Disentangling atmospheric and soil drought: a new stand-level manipulation approach for mature forests

Atmospheric and soil drought are major, yet typically confounded, drivers of tree stress. Rising vapor pressure deficit (VPD) may impair forest function independently of soil moisture decline, yet stand-scale experimental separation in mature forests has remained unresolved. We tested whether atmospheric demand can be manipulated independently of soil water supply and how such manipulation propagates through the soil-plant system. We established a factorial field manipulation in a c. 130-yr-old Pinus sylvestris forest that independently modifies canopy VPD and soil water input using fine-mist VPD reduction, irrigation (+600 mm), and throughfall exclusion (-50% precipitation). Atmospheric and soil conditions were continuously monitored across depths over two growing seasons. The experiment specifically targeted periods of high atmospheric evaporative demand, reducing VPD by an average of 21% when ambient VPD exceeded its 90^th percentile. Soil drying followed sustained high-VPD phases, beginning in the topsoil and progressively later at depth. Despite identical water inputs, reduced VPD particularly altered deeper soil water trajectories, indicating plant-mediated feedbacks. Atmospheric drought propagates through the soil-plant-atmosphere continuum rather than acting solely as an aboveground stressor. Explicit integration of atmospheric demand into experiments and ecosystem models is essential for predicting forest resilience under rising atmospheric drought.