Progressing from ‘functional’ to mechanistic traits

This issue of New Phytologist contains a very exciting paper by Larter etal. (2017, pp. 97–112) showing how millions of years of progressive aridification in Australia led to dramatic evolution in water transport capabilities in this country’s most successful extant conifer clade. The genus examined is Callitris, a remarkably resilient clade of Cupressaceae that ranges from the rainforested slopes of New Caledonia, across much of the parched Australian continent. Indeed the current world title for the most water-stress tolerant xylem comes from a small tree species of Callitris that inhabits desert margins in southern Australia (Larter et al., 2015). In their latest work on the Callitris clade, Larter et al. (2017) find that a single functional trait (xylem resistance to cavitation) is very strongly correlated with species distributions in terms of aridity across the extensive climatic range of the group. Together with similar studies from different scales (Blackman etal., 2012; Pittermann et al., 2012; Brodribb et al., 2014) this work paves the way for using mechanistic traits to explain broad patterns of evolution and ecology in plants. By ‘mechanistic traits’, I mean traits whose function can be clearly physiologically defined, as opposed to the more abstract ‘functional’ traits, such as leaf mass per area (LMA), that have been used to great effect in explaining plant economics over the last 15 years (Wright et al., 2004).