The role of environmental gradients in the shift of wood traits from seedlings to adult trees

Abstract

Tree development, or ontogeny, involves concurrent changes in plant size and environmental conditions, both of which can influence wood structure and function. Disentangling the relative roles of intrinsic (height) and extrinsic (environment) drivers of wood trait variation remains a major challenge. This is important because wood underpins hydraulic efficiency, hydraulic safety, and mechanical support—functions critical for whole-tree performance. In this study, I sampled twigs at a fixed distance from the apex from sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) to quantify how wood traits, light availability, and water availability change across seedling, sapling, and adult developmental stages. Across both species, wood structure shifted toward greater hydraulic safety with ontogeny, demonstrated through decreased vessel diameter (Dh), increased vessel number (VN), and increased vessel reinforcement ((t/b)²) in adult trees. Despite shifts towards greater hydraulic safety, hydraulic efficiency (Ks) was maintained across developmental stages, indicating that increases in VN and lumen fraction (F) compensated for reductions in efficiency typically associated with smaller Dh. Wood trait covariation was structured by a hydraulic efficiency–safety trade-off, however traits such as F varied more independently, enabling compensations such that this trade-off did not constrain tissue-level hydraulic function. Contrary to expectations, I did not detect differences in water availability across developmental stages. Although light availability increased from seedlings to adults, it did not explain changes in wood traits with ontogeny. Instead, tree height emerged as the dominant driver of trait values, reflecting increasing hydraulic constraints associated with longer water transport distance. The shift toward smaller Dh and maintained Ks in adult apices contrasts with expectations of increasing transport efficiency in taller trees but is consistent with selection for resistance to freeze–thaw embolism in temperate environments.