Tree functioning under different soil water availability and atmospheric water demand

As a consequence of ongoing climate change, trees are increasingly exposed to atmospheric and soil drought. The project “VPDrought - a novel approach to disentangle atmospheric and soil drought” investigates how trees respond to distinct and combined changes in below- and above-ground conditions by manipulating precipitation and vapor pressure deficit (VPD) in a drought-prone natural pine ecosystem in the Swiss Rhone valley. In particular, the effects of changing VPD on mature trees have not been well studied yet, and the VPDrought project, extending a 20-year long-term irrigation project, provides worldwide unique conditions for integrated studies from the cell to the ecosystem level on an eLTER research platform. The manipulation of the environmental conditions began in late spring 2024 and will continue until 2028. We will demonstrate the effectiveness of our approach and present initial results on the short- and long-term responses of mature trees to altered soil- and atmospheric drought conditions, focusing on the temporal dynamics of key indicators for tree functioning, such as stem sap-flow, tree water deficit, stem shrinkage and expansion, and photosynthetic activity. To the extent possible at this early stage of the project, we will explore thresholds and tipping points under extreme conditions and discuss potential implications for forest health and tree mortality under future climate conditions. Our presentation highlights novel opportunities for studying tree responses to changing water availability and atmospheric demand. Thanks to comprehensive and often continuous high-temporal-resolution observations, this project allows us to investigate tree functioning in ways that have not been possible before, paving the way for new insights into forest ecology under climate change.