Trees with larger xylem vessels can move water through their bodies better and can sustain higher metabolic rates than trees with smaller xylem vessels. But what controls xylem vessel size? This is the question that Olson and Rosell set out to answer in this month's New Phytologist.
Two potentially-conflicting views have been presented, the physical/physiological and the environmental/ecological. Researchers have argued that either bigger trees have larger vessels or trees in wetter environments have larger vessels.
Olson and Rosell measured vessel and stem diameters in over 200 angiosperm tree species at 5 sites around the world. They found that trees with bigger stem diameters have larger xylem vessels, matching the expectation from previous research. The scaling relationship was consistent across sites, with no significant differences in slopes or intercepts. This finding suggests that observations of smaller vessels in drier environments are an artifact of smaller tree sizes.
The apparent dependence of vessel dimensions on stem dimensions means that if selection acts to increase cavitation resistance via narrower vessels, it will likely do so via smaller trunks. Similarly, selection favoring increased stature will favor wider vessels. Stem length, diameter and vessel diameter are therefore intimately associated through the action of selection.This is exciting because it reinforces the idea that selection has favored an optimal balance between moving water well and safely in trees. Being bigger and having a higher metabolic rate is important in wet places where competition for light and space are important but is less important in dry places, especially if it compromises metabolic functioning.