Water consumption of mature European beech and Douglas fir trees growing in pure and mixed stands

In recent decades, the frequency and intensity of hotter droughts have increased, posing a serious threat to our forests. During hot droughts, increasing evapotranspiration depletes soil moisture reserves and thus exposes the trees’ xylem to critical water potentials. Several of Central Europe’s major timber species have been found to be especially susceptible to repeated summer droughts. Therefore, the forestry sector is increasingly considering the establishment of mixed stands and the inclusion of putatively more drought-resistant non-native tree species. However, silvicultural decisions about increasing the cultivation of non-native species and planting them in mixture requires empirical data on species-specific water consumption in pure and mixed culture in order to assess climate risks and to avoid potential negative competition effects.

To address these questions, we installed 32 dual-method-approach type sap-flow sensors capable of measuring the entire range of sap flow rates in pure European beech and Douglas fir stands as well as in a nearby mixed beech-Douglas fir stand on deep sandy soil in northern Germany. Additionally, heat-field-deformation type sap-flow sensors were used for measuring the radial sap flow profile in the xylem of each individual tree. The trees equipped with sensors covered a broad DBH range which allowed extrapolating to stand-level water consumption. Sap flow, soil moisture, soil matric potential and weather conditions were monitored over the wet year 2021 and the dry year 2022. We further applied time-dependent probe misalignment correction to account for measurement errors related to sensor installation and to changes in stem water content over the growing season.

Sapwood depth increased with the increasing DBH of a tree and ranged for beech from 6.5 to 15.5 cm and for Douglas fir from 7.5 cm to 14.5 cm. In general, both beech and Douglas fir in mixture had deeper sap flow profiles compared to their pure stands. Stand-level water consumption was higher in the pure beech than in the Douglas fir stand; the mixed stand consumed even more water than the two pure stands. Further, tree-level water use was related to tree size and the radial sap flow profile. Total tree water consumption was markedly higher in the dry year 2022 than in the moist year 2021 due to a higher evaporative demand.

The findings of this study are crucial for supporting foresters in silvicultural decision making and for better understanding the water cycle dynamics in forest ecosystems in the face of climate change.