Climate change-induced tree mortality in dryland forests and its consequences for ecosystem functioning

Trees die off as a consequence of climate change-induced drought and heat has been observed worldwide, including in drought-adapted dryland forests. Despite intensive research on tree mortality, major topics remain unsolved so far, such as the following questions: What conditions promote forest vulnerability to drought? What are the consequences of tree mortality for ecosystems? Selected case studies showed that mortality rate of oak trees in a Mediterranean maquis increased linearly with decreasing annual rain amounts. On the other hand, pine trees in a semi-arid plantation died off following an extreme drought event, though the differences among stands in mortality could not be explained by differences in topography, soil type, tree age or stand density. Lower mortality rate in this forest was related to higher surface rock cover and stoniness across the soil profile, leading to a higher soil water content. Four years after the extreme drought event, soil organic matter and water content did not differ between plots of high and low tree mortality. However, phosphorus availability in the top soil was higher in stands where many trees died, opening up opportunities for exploitation of this key soil resource by successive vegetation. A litter decay study in pine forests showed that tree mortality and the predicted prolongation of dry periods due to climate change may enhance decomposition, possibly reducing ecosystem carbon stocks. Sustainable use of forests in climate change-affected drylands requires adaptive land management and active interventions to prevent massive tree die-off and to maintain vital ecosystem functions.