DISTRESS · Understanding the mechanisms behind tree responses to drought-induced stress with increasing tree size

Understanding the mechanisms behind tree responses to drought-induced stress with increasing tree size

Plants adjust their water status (the leaf water potential Ψleaf; i.e., the force that allows plants to draw water from the roots and transport it to the leaves) by opening and closing stomata, tiny pores on the leaf surface which are also the gateway for CO2 entering the leaf for photosynthesis. Stomatal closure regulates transpiration and sap flow to control water loss and prevent xylem embolism (gas-filled conduits that disrupt water transport) during dry conditions. Hence, under drought conditions, caused by either drying soils, water-demanding atmospheric conditions (i.e., with high vapour pressure deficits, VPD) or both, canopy-scale water conductance (G, a measure of stomatal openness) and Ψleaf are expected to decline. In addition, as trees grow taller, they must pull water along a longer transport pathway and against a stronger gravity gradient, potentially requiring a greater force (i.e., Ψleaf) to maintain transpiration. Given these higher limitations to water transport, hydraulic theory predicts that tall trees will presumably be more sensitive than shorter ones to increases in VPD and drought severity following climate change. Because of the value of large trees to biodiversity and ecosystem biogeochemical cycles, further work is needed to understand the effect that tree height has on tree response to increased VPD and drought. DISTRESS aimed at (1) testing whether trees adjust different structural and functional traits to compensate for the predicted negative effect of height on G and Ψleaf, and (2) describing the mechanisms behind these adjustments and the potential interactions with other functional processes that may impair tree response to drought stress with increasing size. To achieve these goals, height-driven changes in multiple functional traits were assessed to test whether tall trees are more vulnerable to drought and evaluate how compensatory responses and trade-offs among traits may influence height-driven patterns in tree drought responses (WP1). In addition, the SAPFLUXNET database was used to perform a global-scale analysis of sap flow and G responses to VPD as a function of tree height (WP2). This analysis complements the approach in WP1, which used ‘static’ functional traits, by providing a more dynamic perspective on whole-tree responses to atmospheric drought. This integrated analysis has improved our understanding of the role that height plays in water-use regulation and tree vulnerability to drought. This knowledge may be used to improve mechanistic models of tree response to climatic variability. Such information is essential to better simulate the impact that climate change may have on forest ecosystems and thus adapt forest management strategies.

Data: CORDIS, © European Union