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A major concern in recent years has been the widespread tree mortality seen in many parts of the world associated with droughts and heat waves (Hartmann 2011). Although rising temperatures and shifting precipitation regimes are expected to cause more of these mortality events, the associated rise in atmospheric CO2 concentrations could improve tree water use efficiency, thus mitigating the effect of heat and drought on trees. In this issue, Warren et al. (2011) report the opposite— elevated CO2 concentrations promoted greater senescence in temperate trees (Liquidambar styraciflua, sweetgum) experiencing a severe summer drought when compared with ambient CO2 conditions. Stands exposed to elevated CO2 had greater root standing biomass and lower stomatal conductance (gs) than ambient stands, which should increase water uptake ability and soil moisture, respectively, thereby improving drought tolerance. However, trees in high-CO2 plots were hit harder by the drought: they shed more foliage and had lower modeled net photosynthetic rates during the drought than trees from ambient CO2. The paper synthesizes a wide range of data across a number of scales, including leaf-level gas exchange, branch hydraulics, stem sap flow, fine root dynamics and standlevel biomass estimates, to provide a multi-faceted picture of how carbon and water fluxes were affected by this climatic event. The results also concur with another recent report, where Populus deltoides grown at elevated CO2 for 3 years were more susceptible to water stress, and shed more total leaf area during a drought, than trees from ambient CO2 (Bobich et al. 2010). Could rising CO2 actually increase the susceptibility of forests to droughts? A reduction in gs at elevated CO2 is a common response across many species, with an average decrease of 22% (Ainsworth and Rogers 2007), leading to the expectation

The bigger they are, the harder they fall: CO2 concentration and tree size affect drought tolerance