Patterns of Intra-Order Variation in Shoot Traits Are Order-Specific Along the Branch Basal Height Gradient of Larix principis-rupprechtii

Intra-order trait variation is a key driver of aboveground shoot performance at different branch basal heights. Although the basic light exposure and nutrient supply to shoots vary with branch basal height, most studies have focused on inter-order variation in shoot traits. However, how and to what extent shoot traits change with branch basal height, as well as whether a general intra-order pattern exists among different shoot orders, remain largely unclear. We compared intra-order variation in shoot diameter, length, specific stem length (SSL), and stem tissue density (STD) across four branching orders of Larix principis-rupprechtii along a vertical height gradient of 5.5–6.0 m. We tested (a) the degree of intra-order versus intra-order variation in shoot traits along the gradient and (b) whether intra-order trait patterns and their relationship with branch basal height were consistent across the four branching orders. Specifically, we hypothesized that within a branching order, shoot traits would undergo adjustments: shoots at higher positions would focus on growth (by increasing diameter and length), whereas shoots at lower positions would enhance resource acquisition (by increasing SSL) and protection (by increasing STD). Branching order explained most of the overall variation in shoot traits, including shoot diameter and length, but accounted for only a small portion of the variation in SSL and STD. Branch basal height explained only a small fraction of intra-order shoot trait variation, which was larger within than between basal heights. Moreover, the relationships between traits and branch basal height rarely aligned with our hypotheses and varied considerably across different shoot orders. Along the complex branch basal height gradient, where multiple traits change simultaneously, shoots of different shoot orders exhibit distinct patterns of variation, leading to specific intra-order trait variation. The lack of support for our hypothesis may result from the multifaceted interactions between light availability, spatial constraints, nutrient heterogeneity, and dynamic branch-order interactions. Our findings suggest that to better understand the impact of environmental variation on shoot performance, future research should integrate a more comprehensive analysis of shoot responses to change and measure a broader range of shoot traits and environmental variables.