Abstract: Climate change is causing range shifts of tree species to track their climatic optima. Despite a general tendency of upward shifts in alpine treelines with warming climate, they often show lagged responses and are contingent on species-specific and microsite conditions. However, it is unknown how plant functional traits mediate treeline species responses to climate warming. Herein, we investigated intraspecific variations in leaf traits associated with resource acquisition strategies of a treeline species, Smith fir (Abies georgei var. smithii), along a wide elevational gradient (3,700–4,400 m) on the southeastern Tibetan Plateau. We analyzed these traits across 12 leaf age classes (1 to 12-year-old leaves). Leaf lifespan, thickness, and tissue density increased with elevation, whereas specific leaf area (SLA) and leaf area decreased. In contrast, leaf nutrient content (C, N, P) in current-year leaves remained similar across elevations, while nitrogen content peaked at mid-elevations in the case of older leaves. Principal component analysis revealed a shift along the leaf economics spectrum from acquisitive to conservative strategies with increasing elevation. Piecewise structural equation modeling highlighted elevation- mediated leaf nutrient status by interactions with leaf age, leaf morphological traits and soil properties. Elevation extends leaf lifespan and amplifies age-related conservative traits, enabling trees to cope with shorter growing seasons and nutrient constraints at alpine treelines. By integrating leaf age into a trait-based framework, this study advances a mechanistic understanding of how ontogeny and environmental constraints jointly regulate functional strategies in treeline species under a warming climate.

Key words: Abies georgei var. smithii, alpine treeline, leaf functional traits, leaf age, elevation gradient, resource use strategy