Axial and Radial Variations in Xylem Anatomy of Angiosperm and Conifer Trees in Yunnan, China

The model of West, Brown and Enquist (WBE model) showed that lumen hydraulic resistance in trees can be independent of path length, provided that vascular conduits widen basipetally and sufficiently, and the number of functional conduits is constant from twigs to trunk. To test the anatomical realism of this model, we investigated axial and radial changes in xylem characteristics of six angiosperm tree species (fourteen trees, 15–25 m tall) and three coniferous tree species (seven trees, 16–45 m) in Yunnan, China. Conduit lumen diameter increased linearly with distance from the top of the bole to its base in six trees. In all but one of the fifteen remaining trees, lumen diameter increased from the top to near the base of the crown but remained constant or increased more slowly below the crown. Similar, but inverse patterns were found for axial variation in vessel density of the angiosperms, which indicated that the number of conducting vessels increases from trunk to twig as they become narrower. Vessel diameters increased almost linearly with cambial age for the first 20 to 40 years, and then stabilized or increased more slowly in eight trees of Fagaceae. Axial trends in conduit lumen diameter and number were consistent with only partial buffering of hydraulic resistance from pathlength effects, generally confined to the crown region. Neither conduit taper nor number was consistent with the WBE model.

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