Water consumption of beech, spruce and Douglas fir in pure and mixed stands in a wet and a dry year – Testing predictions of the iso/anisohydry concept

A rising atmospheric vapour pressure deficit (VPD) increases forest transpiration and depletes soil moisture reserves, exposing trees to stress and reducing groundwater recharge. How stand water consumption varies with the species composition, is not well known, but is crucial for managing water resources. We measured stand-level transpiration of nearby pure European beech, Norway spruce and Douglas fir stands and a beech-Douglas fir mixture on deep sandy soil with sap flux systems during a wet and a dry year to compare the species' water use patterns under varying water availability and examine species mixing effects. In the wet year, pure Douglas fir consumed 123 % more water (472 mm yr^-1) than pure beech (212 mm yr^-1) and 50 % more than pure spruce (estimated at 307 mm yr^-1), with the mixed stand being intermediate (295 mm yr^-1). In the dry year, isohydric Douglas fir and spruce reduced water use by 38 % and 26 %, respectively; yet, their water consumption still exceeded the beech stand. In contrast, beech transpiration increased in the dry year by 2 % due to elevated VPD. In the mixture, Douglas fir reduced transpiration in the dry year less than in the pure stand (-28 % vs. -38 %), suggesting the species profited from beech admixture. We conclude that forest water consumption is determined by both stand structural properties and tree species identity, with the degree of isohydricity largely determining interannual transpiration variation. High water consumption of Douglas fir rapidly depletes soil moisture, which may reduce groundwater recharge and threaten the species in drier regions.