Mechanisms linking drought, hydraulics, carbon metabolism, and vegetation mortality.

Events of regional-scale vegetation mortality appear to be increasing in a variety of biomes throughout the Earth and are frequently associated with increased temperatures, droughts, and often (but not always) with outbreaks of biotic agents such as insects and pathogens (for review, see Allen et al., 2010; Supplemental Information S1). The precise physiological mechanisms underlying plant mortality are poorly understood, which limits prediction of where, when, and how vegetation will change with future climatic variation (for review, see Supplemental Information S2). In particular, hypotheses regarding carbon starvation (failure to maintain metabolism or fend off biotic agents due to prolonged negative carbohydrate balance) and hydraulic failure (desiccation from failed water transport) have stimulated recent debate (Adams et al., 2009a, 2009b; Leuzinger et al., 2009; Sala, 2009; McDowell and Sevanto, 2010; Sala et al., 2010). In this Update, I offer a perspective on the mechanisms of drought-induced mortality by providing an integrated view of carbonmetabolism in relation to stress and mortality, linking carbon metabolism with new evidence on the role of hydraulic failure in mortality, and identifying associated uncertainties. Drought is the predominant climate characteristic associated with mortality (Allen et al., 2010), but there are parallels with low temperature, light, and paleo-CO2 concentrations, from which I have cautiously drawn evidence regarding mechanisms of drought-induced mortality. Two novel results emerged from this review. First, well-known carbohydrate partitioning patterns are consistent with observations of carbohydrate concentrations in stressed but surviving plants as well as those that are dying, and these results support carbon starvation as a threat to survival during drought. Second, carbon metabolism and hydraulics are coupled via multiple interactions that are only recently becoming appreciated, including an enhanced risk of mortality in species that maintain narrow margins of hydraulic safety.