4. Dr. Nadine Ruehr

Reports of tree mortality in response to drier and hotter conditions are becoming increasingly widespread. Particularly in already drought-prone environments tree death triggered by prolonged dry periods alongside hotter conditions could be further intensifying in the coming decades due to climate change, which might also put efforts of forest regeneration at risk. But we currently lack the capability to make consistent assessments of tree mortality and linking it to the underlying reasons and long-term consequences. To overcome such challenges, the International Tree Mortality Network (www.tree-mortality.net) was formed, which aims to bring together the global ecological, forestry, remote-sensing and Earth-system science communities to: (1) quantify trends in tree mortality rates globally, (2) attribute the causes of tree mortality and (3) help develop the capability to accurately predict tree mortality trends, in a changing world. Here, we present results from a recently compiled wide-ranging survey of data sources potentially suitable for assessing tree mortality rates globally. We then outline some of the challenges, and potential solutions, to combining these data sources into consistent metrics of tree mortality rates. In addition, we present recent observations of drought and hotter-drought triggered tree mortality in Mediterranean and semi-arid forests and discuss some of the underlying physiological reasons and potential longer-term consequences.