Temperature is not the main driver of vessel diameter(when standardized by height) across climatic gradients in Viburnum
It is generally assumed that tree-rings, and their vessel diameters, are wider in warmer and wetter years. To maintain constant conductance per unit leaf area as trees grow taller, stem vessels should widen from tip to base. But wider vessels are more susceptible to embolism, so taller plants are progressively becoming more vulnerable to drought or cold as they grow. The traditional theory of vessel hydraulic adaptation postulates that vessel diameter is affected by climate, with cold environments selecting for narrower vessels. Recent work suggests that plant height is an important driver, hence the relationship between vessel diameter, climate and height remains to be explored. In order to do this, we installed 16 climate stations along 5 strong elevational gradients in Mexico and measured vessel diameter of the most recent tree ring along the stems of 57 trees of 10 species of Viburnum (Adoxaceae). We examined the possibility that there could be adaptive variation in the Y-intercept of within-plant tip-to-base vessel widening curves, with plants in dry and cold areas having lower intercepts (narrower vessels for a given height) than in moist-warm places. We found no evidence that temperature affects vessel widening, though total plant height, leaf area, and possibly wood density appears to affect the Y-intercept of vessel widening profiles of individual stems. These findings stimulate new questions including the need to understand how vessel widening in the leaves is coordinated with that in the stem.