Global convergence in the vulnerability of forests to drought

Analysis of data from forest plants worldwide shows that margins between threshold xylem pressures at which plants suffer damage and the lowest xylem pressures experienced are small, with no difference between dry and wet forests, providing insight into why drought-induced forest decline is occurring in both arid and wet forests. Forest dieback resulting from extreme drought events has the potential to cause widespread loss of biodiversity, with a major impact on the global carbon balance. Plants undergoing drought stress experience reduced xylem pressure, and each species can tolerate a different degree of reduction before xylem damage and, eventually, hydraulic failure occur. This study looks at the safety margin between minimum experienced xylem pressure and the damage threshold for 226 forest species from 81 sites worldwide and shows that most species across both dry and wet biomes have small safety margins against injurious levels of drought stress. These plants are potentially vulnerable to the combination of rising temperatures and declining rainfall that is predicted to cause droughts of increasing intensity and duration in the near future. 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 severity1. One primary cause of productivity loss and plant mortality during drought is hydraulic failure2,3,4. 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 Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely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 (<1 megapascal) 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 globe5,6. 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 risk7,8.