Leaf area compounds height‐related hydraulic costs of water transport in Oregon White Oak trees

Summary The ratio of leaf to sapwood area generally decreases with tree size, presumably to moderate hydraulic costs of tree height. This study assessed consequences of tree size and leaf area on water flux in Quercus garryana Dougl. ex. Hook (Oregon White Oak), a species in which leaf to sapwood area ratio increases with tree size. We tested hypotheses that Q. garryana individuals of greater size and leaf area show reduced leaf‐specific hydraulic conductance ( K L ), crown water flux per leaf area ( E L ), and carbon isotope discrimination (Δ). K L , E L and Δ differed between trees of two size classes examined, with 25 m trees showing evidence of lower water flux and carbon isotopic discrimination compared to 10 m trees. Whole‐tree water fluxes were smaller in 25 m than in 10 m trees both per unit sapwood and per leaf area, but more so per unit leaf area: 25 m trees had a leaf to sapwood area ratio 1·6 times greater than that of 10 m trees. The findings from this study are unique in that increased leaf to sapwood area ratio of larger trees compounded hydraulic constraints on water transport due to tree height. These results provide further support and generality to the hypothesis of hydraulic limitations to tree‐water flux, and show that limitations to water flux are not necessarily accompanied by the structural compensation of reduced crown leaf area in larger trees.

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