Branch height rather than diameter at breast height drives intraspecific leaf trait variation in large trees of a subtropical evergreen broad-leaved forest

Abstract For woody plants, intraspecific trait variation (ITV) serves as a core adaptive strategy to resist hydraulic failure and physical damage. However, how ITV varies with branch height and diameter at breast height (DBH) remains relatively underexplored compared with interspecific trait variation. To investigate ITV across different branch heights and DBH, we selected 14 large individuals of Castanopsis chinensis in an old-growth subtropical evergreen broad-leaved forest in southern China. We collected branch samples from the bottom to the top of each tree and measured seven functional traits related to leaf carbon gain, hydraulics, and mechanical strength, including leaf size (LS), leaf non-structural carbon concentration (NSC), leaf area to sapwood area ratio (AL/AS), leaf petiole length (LPL), leaf folding angle (LFA), leaf punching strength (Fp) and leaf mass per area (LMA). Our results showed substantial ITV in plant traits, with AL/AS exhibiting the greatest variation and LFA the least. LS, AL/AS and LFA decreased significantly with branch height, whereas NSC and LPL increased significantly with branch height. Additionally, both Fp and LMA exhibited significantly positive correlations with branch height, with the strongest associations observed across all measured functional traits. However, none of the traits were significantly correlated with DBH. Our study demonstrates substantial trait variation within a single species and highlights branch height as the primary driver of this ITV. These findings reveal distinct differences in carbon allocation, water use, and physical damage resistance across branch heights, which are vital for plants to cope with external stresses at different vertical branch layers.