Metabolic Scaling Theory (MST) predicts the allometry of tree hydraulic conductance (K) with mass (M), and in its simplest form assumes that the same scaling will follow for rates of tree water use (Q) and biomass growth (B). Here we apply and test an allometric model for K (and by proxy Q and B) in two co-occurring species with contrasting vascular anatomy: diffuse-porous Acer grandidentatum and ring-porous Quercus gambelii. We tested model assumptions of a) elastic similarity of tree height-by-diameter growth, b) area-preservation of branching, c) similarity of axial and radial taper of xylem vessel diameter, d) vessel space-filling according to a "packing function", e) proportionality between modeled and actual K, and f) proportionality between K and both Q and B. Assumptions e-f were tested across a wide range of tree sizes to determine the allometry of K, Q, and B with M. Eighteen trees per species were measured in mixed stands of riparian woodland to minimize confounding effects of soil type and water availability. Assumptions a-d were tested on separate trees from the same stands.
Model assumptions a-d were largely confirmed in both species. As trees increased in size, their height scaled with their basal diameter to approximately the 2/3 power as predicted for elastic similarity. Area-preserving branching was confirmed, with deviations primarily limited to smaller twigs in the direction expected for occasional twig dieback. Vessel taper was similar axially and radially. However, the "taper function" differed between species, with Q. gambelii achieving much wider trunk vessels than A. grandidentatum. Both species followed a packing function wherein the frequency of vessels was inversely related to their average diameter as required to achieve approximately constant space-filling (log-log slope = -2). However, Q. gambelii deviated towards having more large trunk vessels (slope = -1.69), and A. grandidentatum to having fewer (slope = -2.25). Modeled K scaled with trunk diameter (D) to the 1.96 power in Q. gambelii, predicting Q and B µ M0.71; In A. grandidentatum, modeled K µ D1.7, Q and B µ M0.64. Data on K, Q, and B scaling were being analyzed at submission, but preliminary Q results appear to confirm model predictions for A. grandidentatum.