Canopy‐scale VPD reduction in a mature forest alters stomatal response to soil drought in Pinus sylvestris L.

Summary Understanding how trees respond to interacting soil and atmospheric droughts is crucial for predicting forest resilience under climate change. However, no experiment has yet jointly manipulated vapor pressure deficit (VPD) and soil moisture in forests, limiting our ability to disentangle their effects. We investigated how c. 130‐yr‐old trees respond to coupled soil and atmospheric drought with a rainfall and VPD manipulation in an unmanaged forest. Using fine mist sprayers installed at canopy height, we reduced VPD by 24% in combination with throughfall exclusion (−50% precipitation) or irrigation (+100% precipitation) in mature Pinus sylvestris trees. We monitored tree water status, canopy conductance, and stomatal responses throughout the growing season. With irrigation, VPD reduction did not affect tree water relations. Under soil drought, trees exposed to reduced VPD used more water and reached lower predawn water potentials (Ψ PD ). This was accompanied by two parallel mechanisms: more frequently open stomata (at low‐VPD intensities); and a modification in the stomatal response to VPD. Moreover, reduced VPD resulted in shifted stomatal closure from a predawn water potential of −1.2 to −1.5 MPa. Exposure to low atmospheric demand affected trees' response to soil drought, providing observational evidences from joint air‐soil drought manipulation in natura .