Sensing stresses adaptation at the whole-tree scale at VPDrought in Pfynwald

The hydraulic system is the central hub that steers the whole-tree water and carbon fluxes. During periods of low water availability, trees differently modulate these fluxes to balance photosynthesis, growth, and other metabolic activities. To efficiently manage these processes, trees have developed various water-use strategies, regulating the closure of stomatal pores on their leaves at different environmental thresholds. Recent observations during increased vapor pressure deficit (VPD) and reduced soil water availability periods, raise the question of whether trees can acclimate their hydraulic functioning at the whole-tree level. Additionally, it is largely unknown if such acclimations are driven by VPD or reduced soil water availability, hindering our ability to provide robust predictions on the stress thresholds and tree growth. Novel sensor technologies, such as stem dendrometers, sap flow sensors, and cavicams allow us to continuously monitor the secondary growth, sap transport rates, and stem water potential, respectively. This information is critical for quantifying the seasonal dynamics of whole-tree water use and calculating critical parameters such as canopy conductance. In this project, we investigate the changing sensitivity of stomatal control in the canopy of Scots pine trees (Pinus sylvestris) at the VPDrought experimental research site. We couple these continuous monitoring to daily dynamics of gas exchanges (CO2 and H2O) and leaf water potential. The unique research setup of VPDrought, which includes a factorial design of VPD manipulation and rain exclusion, enables us to disentangle the acclimation potential of whole-tree water use to atmospheric and soil drought. These acclimation results will be compared with the large-scale rain exclusion experiment at the Swiss Canopy Crane II site to confirm the importance of such acclimation in a more diverse forest type. Through this project, we aim to provide new insights into the strategies Scots pine implements to cope with either atmospheric or soil droughts. The data generated are of great value for understanding whole-tree adaptation to different environments and mechanistic modeling of water transport and turgor-driven growth. Moreover, due to the large number of monitored trees, the data are particularly valuable for studies utilizing remote sensing. Coupling different sensors and technologies makes it possible to study water use at unprecedented temporal resolutions, enabling us to address novel research questions relevant to developing science-driven forest management strategies.