Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many individual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk

This study combines existing hydraulic principles with recently developed methods for probing leaf hydraulic function to determine whether xylem physiology can explain the dynamic response of gas exchange both during drought and in the recovery phase after rewatering. Four conifer species from wet and dry forests were exposed to a range of water stresses by withholding water and then rewatering to observe the recovery process. During both phases midday transpiration and leaf water potential (C leaf ) were monitored. Stomatal responses to C leaf were established for each species and these relationships used to evaluate whether the recovery of gas exchange after drought was limited by postembolism hydraulic repair in leaves. Furthermore, the timing of gas-exchange recovery was used to determine the maximum survivable water stress for each species and this index compared with data for both leaf and stem vulnerability to water-stress-induced dysfunction measured for each species. Recovery of gas exchange after water stress took between 1 and .100 d and during this period all species showed strong 1:1 conformity to a combined hydraulic-stomatal limitation model (r 2 = 0.70 across all plants). Gas-exchange recovery time showed two distinct phases, a rapid overnight recovery in plants stressed to ,50% loss of leaf hydraulic conductance (K leaf ) and a highly C leaf -dependent phase in plants stressed to .50% loss of K leaf . Maximum recoverable water stress (C min ) corresponded to a 95% loss of K leaf . Thus, we conclude that xylem hydraulics represents a direct limit to the drought tolerance of these conifer species.Photosynthesis occurs in an aqueous environment and until evolution comes across a solid-state means of fixing atmospheric CO 2 , terrestrial plant species, even those in humid tropical rainforests (Engelbrecht et al., 2007) will be exposed to potentially lethal desiccation. The reason for this is that in most environments competition between plants forces them to engage in a dangerous balancing act between trading water for carbon at the leaf while minimizing costs associated with replacing this transpired water with water pulled from the soil. The job of seeking and transporting water falls upon the roots and vascular system, and reduced investment in these systems comes at a cost in terms of the safety and efficiency of water carriage. These conflicting demands mold the form and function of vascular plants and have yielded a diverse spectrum of vascular anatomies, each tuned to a specific flow capacity and drought tolerance.Desiccation tolerance is at the center of the vascular cost/benefit equation. The reason for this is that a more desiccation-tolerant vascular system (one that resists embolism better during soil drying) is distinctly more costly to build than a sensitive system (Hacke et al., 2001a), yet the repercussions of vascular failure are likely to be fatal. This trade-off, as with many other systems in biology, leads to functional diversity and hence there is a great range in the ability of...