Will rising CO2 and temperatures exacerbate the vulnerability of trees to drought?

Large-scale tree dieback events are increasing in frequency around the world, with many of them attributed to the effects of climate change-related droughts (i.e., droughts that co-occur with heat stress) (Adams et al. 2010, Allen et al. 2010). As mean global temperatures are expected to rise by 2–4 °C in the next 85 years (Christensen et al. 2007) and elevated temperatures are correlated with higher vapor pressure deficits and evaporative driving forces (Oishi et al. 2010), we may expect a rise in tree mortality due to drought in a warmer future. However, climate warming is mainly driven by rising atmospheric CO 2 concentration, a factor that can increase plant drought tolerance (Drake et al. 1997). The degree to which these two global change factors will alter the vulnerability of trees to drought is unclear, and combinatorial experiments studying the effects of both warming and elevated CO 2 on tree drought tolerance are rare (but see Wertin et al. 2010, 2012, Zeppel et al. 2012, Lewis et al. 2013). Research on what causes tree mortality during climate-change drought events tends to focus on two hypotheses: direct hydraulic failure or carbon starvation (McDowell et al. 2008). The first hypothesis acknowledges that if stomata remain open during drought to maintain carbon fixation for metabolism, then the associated water losses from transpira-tion will eventually cause catastrophic cavitation (Anderegg et al. 2012). The second hypothesis focuses on the role of stomata in maintaining the integrity of the hydraulic pathway of trees. As stomata close during drought, respiration continues to burn carbohydrates without new photosynthetic carbon fixa-tion and the plant's carbohydrate stores are depleted until they cannot maintain metabolic needs (Adams et al. 2009). The role of carbon starvation in limiting tree survival and responses to climate is clearest during glacial periods with low CO 2 concentrations (~200 ppm) (Gerhart et al. 2012), but low stomatal conductance during drought may generate analogously low intercellular CO 2 concentrations under future high CO 2 conditions. Lastly, the interdependence of water transport and carbohydrate status in trees has also received increasing recognition (McDowell et al. 2011, Sala et al. 2012): phloem transport of sugars to sink tissues requires adequate water transport, and there is recent evidence that embolism repair may depend on carbohydrate availability (Secchi and Zwieniecki 2011). In this issue of Tree Physiology, Duan et al. (2013) add to our growing knowledge of how climate change may alter …