Understanding the mechanisms behind tree responses to drought-induced stress with increasing tree size
Plants adjust leaf water potential and hydraulic conductance under drought through stomatal behaviour, reducing sap flow and protecting plants from extensive water loss and embolism. Due to the negative effect that vapour pressure deficit (VPD) and tree height have on canopy-scale water conductance (G), Darcy’s law predicts a decline in G due to the expected increase in VPD following climate warming, to which tall trees would be presumably more sensitive. Further work is thus needed to understand the effect that tree size has on tree response to increased VPD and drought. This project aims at (1) testing whether, at a given VPD, trees adjust different functional traits to compensate for the negative effect of height on G in (a) tropical forests and (b) at a global scale, 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. We will first measure multiple functional traits (including sap flux, gas exchange and leaf and xylem water potential) on trees of different heights to test Darcy’s law predictions and evaluate the role that the trade-offs among traits play on enhanced vulnerability to drought with increasing tree size in tropical forests. In order to assess whether the studied mechanisms prevail across species and ecosystems, we will perform a global-scale analysis of sap-flow and, thus, G responses to VPD as a function of tree height using the sap-flux data from 159 species and nine different biomes gathered within SAPFLUXNET. This integrated analysis will provide a better understanding of the role that tree size plays in tree vulnerability to drought in the short (temporary physiological response) and long term (legacy effects), allowing the improvement of 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.