Assessment of drought stress and growth responses in European forests using high-precision dendrometers

The exceptional dry periods between 2018 and 2022 emphasize the growing challenge for European forests, as they experience increasingly dry and warm conditions that impact their growth and resilience. Recent observations reveal that climate change is driving up the vapor pressure deficit (VPD) and reducing soil water availability, underscoring the urgent need for effective tree health monitoring, especially at the whole-tree level. Signals such as tree water deficit (TWD) offer crucial insights into the water status of trees, but the precise TWD level that triggers physiological stress remains unknown. Through advanced analytical techniques, we have identified consistent shrinkage thresholds for common European tree species, allowing us to monitor TWD continuously to detect the timing and severity of drought stress. However, a central question remains: how stable are these thresholds across various regions and over time?

This PhD project utilizes cutting-edge automated dendrometers, which are installed on tree bark to continuously measure TWD and growth signals of widespread European tree species. Complementary measurements of leaf water potential will also be collected to accurately assess drought stress levels across a broad spatial range in Europe. Our primary goal is to determine species-specific drought stress thresholds that hold true, both spatially and temporally. At selected sites, we will collect additional ecophysiological data on critical hydraulic thresholds, such as the point at which the tree's water transport system begins to incur damage, contextualizing leaf water potential dynamics.

Data will be gathered from multiple sites across Europe, including the VPDrought experiment in Valais, Switzerland; the Swiss Canopy Crane II site near Basel, Switzerland; the Kroof experiment near Freising, Germany; and the FeMoPhys monitoring site in Demmin, Germany. The unique design of VPDrought, which combines VPD manipulation with rain exclusion, allows us to examine the adaptability of trees to both, atmospheric and soil drought. The primary experimental work will occur at the KROOF site, where we simulate various drought intensities through rain manipulation. After refining methods to interpret dendrometer signals, we will apply these techniques across a Europe-wide dendrometer network (spanning over 1,500 trees) to map drought stress patterns across Central Europe and identify areas of heightened vulnerability.