Learning from a century of droughts

Tree ring records show that sensitivity of conifer trees to drought has generally increased during the last century, but so has post-drought recovery. Mature trees today must grow under atmospheric conditions that are very different to those into which they germinated last century, due largely to anthropogenic changes to the atmosphere over the last 100 years. Our sluggish action in curbing anthropogenic CO2 emissions means that sustained increases in temperature and global atmospheric CO2 will continue for most of the decade to come. This prediction is now uncontroversial, but what remains debatable is how forests are likely to respond to this atmospheric trajectory, especially in terms of drought mortality of trees. Two rather different views have emerged; one emphasizes the beneficial effects of high CO2 in improving photosynthetic efficiency and water-use economy; while the other suggests that an increase in the frequency of combined hot and dry weather is likely to cause severe injury to trees and forests around the world. In support of the optimistic former view, studies of changing water-use economy in trees appear to confirm a general increase in photosynthetic efficiency in parallel with CO2 over the last century1. However contrary evidence from manipulative studies indicates that CO2-dependent increases in water-use efficiency provide little benefit under water limitation2,3. Meanwhile, the rate of forest mortality appears to be increasing4. Annual tree rings, traditionally used to reconstruct global temperature, can also provide a record of the long-term health of trees, potentially contributing insights into how individuals have responded to droughts as global CO2 levels climbed through the last century. Writing in Nature Ecology & Evolution, Li et al.5 use tree ring records from 28 conifer and 10 angiosperm species to evaluate how significantly tree growth was impacted by drought (resistance) and how well tree growth recovered (resilience), during time periods spanning 100 years, and an increase of approximately 100 ppm CO2.