Understanding water use strategies of Central European tree species in dependency on groundwater depth

In 2003, 2015, and 2018 extreme droughts caused severe depletion in soil water storage, decreased groundwater tables, and severe damage to forest ecosystems in Europe, such as increased mortality rate. In the future, such extreme droughts will be more likely to occur due to climate change. Therefore, it is crucial to understand and develop mitigation strategies and responses to reduced water availability of European trees for drought resilience.

Tree species significantly differ in their response to drought. Isohydric trees, for example, which are often deep-rooted, close their stomata when sensing a change in soil water potential while anisohydric trees - often shallow-rooted - continue to transpire even when soil moisture declines. As a result, anisohydric trees have an increased risk of hydraulic failure under drought stress because of their high stomatal conductance. Moreover, it is assumed that deep-rooted trees are more resilient to droughts than shallow-rooted trees because these trees possess an enhanced capacity to better withstand periods of drought. However, when naturally deep-rooted and isohydric trees lose their stable water source connection they might be strongly susceptible to drought. In this study, we examine the different below and above-ground mitigation strategies of common central European tree species in a temperate climate.

We chose a mixed forest stand composed of Fagus sylvatica L., Carpinus betulus L., Fraxinus excelsior L., and Quercus spp. trees on a hillslope in NW Germany where a natural gradient of groundwater distance (> 4 m top site, ~ 1.50 m valley) exists and variable rooting depths are found. Observations of soil and plant water status, as well as groundwater level at three hillslope positions (top, slope, valley), started in May 2023. Two point-dendrometers per tree species and hillslope position providing annual tree growth were used to determine tree water potential. Sap flow sensors (three per tree species and position) were installed to estimate plant water use and stem water content as well as for upscaling to tree and stand-level photosynthesis. All sensors will run another growing season for comparative analysis.

Although the year 2023 was not particularly dry and no severe soil water depletion was observed, the first growing season measurements indicate that e.g. C. betulus and F. excelsior are performing better - i.e. showing higher sap flux velocities and higher growth rates - in the valley than at the top position. Potential reasons could include the proximity of the groundwater table in the valley, trees being less limited in their transpiration efforts. However, other factors such as differing shading and less competition to C. betulus trees, which are more abundant uphill, need to be explored further.