New insight into leaf drought tolerance

Abstract

Drought-induced forest dieback appears to have increased in frequency over the last decade (Van Mantgem et al. 2009; Allen et al. 2010; Peng et al. 2011). These diebacks have occurred across a wide diversity of forest types, suggesting that no forest biome is invulnerable to climate change (Choat et al. 2012). However, drought-influenced mortality rates differ substantially between species growing at the same site (Breshears et al. 2005) and populations within the species’ distribution range (Benito-Garz on, Ruiz-Benito & Zavala 2013). These dieback episodes may therefore lead to major vegetation shifts, substantial decreases in carbon uptake by the ecosystem (Ciais et al. 2005), and interactions, with invading alien species, for example. We need to understand the mechanisms by which drought stress affects tree performance and causes tree dieback if we are to identify the forests most vulnerable to climate change and to predict important vegetation processes, such as changes in community composition caused by differential plant mortality among species. Many studies have tried to identify the traits associated with improved plant survival during periods of drought. Three main strategies have been identified: water stress tolerance, avoidance and escape (see Fig. 1). Escape is when plants are temporarily absent during the drought period, due to mechanisms such as the shedding of leaves, as in ephemerophytes that complete their life cycle very rapidly and enter a dormant state during the long dry period in deserts. Avoidance includes a variety of mechanisms delaying or preventing the negative impact of stress. For drought stress, this involves minimization of water loss in plants experiencing drought, for instance, through strong stomatal closure to prevent decreases in water potential and subsequent runaway embolism. Tolerance involves xylem vascular system and/or leaf cells resisting dehydration (in plants able to sustain very negative water potentials) through the construction of xylem conduits resistant to cavitation and the maintenance of cell turgor. New technologies developed over the last 15 years have made it possible to assess xylem resistance to cavitation in many species (Delzon et al. 2010; Cochard et al. 2013) and populations within species (Lamy et al. 2014). However, the broad characterization of turgor loss point has proved more of a challenge. In this issue of Functional Ecology, Mar echaux et al. 2015; report an investigation of the leaf water potential at turgor loss point (ptlp) for a broad range of tropical tree species (71 in total). They report significant differences in ptlp values between species and show that ptlp might be associated with ecological drought tolerance (species with high ptlp are more frequently distributed in the tropical forest studied). Species growing in drier biomes have also been shown to have more negative values of ptlp (Bartlett, Scoffoni & Sack 2012b). Mar echaux et al. 2015 also provide evidence for the absence of a link with leaf economic spectrum, particularly as concerns specific leaf area (SLA). Together with the lack of relationship between SLA and forest water availability, these results strongly suggest that the broad and untimely use of SLA to predict species drought tolerance and distribution is likely to yield misleading results. The inclusion of ptlp in process-based vegetation models could therefore improve the accuracy of predictions of climate change impacts on forest ecosystems. Future studies are nevertheless required to confirm the correlation between leaf water potential at turgor loss point and species habitat suitability over a broader range of soil water availability. In this study, Mar echaux et al. 2015; used a recently developed method based on the use of an osmometer to predict ptlp (Bartlett et al. 2012a). This method is based on the accurate prediction of ptlp by leaf osmotic water potential at full hydration, as experimentally determined for 30 species by Bartlett et al. (2012a). This is consistent with interspecific differences in ptlp being driven principally by osmotic rather than elastic adjustments (Lenz, Wright & Westoby 2006; Bartlett, Scoffoni & Sack 2012b). This technique is much faster than the commonly used pressure– volume curve approach, opening up new possibilities for studying leaf drought tolerance within (phenotypic plasticity and genetic differentiation) and between (at the community and biome levels) species. It is particularly useful for studies in tropical forests, in which most of the plant species present have long vessels, making it more difficult to measure xylem resistance to cavitation, and making it all the more necessary to identify traits useful for the assessment of drought tolerance. Many studies have shown that leaf water potential at turgor loss point (ptlp) plays a key role in maintaining gas exchanges and growth rate under moderate water stress (Brodribb et al. 2003; Mitchell et al. 2008), but the direct relationship between ptlp and drought-induced tree mortality has never been investigated. As in the xylem hydraulics community, which has used the xylem *Correspondence author. E-mail: sylvain.delzon@u-bordeaux.fr