Worldwide temperature increases and more frequent droughts will critically modify forest transpiration, thereby affecting the global water cycle. Yet, tree responses to increased atmospheric vapour pressure deficit (VPD) and reduced soil water availability (SWC) are not fully understood due to long-term adjustments of trees to local climatic and edaphic conditions that can modify species-specific 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 (βVPD) and the maximum SFD (S95), using the SAPFLUXNET database. Furthermore, we tested whether long-term adjustments to site climatic conditions and stand characteristics affect βVPD and S95.
The species showed contrasting sensitivities and maximum SFD with the highest values in F. sylvatica, followed by Q. ilex, which both outperformed the two conifers that showed low βVPD and low S95. We observed that βVPD and S95 significantly decreased with lower SWC in F. sylvatica, P. sylvestris, and Q. ilex, but not in P. abies. Both βVPD and S95were to some degree driven by tree height, and the temperature and precipitation at the sites. However basal area was the most important driver of βVPD and S95, explaining 30% of the variance.
We observed significant short-term transpiration responses to VPD and SWC in three species and considerable adjustments to long-term climate and stand characteristics. A future warmer and drier climate will induce restrictions on tree transpiration and thereby heavily affect the soil-plant-atmosphere water cycle. However, basal area being the largest driver of tree transpiration sensitivity to VPD across a broad range of conditions provides the opportunity to adapt European forests to future climate conditions.