation. • RDA indicates species-specific WUE-soil moisture links: positive for S. purpurea , C. moorcroftii , A. arnoldii ; negative for L. nanum , K. pygmaea . • Taxonomy and niche differentiation drive WUE variations, facilitating coexistence in alpine ecosystems.