Drought stress in European forests: Integrating novel experimental data to improve vegetation modelling

Drought events are threatening forest ecosystems worldwide and are also expected to increase in intensity and frequency in the future. Around Europe, multiple experimental sites have been set up to investigate the impacts of drought, for example, by excluding rainfall from trees. Over the years, these experiments have increasingly incorporated high-resolution temporal sensors. These sensors collect data on tree physiology—such as changes in diameter, sap flow, and stem water potential — at intervals as frequent as once every 30 minutes. While these experiments provide valuable insights into the impacts of drought on tree function, they are typically limited to the specific environmental conditions and species present at the study site.

Vegetation modelling offers a way to generalise from experiments. Multiple state-of-the-art vegetation models now incorporate plant hydraulics which (1) allows for simulating water movement throughout whole trees in detail and (2) introduces a hydraulic failure-based mortality process that describes how trees may succumb to extreme drought stress. However, representing plant hydraulic processes comes at the expense of introducing additional, often difficult to constrain, parameters to models.

Here, we show how new experimental data can be integrated into process-based vegetation modelling. More specifically, we use high-resolution sapflow and stem water potential data to effectively constrain the most crucial plant hydraulic parameters of the terrestrial biosphere model QUINCY: saturated xylem hydraulic conductivity, and stem and leaf water storage capacitance.

We further apply QUINCY to several FLUXNET sites and show that our parameterizations are consistent across different environmental conditions. We also discuss how the incorporation of hydraulic failure-based mortality mechanisms may alter modelled carbon dynamics and how future experiments could help reduce uncertainty in modelling drought-induced mortality.