On research priorities to advance understanding of the safety–efficiency tradeoff in xylem

Abstract

We appreciate Bittencourt et al.'s (2016) constructive contributions following our paper, Gleason et al. (2016), on the proposed tradeoff between hydraulic safety and efficiency. To continue this dialog we would like to comment on which of the research directions proposed by Bittencourt et al. seem most promising to us. We agree that various xylem tissue fractions could potentially modify the safety–efficiency relationship. In principle, any tissue fraction could trade off with any other tissue fraction. However, as a matter of observation, parenchyma fraction is negatively correlated with fiber fraction, whereas parenchyma and fiber fractions are not strongly correlated with vessel lumen fraction (Ziemińska et al., 2015; Morris et al., 2016). As such, vessel lumen fraction, vessel diameter, and vessel frequency are largely uncoupled from nonvessel tissue fractions across self-supporting angiosperm species (Zanne et al., 2010), and are therefore unlikely to trade off with mechanical safety and hydraulic efficiency (or safety). Furthermore, vessel lumen fraction itself does not vary markedly across angiosperms, ranging from c. 5% to 20% (mean ≈ 15%) (Zanne et al., 2010; Morris et al., 2016), although larger fractions are not uncommon in ring-porous and climbing species. Contrasts between climbing (e.g. lianas) and freestanding growth forms are more likely to show differences in allocation to vessel vs nonvessel space, and the climbing habit therefore may offer a more appropriate system for evaluating this idea (Gartner, 1991).