Convergent Vessel Diameter–Stem Diameter Scaling across Five Clades of New and Old World Eudicots from Desert to Rain Forest

<i>Premise of research</i>. Variation in average xylem vessel diameter across species has important functional consequences, but the causes of this variation remain unclear. Average vessel diameter is known to scale with stem size within and across species. Vessel diameter also seems to differ between clades and across environments, with dryland plants having narrower, more cavitation-resistant vessels. As a result, it is not clear to what extent phylogenetic affinity and environment are associated with differences in the vessel diameter–stem size relationship. <br> <i>Methodology</i>. With linear models and correlations, we explored the influence of environment and phy- logeny on the vessel diameter–stem diameter relationship in a molecular phylogenetic context across 83 species in four families spanning desert to rain forest in the Americas, Africa, Asia, and Madagascar. <br> <i>Pivotal results</i>. Mean species vessel diameter was strongly predicted by trunk diameter (slope ∼ 0.33), and this slope was not affected by either phylogenetic affinity or environment. Clades differed only slightly in mean vessel diameter when controlling for stem size, and there was no tendency for plants of moist environments to have wider vessels. Of four climate indexes, only the temperature index contributed to explaining vessel diameter, although very weakly. <br> <i>Conclusions</i>. Our results are congruent with models suggesting that natural selection should maximize vessel conductivity while minimizing cavitation risk via vessel taper in the context of conductive path length. Because neither environment nor phylogeny contributed to explaining vessel diameter–stem diameter scaling across species, our results appear congruent with the notion that selection favoring cavitation resistance via narrow vessels should lead to shorter statures independently of ancestry or habitat. The repeated finding of narrow vessels in dryland plants might therefore reflect the smaller average stem size of plants in drylands rather than the plants having vessels that are narrow for their stem diameters.