A species-level model for metabolic scaling of trees II. Testing in a ring- and diffuse-porous species

A 17-parameter ‘species model’ that predicts metabolic scaling from vascular architecture was tested in a diffuse-porous maple (Acer grandidentatum) and a ring-porous oak (Quercus gambelii). Predictions of midday water transport (Q) and its scaling with above-ground mass (M) were compared with empirical measurements. We also tested the assumption that Q was proportional to the biomass growth rate of the shoot (G). Water transport and biomass growth rate were measured on 18 trees per species that spanned a broad range in trunk diameter (4-26 cm). Where possible, the same trees were used for obtaining the 17 model parameters that concern external branching, internal xylem conduit anatomy, and soil-to-canopy sap pressure drop. The model succeeded in predicting the Q by Mb scaling exponent, b, being within 8% (maple) and 6% (oak) of measured exponents from sap flow data. In terms of absolute Q, the model was better in maple (16% Q overestimate) than oak (128% overestimate). The overestimation of Q was consistent with the model not accounting for cavitation, which is reportedly more prevalent in oak than in maple at the study site. The modelled and measured Q by M b exponents averaged within 3•6% of the measured G by M b exponents, supporting the assumption that G α Q1. The average b exponent was 0•62 ± 0•016 (mean ± SE) across species, rejecting b = 0•75 for intraspecific scaling. The performance of this species model, both for scaling purposes as well as for predicting rates of water consumption within and between species, argues for its further refinement and wider application in ecology and ecosystem biology.