Functional ratios among leaf, xylem and phloem areas in branches change with shade tolerance, but not with local light conditions, across temperate tree species

Summary

Leaf, xylem and phloem areas drive the water and carbon fluxes within branches and trees, but their mutual coordination is poorly understood. We test the hypothesis that xylem and phloem areas increase relative to leaf area when species are selected for, or branches are exposed to, higher levels of light intensity. Trees of 10 temperate, broadleaved and deciduous, tree species were selected. Fifty-centimetre-long branches were collected from shaded and exposed conditions at a height of 3-4 m. We measured the total leaf area, xylem area, phloem area and leaf traits, as well as the area of the constituent cell types, for a stem section at the branch base. Xylem area : leaf area and phloem area : leaf area ratios did not differ consistently between sun and shade branches, but, as expected, they decreased with species' shade tolerance. Similar trends were observed for conductive cell areas in xylem and phloem. Trees of light-demanding species maintain higher water loss and carbon gain rates per leaf area by producing more xylem area and phloem area than shade-tolerant species. We call for more comparative branch studies as they provide an integrated biological perspective on functional traits and their role in the ecology of tree species.