The role of environmental gradients in the ontogenetic shift of wood traits from seedlings to adults in two temperate tree species

Hanna Hickey, Angela Luisa Prendin, Julie Messier

Tree development, or ontogeny, involves concurrent changes in plant size and environmental conditions, both of which can influence wood structure and function. Disentangling the factors underlying ontogenetic variation in wood traits remains a major challenge. This is important because wood underpins hydraulic efficiency, hydraulic safety, and mechanical support - functions critical for tree performance. In this study, we sampled twigs from 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 this shift, hydraulic efficiency (Ks) was maintained across developmental stages, indicating that increases in VN and lumen fraction (F) compensated for reductions in efficiency 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, we did not detect differences in water availability across developmental stages. Although light availability increased from seedlings to adults, it explained little variation in wood traits, while height was associated with only a subset of traits in sugar maple and none in yellow birch. 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.