Understanding tree-water relations under drought stress – a case study

Climate warming and the associated rise in atmospheric vapor pressure deficit (VPD) have increased transpirational demand and coupled with droughts have increased susceptibility of plants. Plant water use strategies along the isohydry gradient during hot-drought determines maintenance of plant water status. While a number of studies have investigated drought response of different tree physiological processes and found species-specific behavior, studies with a holistic approach to understand growth, water use and stem water status under different neighborhood constellations in mature trees are still lacking.

We measured stem growth and water consumption in pure and mixed European beech and Douglas fir stands during two moist (2021, 2023) and one dry year (2022) on deep sandy soil in northern Germany, using a dataset from 16 trees equipped with high-resolution band dendrometers and 32 trees with sap-flow sensors (dual-method approach). In addition, radial sap flow profiles were measured in each tree with heat-field-deformation sensors, canopy structure analysed with mobile laser scanning, soil moisture content and soil matric potential recorded at multiple depths to interpret the growth and water use patterns.

During a period of persisting drought, pure Douglas fir reached 50% soil desiccation nearly twice as fast as the pure beech and mixed beech-Douglas fir stand. However, compared to the previous, wet year (2021), pure Douglas fir had the lowest reduction in growth (12%) and mixed Douglas fir the highest (36%). Beech ranged in between, with lower growth reduction in the mixed stand. Daily sap flow rates increased with higher VPD, but decreased at <20% of relative extractable water (REW) with greatest reduction in isohydric Douglas fir compared to beech. Stem water content remained relatively high (>50%) until 20% REW, but showed a sharp decrease afterwards, along with increasing tree water deficit.

We show how different tree physiological processes and their relation change in interaction with soil moisture and VPD. The partially contrasting patterns in water use can be explained by the relatively more isohydric and anisohydric behaviour of Douglas fir and beech, respectively. Furthermore, canopy structural traits may play a key role in shaping tree species-mixture effects, favoring beech in mixture with Douglas fir under drought. Our results demonstrate how tree functional traits are influencing the forest water cycle in the face of climate change.