Stand structure of Central European forests matters more than climate for transpiration sensitivity to VPD

Temperature rise and more severe and frequent droughts will alter forest transpiration, thereby affecting the global water cycle. Yet, tree responses to increased atmospheric vapour pressure deficit (VPD) and reduced soil water content (SWC) are not fully understood due to long-term tree adjustments to local environmental conditions that modify transpiration responses to short-term VPD and SWC changes. We analysed sap flux density (SFD) of Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus ilex from 25 sites across Europe to understand how daily variation in SWC affects the sensitivity of SFD to VPD (β_VPD) and the maximum SFD (S₉₅). Furthermore, we tested whether long-term adjustments to site climatic conditions and stand characteristics affect β_VPD and S₉₅. The studied species showed contrasting β_VPD and S₉₅ with the largest values in F. sylvatica, followed by Q. ilex, which surpassed the two conifers that showed low β_VPD and low S₉₅. We observed that β_VPD and S₉₅ dropped during days of low SWC in F. sylvatica, P. sylvestris and Q. ilex, but not in P. abies. Both β_VPD and S₉₅ were driven by tree height, and the temperature and precipitation at the sites. However, stand basal area was the most important driver of β_VPD and S₉₅, explaining 30% of their total variance. Synthesis and applications: A future warmer and drier climate will restrict tree transpiration and thereby heavily affect the soil–plant-atmosphere coupling. However, the effect of basal area, being the largest driver of tree transpiration sensitivity to vapour pressure deficit across a broad range of conditions, provides the opportunity to pre-adapt European forests to future climate conditions. While stand thinning can increase the soil water availability for remaining trees, it also increases transpiration sensitivity to high air temperatures and may thereby amplify tree vulnerability to heat and drought.