Abstract:

Climate-driven aridity threatens agroforestry systems (AFS) by reducing canopy conductance (Gc) sensitivity to vapor pressure deficit (VPD) (m), increasing hydraulic risks. While multidimensional water regulation in agroforestry enhances drought resilience, its mechanistic basis remains unclear. Integrating sap flow measurements, hydro-meteorological data, and plant functional traits, we determined that AFS enhance drought resilience by regulating m value, thereby restructuring ecological functions of tree water use strategies. Our findings demonstrated that AFS without drought (AFS-ND) doubled transpiration (EL) versus monocultures (FS-ND) through optimized dry-season (morning-enhanced/afternoon-suppressed) and wet-season (nighttime-enhanced) sap flow density (SFD) regulation, coupled with canopy structural optimization (increased leaf area index, crown diameter), enhancing m and developing a drought-preventing strategy. AFS with moderate drought (AFS-MD) maintained elevated m through dynamic shifts in water uptake strata (deep-to-shallow soil layers from dry to wet seasons), implementing an effective drought-buffering strategy. For AFS with severe drought (AFS-SD), the paradoxical coexistence of enhanced SFD coupled with reduced EL revealed that trees sustained robust canopyatmosphere coupling through coordinated exploitation of middle-deep soil water reserves, thereby offsetting canopy structural degradation. This sophisticated drought-avoidance strategy, rooted in stomatal-biomass tradeoffs, successfully transitioned the regulatory regime from the risky anisohydric behavior characteristic of monocultures to a conservative isohydric behavior, effectively mitigating hydraulic failure risks. However, heightened Gc sensitivity may induce long-term hydrological debts via excessive deep layer water depletion. The study advances our understanding of using the sensitivity of Gc to VPD to assess ecosystem drought resilience.