Belowground responses to soil and atmospheric drought imaged with time-lapse geoelectrics

Forests are experiencing increasing stress from the combined effect of atmospheric warming and the air's capacity to hold more moisture. Vapour pressure deficit (VPD), a key measure of atmospheric moisture demand, increases with warming due to the nonlinear rise in saturation vapour pressure, amplifying transpiration demand and drought stress and increasing mortality risk. VPD may cause large-scale physiological stress in trees and can lead to forest die-backs. The Pfynwald Research platform (Southern Switzerland) is located in a natural forest reserve dominated by (> 100-year-old) Scots pines growing on shallow soils with low water-holding capacity. The region has experienced pronounced drought stress and tree mortality in recent decades, making it a natural laboratory to investigate drought impacts on tree physiology, ecosystem functioning, and resilience. The Pfynwald research team, led by WSL, has initiated a long-term irrigation study in 2003 to quantify ecosystem responses to alleviation of chronic drought stress. A more recent experiment, inaugurated in 2024, studies the effect of soil and atmospheric drought by a unique setup that (1) intercepts rainfall through a throughfall exclusion and (2) manipulates atmospheric VPD by regulating air humidity in the forest canopy. In this contribution, we focus on the belowground by showcasing a time-lapse quasi-3D geoelectric experiment that we started in May 2025 including daily repeated subsurface electrical resistivity surveys to track belowground moisture variations in time and space. Our setup allows us to investigate deeper belowground effects of aboveground manipulations, namely the VPD, irrigation, and drought treatments. We present a multi-disciplinary investigation that integrates our geophysical findings with the dense, aboveground observations of the site. Our work stresses the importance of distributed and continuous monitoring of belowground states in natural manipulation experiments, for a holistic understanding of how climate change will affect forest ecosystems.