Linking tree water status to the carbon budget: identifying mechanisms of drought-induced tree mortality in Pinus nigra and Picea abies

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Forest ecosystems foster the bulk of terrestrial biodiversity and are a major driving force of the cycling of life-supporting elements (e.g., carbon, nitrogen). However increasingly frequent occurrences of increasingly intense heat and drought events have led to widespread forest mortality around the globe. Anticipated future increases of drought events may further threaten the survival of forests and hence jeopardize the sustained flow of the life-supporting services they provide. Surprisingly, there are large gaps in the fundamental understanding of how drought kills trees. Current working hypotheses (carbon starvation, impeded carbon translocation, hydraulic/symplastic failure) all lack experimental support but a thorough understanding of these mechanisms is central to predicting impacts on forest ecosystems, future vegetation distribution and element cycling. Here I propose a series of experiments explicitly designed to partition the different mechanisms of drought-induced tree mortality. The experimental design will cover morphological, physiological and functional aspects of the studied trees and will include manipulations of the atmospheric composition. The results of these experiments will improve our understanding of drought-induced tree mortality and will be helpful for developing adaptation and mitigation strategies. Thus they are essential for the conservation of forest ecosystems in a warmer and dryer, CO2-enriched world.