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.
