High VPD mitigates the impact of soil drought in Pinus sylvestris L. through earlier stomatal closure

Climate change scenarios foresee a drastic increase in atmospheric and soil droughts, driven by elevated vapor pressure deficit (VPD), in the future. However, the impact of coupled high VPD and soil water deficit on tree physiology is mainly untested, especially in the field. To disentangle VPD and soil moisture effects on mature trees, we installed a unique experimental setup in a 130-year-old natural pine tree forest. By misting water vapor towards tree canopies and excluding precipitation, we decreased VPD by 20-30% and/or precipitation by 50%. We continuously measured sap flow and trunk growth with punctual dial dynamics of gas exchange and leaf water potential for one growing season. We found that high VPD under well-watered (WW) soil conditions increased tree water loss and reduced secondary growth compared to WW trees at reduced VPD. Oppositely, high VPD at dry soil water conditions (WD) reduced whole-plant transpiration by earlier stomatal closure, and increased secondary growth compared to WD trees at lower VPD. Consequently, high VPD can mitigate the adverse effects of soil drought through its impact on stomatal sensitivity, with significant consequences on tree growth. These results highlight the complexity of the physiological response of mature trees under (un-)coupled stresses.