Periodic Reporting for period 2 - DISTRESS (Understanding the mechanisms behind tree responses to drought-induced stress with increasing tree size)
Période du rapport: 2021-12-16 au 2022-12-15
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.
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.
During the outgoing phase of this MSCA at the Pacific Northwest National Laboratory (PNNL; 12/2019 – 12/2021), a quantitative synthesis on how different structural and functional traits change with height was performed to assess the implications those adjustments may have on tall-tree vulnerability to drought. This review showed that both Ψleaf and G decrease with tree height. However, taller trees have developed several structural and functional adjustments, including enhancements in carbon and water storage and water uptake and transport efficiency, that minimize this drop in Ψleaf, allowing them to resist episodic water stress. Yet, these shifts may not be sufficient to prevent increasing mortality rates in taller trees during more severe droughts. Our review shows that we still lack conclusive evidence to disentangle the mechanisms (i.e. hydraulic dysfunction, carbon depletion and/or biotic attacks) behind tall-tree increased mortality during drought. In addition to this literature review, the carbon reserves of Bornean trees vulnerable to drought-induced mortality and subjected to stem girdling (to hinder carbon transport from the leaves to the roots) were analysed. Preliminary results indicate that girdling led to tree mortality. However, larger trees tended to have greater non-structural carbon reserves than smaller ones, and tree size did not have a significant effect on the time to tree death.
Work during the MSCA’s return phase at the Centre for Research on Ecology and Forestry Applications (CREAF; 12/2021 – 12/2022) involved working on sap flow data from the global dataset SAPFLUXNET (WP2). The effect tree height has on sap flow rates per tree and per sapwood (i.e. living section of the wood) area and G, as well as on their response to VPD, was assessed within and across species. Tree height has a significant effect on transpiration, although this effect is stronger at the tree level than on a per sapwood area basis. Taller species have greater water fluxes. At the intra-specific level, total sap flow per tree increases with tree height, while height-related changes in G and sap flow per sapwood area were highly species-specific. Transpiration increases with VPD were stronger in taller trees, while G decreased with VPD more steeply in shorter ones.
Results from this project were disseminated through four scientific manuscripts (three with the fellow as first author, two in preparation and one under review at the reporting time) as well as through oral presentations at international conferences (two), internal (three) and external (one) seminars and the 2022 European Researchers’ Night.
Work during the MSCA’s return phase at the Centre for Research on Ecology and Forestry Applications (CREAF; 12/2021 – 12/2022) involved working on sap flow data from the global dataset SAPFLUXNET (WP2). The effect tree height has on sap flow rates per tree and per sapwood (i.e. living section of the wood) area and G, as well as on their response to VPD, was assessed within and across species. Tree height has a significant effect on transpiration, although this effect is stronger at the tree level than on a per sapwood area basis. Taller species have greater water fluxes. At the intra-specific level, total sap flow per tree increases with tree height, while height-related changes in G and sap flow per sapwood area were highly species-specific. Transpiration increases with VPD were stronger in taller trees, while G decreased with VPD more steeply in shorter ones.
Results from this project were disseminated through four scientific manuscripts (three with the fellow as first author, two in preparation and one under review at the reporting time) as well as through oral presentations at international conferences (two), internal (three) and external (one) seminars and the 2022 European Researchers’ Night.
DISTRESS findings highlight the need for accounting for the size-related variability in functional traits, particularly within species, which is often neglected as most studies on tree responses to drought focus on inter-specific differences. The literature review identified knowledge gaps that should inform future research needs, particularly in disentangling the roles hydraulic failure, carbon depletion and biotic attacks (and their interaction with tree defence and physiological status during drought) play on size-dependent vulnerability to drought-induced mortality. The better understanding of tree function gathered through DISTRESS could be used to improve ecosystem models, and thus their projections of vegetation responses to climate change. The results of this project did not only advance the field of plant ecophysiology but should also serve policy makers and forest managers in their task of developing strategies that minimize the impact of drought on forest ecosystems.