In a context of climate change and forest fragility, an international consortium of scientists led by INRA1 have identified the world’s most drought-resistant tree species in the world. Results of the study, which appear in the June issue of Plant Physiology, were obtained using an instrumental analysis of the cavitation process, and provide new leads for the adaptation of trees to future climate change.
Drought-induced hydraulic failure is a leading cause of mortality of trees in severe drought conditions. It has become a major concern in light of future climate predictions, with forests across the world showing signs of vulnerability to intense and prolonged drought events.
In a study of the Callitris, a conifer species highly adapted to extreme drought, an international team led by INRA scientists1 have proven that Callitris tuberculata, found in extremely dry areas of Western Australia, is the most drought-resistant tree in the world. Hydraulic conductance (the circulation of water) is always possible inside these trees, even at pressures close to the practical limit of water metastability, suggesting that the vascular system which transports the water has evolved under drought stress to obtain the absolute limit. This research has identified a limit up to which trees can adapt to drought. Researchers also suggest that Callitris tuberculata, a desert species, will not be able to adapt to a drier climate, despite forecasts from climate specialists that the tree’s habitat will become even more arid.
While other drought resistance mechanisms exist, such as avoidance (seen in ephemerals2) or protection via thick and fleshy parts (seen in succulent plants such as cacti), these are rarely observed in trees.
Cavitation: key to understanding drought resistance
Cavitation is the process by which an air bubble forms in the vascular system of a tree (its ‘xylem’) and bursts the water column, preventing the circulation of sap. Cavitation occurs during periods of severe drought and, at high rates, can kill a tree. Scientists used a prototype for measuring this phenomenon, the Cavitron, to assess drought tolerance in trees (particularly Callitris tuberculata) by examining hydraulic conductance in a tree branch under negative pressure to obtain a cavitation vulnerability curve and information about drought tolerance in different species, populations and genotypes.
Finding ways to adapt to climate change
Findings on drought tolerance in Callitris tree species show that significant leeway exists for improving drought resistance in species found in our temperate climates (oak, pine, spruce, beech). Determining the genetic properties of these characteristics and identifying QTLs (quantitative trait loci, regions in a genome associated with a quantitative characteristic) and genes of interest will be important areas to explore for the future.