Phenotypic Trait Variation and Adaptation Strategies in Leaves of Pinus densata in Southeastern Xizang

To explore the variation in leaf phenotypic traits and environmental adaptation strategies of Pinus densata in southeastern Xizang, 15 plots were established across five regions—Gongbujiangda County (GB), Bomi County (BM), Bayi District (BY), Milin City (ML), and Lang County (LX)—and 11 leaf traits were measured, including leaf length (LL), width (LD), area (LA), volume (LV), fresh weight (LFW), dry weight (LDW), tissue density (LTD), specific leaf area (SLA), and leaf greenness index (SPAD). Results showed that all traits except LL varied significantly among regions, with moderate variation overall; SPAD exhibited the highest coefficient of variation, while leaf water content was the most stable. Extensive correlations were detected among traits: leaf size and weight traits were positively intercorrelated and all negatively correlated with LTD, and SLA correlated negatively with LTD but positively with SPAD. Principal component analysis and hierarchical clustering further revealed that phenotypic variation aligned with the leaf economic spectrum and grouped the populations into three strategy types. Specifically, GB populations approached the “slow investment–return” end of the spectrum, BY and BM populations the “fast investment–return” end, while ML and LX occupied intermediate positions (transitional strategies), with ML leaning toward the slow end. These findings demonstrate that P. densata in southeastern Xizang has evolved diverse resource use and adaptation strategies through synergistic and trade-off relationships among leaf traits, enabling its persistence in complex high-altitude environments.