Comparison of vascular networks, water use and growth rates in two tree species of contrasting functional type

The curious pattern of metabolic rate scaling with mass to the ¾ power has been observed across organisms and has eluded biologists for nearly a century. Metabolic scaling in trees has recently attracted attention as scientists try to model ecosystem dynamics of the hydrologic cycle and the carbon cycle. In this study, we attempt to gain greater understanding about the mechanical and hydraulic principles that govern vascular networks, how water transport through these networks scale with tree size, and how water use relates to growth rates in functionally diverse ring-porous Quercus gambelii and diffuse-porous Acer grandidentatum. We parameterized a numeric network model with species-specific vascular and structural characters to predict water use and growth rate scaling with tree size. The network model currently is confined to optimal water supply. To better understand water use and growth rate patterns during variable season conditions, we measured whole-tree sapflow, conductance and growth rates over one growing season in these two species. The numeric network model did exceptionally well at predicting species-specific scaling of water use and growth rates with tree size. In addition, it accurately predicted relative water use per species. Comparison of these two sympatric species over the growing season suggested that ring-porous Q. gambelii has relatively stable (isohydric) water use patterns and similar growth rates to diffuse-porous A. grandidentatum which has more flexible water use strategy leading to variable growth rates. These two species are able to be co-dominant in this region due to unique water use niches and vasculature. The accuracy of the numeric model predictions tested here suggest that scaling models such as these could be valuable in making ecohydrological predictions enabling the prediction of water use and growth rates with tree size and scaling this up to the stand and ecosystem level. We hope this work infusing hydraulic and mechanical constraints driving water use and growth rates of individuals within and between species contributes to better understanding of processes that effect predictions of ecosystem challenges under global change.