Climate change is often associated with increasing vapor pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil‒atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g br), transpiration rate (E) and net photosynthesis (A net) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g br to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the g br and E response to VPD. These results demonstrate that avoiding drought on the supply side (soil moisture) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

Climate change is often associated with increasing vapor pressure deficit (VPD) and decreasing soil moisture (SM). While atmospheric and soil drying often co-occurs, their differential effects on plant functioning and productivity remain uncertain. We aimed to elaborate on the divergent effects and underlying mechanisms of soil and atmospheric drought, based on continuous, in situ measurements of branch gas exchange, with automated chambers, in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmosphere drought (low SM, high VPD) during the rainless Mediterranean summer, and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g br), the rates of transpiration (E) and net photosynthesis (A net) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g br to the massive rise in VPD (to a maximum of 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing that plant water potential is a good predictor of the g br and E response to VPD. These results demonstrate that the release from drought on the supply-side, in combination with plant hydraulic regulation, eliminates the effect of atmospheric demand (VPD) as a stressor and on canopy gas exchange in mature, drought-adapted pine trees.

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify such sequence, we used a 2016 tree mortality event in the semi-arid pine forest of Yatir were dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the living ones, eight months ‘prior to the visual signs of mortality’ (PVSM; e.g., brown needles). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, six months PVSM. Third, cessation of stem sap flow three months PVSM. Eventual mortality could therefore be detected long before visual signs are observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capabilities, and soil water uptake. The results indicated that breakdown of radial stem water flow and phloem functionality is a critical element in defining the ‘point of no return’ in the sequence of events leading to mortality of mature trees.