Developmental contributions to phenotypic variation in functional leaf traits within quaking aspen clones.

Phenotypic variation in plant traits is strongly influenced by genetic and environmental factors. Over the life span of trees, developmental factors may also strongly influence leaf phenotypes. The objective of this study was to fill gaps in our understanding of developmental influences on patterns of phenotypic trait variation among different-aged ramets within quaking aspen (Populus tremuloides Michx.) clones. We hypothesized that phenotypic variation in leaf functional traits is strongly influenced by developmental cues as trees age. We surveyed eight aspen clones, each with eight distinct age classes ranging from 1 to 160 years in age, and selected three ramets per age class for sample collection. Leaf traits measured included photosynthesis, stomatal conductance, water use efficiency, specific leaf area, and concentrations of N, phosphorus, sucrose, starch, condensed tannins and phenolic glycosides. Using regression analysis, we examined the relationships between ramet age and expression of leaf functional traits. The data showed significant correlations between ramet age and 10 of the 12 phenotypic traits measured. Eight of the phenotypic traits demonstrated a non-linear relationship in which large changes in phenotype occurred in the early stages of ramet development and stabilized thereafter. Water relations, nutrient concentration, leaf gas exchange and phenolic glycosides tended to decrease from early to late development, whereas sucrose, condensed tannin concentrations and water use efficiency increased with ramet age. We hypothesize that ontogenetically derived phenotypic variation leads to fitness differentials among different-aged ramets, which may have important implications for clone fitness. Age-related increases in phenotypic diversity may partially underlie aspen's ability to tolerate the large environmental gradients that span its broad geographical range.