Deep Soil Drying Increases Dryland Apple Tree's Eco‐Physiological Sensitivity to Atmospheric Drought

Abstract

Dryland tree survival critically depends on deep soil water (DSW), particularly during dry spells. However, DSW is often limited and can be exhausted during periods of active tree growth, as recharge from precipitation is minimal. The eco‐physiological responses of trees to compound droughts (i.e., combined deep soil and atmospheric droughts) remain poorly understood, particularly the variation of sensitivity to atmospheric drought under deep soil desiccation. This study addresses this gap through a three‐year field experiment (2019–2021) on China's Loess Plateau, a global hotspot for DSW depletion due to extensive afforestation. We found that deep soil drying below 200 cm significantly suppressed daily sap flux density ( J s) by 8%–13%, transpiration rate by 24%–36%, and canopy conductance by 25%–34%, in both normal and dry years. Importantly, deep soil drying heightened the sensitivity of trees to vapor pressure deficit (VPD), as demonstrated by two key physiological responses. First, partitioned trees (i.e., those with physically isolated deep and shallow root systems) exhibited reduced hydraulic buffering capacity, with hysteresis analysis revealing a 16% decrease in hysteresis area compared to controls, indicating stronger J s–VPD coupling. Second, partitioned trees initiated stomatal closure at lower VPD thresholds (1.0–1.4 kPa) compared to controls (1.3–2.8 kPa). These findings underscore the role of deep soil water as ecological insurance against compound droughts, and highlight that its depletion exacerbates tree vulnerability to such stresses. Our results may provide insights into the effect of compound atmospheric and soil drought on the evolution of managed ecosystems.