Allometry of Phloem Thickness and Resin Flow and Their Relation to Tree Chemotype in a Southwestern Ponderosa Pine Forest

Abstract

We measured traits of 470 Pinus ponderosa in northern Arizona and constructed equations scaling phloem thickness and resin flow to tree size. We tested the hypothesis that these traits were related to tree monoterpene chemotype. The monoterpene composition of resin was analyzed for a subset of trees (n 146). Correlation analyses revealed that all tree phenotypic traits were positively correlated. Equations of phloem thickness and resin flow were nonlinear: phloem thickness was a logarithmic function of tree diameter, and diameter explained moderate variation in phloem thickness (R 2 0.234). Similarly, resin flow was a reciprocal function of diameter (R 2 0.214). Resin flow rapidly increased with tree diameter but plateaued as tree diameter exceeded 40 cm. Phloem thickness was only weakly linearly related to resin flow (R 2 0.078). A multivariate regression incorporating diameter, height, and phloem thickness did not explain substantially more variance in resin flow than diameter alone. We found no evidence to suggest that tree phenotype was related to tree chemotype. We conclude that there are positive but weak relationships between tree size, phloem thickness, and resin flow in southwestern ponderosa pine and that trees with thick phloem are not necessarily better defended from herbivory by resin flow.