Responses and driving mechanisms of leaf functional traits of Betula platyphylla along an elevational gradient in the Niyang River Basin, southeastern Tibetan Plateau

Variation in plant functional traits reflects environmental adaptation. Understanding how traits shift along elevational gradients can clarify mechanisms by which species respond to environmental change. Here, we used white birch (Betula platyphylla) in the Niyang River Basin (left bank of the Yarlung Tsangpo River, Xizang) as a model species to quantify leaf trait variation along an elevational gradient and to identify key environmental drivers. Leaf functional traits were analyzed using one-way ANOVA, redundancy analysis (RDA), Pearson correlation, and principal component analysis (PCA). Coefficients of variation ranged from 5.52% to 26.14%, with leaf area showing the greatest variation and leaf water content the lowest. Most traits differed significantly among elevations: leaf area (LA), leaf length (LL), leaf width (LW), petiole length (PL), petiole base width (PBW) and leaf phosphorus content (LPC) declined with increasing elevation, whereas leaf nitrogen content (LNC) and leaf N: P ratio increased. Total chlorophyll content (TChl) and leaf water content (LWC) exhibited a hump-shaped pattern along the elevational gradient. In contrast, leaf dry matter content (LDMC) exhibited a unimodal pattern along the elevational gradient, whereas specific leaf area (SLA) and leaf mass per area (LMA) showed no significant trends with elevation. RDA indicated that soil factors were the primary drivers of leaf trait variation, explaining 50.90% of the total variance; soil total nitrogen (TN), pH, and available potassium (AK) were the most influential variables. Trait patterns suggest contrasting resource acquisition strategies across elevations: high-elevation individuals enhanced photosynthetic efficiency and shifted toward a more acquisitive "fast investment-return" strategy to cope with harsher conditions. Our findings contribute to understanding the adaptive responses of montane tree species to climate warming and improve our understanding of plant trait variation along environmental gradients.