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Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil‐vegetation‐atmosphere‐transport (SVAT) model LWF‐Brook90 taking advantage of regionalized depth‐resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant‐available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT‐derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback‐driven quantification of ecosystem water fluxes in the soil–plant‐atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes. [ABSTRACT FROM AUTHOR]

Trees in seasonal climates may use water originating from both winter and summer precipitation. However, the seasonal origins of water used by trees have not been systematically studied. We used stable isotopes of water to compare the seasonal origins of water found in three common tree species across 24 Swiss forest sites sampled in two different years. Water from winter precipitation was observed in trees at most sites, even at the peak of summer, although the relative representation of seasonal sources differed by species. However, the representation of winter precipitation in trees decreased with site mean annual precipitation in both years; additionally, it was generally lower in the cooler and wetter year. Together, these relationships show that precipitation amount influenced the seasonal origin of water taken up by trees across both time and space. These results suggest higher turnover of the plant‐available soil‐water pool in wetter sites and wetter years. Plain Language Summary: In the middle of a hot and dry summer, we often think that a large rain event can "rescue" a forest from drought. However, it is not clear whether trees can or do consistently use the water from summer rains. A growing body of research indicates that over the course of the summer growing season, trees take up significant amounts of water that originated as winter snow or rain. We studied the seasonal origins of the water taken up by three common tree species (beech, oak and spruce) in forest sites across Switzerland in two different years. Our results demonstrate that trees at most sites use some water from winter snow and rain, even at the height of summer, although there were differences among species and sites. The water sources used by trees likely reflect a combination of the amount and timing of winter and summer precipitation, the rate at which that precipitation moves through the soil, and the distribution of the tree roots that take up the water. Determining the seasonal origin of water used by trees, as well as why it may vary over time and space, can help us better anticipate the effects of global climate change. Key Points: We determined the representation of water originating from winter versus summer precipitation in the xylem of common Swiss treesWater originating from winter precipitation was observed in trees at most sites, even at the height of summer, but varied among speciesClimate and species appear to be key controls over spatial and inter‐annual variations in the seasonal origins of water used by trees [ABSTRACT FROM AUTHOR]