On xylem hydraulic efficiencies, wood space use and the safety efficiency tradeoff: Comment on Gleason et al. (2016) 'Weak tradeoff between xylem safety and xylem specific hydraulic efficiency across the world's woody plant species'
In a recent issue ofNew Phytologist, Gleason et al. (2016) compiled a remarkable data set of wood traits to investigate the tradeoffs between xylem hydraulic safety and xylem hydraulic efficiency, measured as xylem specific hydraulic conductivity (Ks; molH20 mlength s 1 mtranversal 2 Pa This tradeoff is expected and somewhat studied at the conduit level, but how this should propagate to other plant levels is not known. In their paper,Gleason et al. (2016) compared the relationship between two tissue level traits emerging from conduit level traits, hydraulic safety, andKs, to test for this tradeoff. The authors show conclusively that it is not possible to have high efficiency and high safety in plant hydraulic systems. Nevertheless, many species present low efficiency and low safety, which suggests the existence of other axes of variation affecting hydraulic efficiency and safety. In a commentary in the same issue of New Phytologist, Brodersen (2016) highlights the complexity of traits and contexts thatmay be affecting the hydraulic and safety tradeoff. Brodersen (2016) also proposes that there may actually be other traits that can better represent hydraulic safety. Here, we look at other xylem hydraulic efficiencies that may help explain Gleason et al.'s (2016) results and propose new questions. Efficiency may be defined as achieving maximum productivity with minimum wasted effort or expense (Oxford Dictionaries, 2015) What kind of efficiency is measured by Ks? What 'waste' is reduced by increasing this efficiency? Here Ks is flow per unit area and unit pressure gradient. Unit pressure gradient can be considered as longitudinal xylem efficiency, as higher longitudinal efficiency leads to smaller pressure gradients and higherKs. ThenKs integrates transversal and longitudinal water supply efficiency. This can be clearly seen in the following sentence ofGleason et al. (2016, p. 132) 'efficient xylem can transport the same volume of water as inefficient xylem, but does so with a smaller cross section of living wood' or, we add, can transport the same volume of water as inefficient xylem through longer wood section, or, yet, can supply a greater leaf areawith the samewooddiameter and height. ThusKs is a measure of space use efficiency and possible tradeoffs or relations should be analyzed from the viewpoint of wood space use. Many other xylem hydraulic efficiencies could be considered:
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