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Abstract

Trees are exceptional organisms that have evolved over some 385 million years and have overtaken other plants in order to harvest light first. However, this advantage comes with a cost: trees must transport water all the way up to their crowns and inherent physical limitations make them vulnerable to water deficits. Because climate change scenarios predict more frequent extreme drought events, trees will increasingly need to cope with water stress. Recent occurrences of climate change-type droughts have had severe impacts on several forest ecosystems. Initial experimental studies have been undertaken and show that stomatal control of water loss hinders carbon assimilation and could lead to starvation during droughts. Other mechanisms of drought-induced mortality are catastrophic xylem dysfunction, impeded long-distance transport of carbohydrates (translocation) and also symplastic failure (cellular breakdown). However, direct empirical support is absent for either hypothesis. More experimental studies are necessary to increase our understanding of these processes and to resolve the mystery of drought-related tree mortality. Instead of testing the validity of particular hypothesis as mechanisms of drought-induced tree mortality, future research should aim at revealing the temporal dynamics of these mechanisms in different species and over a gradient of environmental conditions. Only such studies will reveal whether the struggle for light will become a struggle for water and/or for carbon in drought-affected areas.