Hierarchical drought responses of canopy photosynthesis and water-use efficiency in Caragana korshinskii Kom.: from optimal function to degradation

Planting the drought-resistant vegetation is important to control desert in the world. However, the resistant mechanism to drought of psammophytes especially at the canopy scale remains unclear. To elucidate the mechanisms of water adaptation in Caragana korshinskii Kom. across various vertical canopy layers, we conducted an experiment in Dawukou District, Shizuishan City, Ningxia Hui Autonomous Region, China to measure hydraulic architecture and photosynthetic traits at different canopy heights under drought. Results showed a distinct vertical gradient in soil volumetric water content (VWC), characterized by water accumulation in deeper soil layers (60–80 cm) and depletion in shallower layers (0–20 cm). Under well-watered condition (70.00%–80.00% field capacity (FC)), the upper canopy achieved the highest photosynthetic rate ( P n ) and transpiration rate ( T r ); under moderate drought stress (40.00%–50.00% FC), photosynthetic amplitudes attenuated across all layers; under severe drought stress (10.00%–20.00% FC), the middle canopy became the primary photosynthetic refuge while lower-canopy photosynthesis nearly ceased. This hierarchical regulation strategy facilitated water conservation and survival under severe drought conditions. Upper-canopy photosynthetic and transpiration traits exhibited the strongest and most significant associations with shallow VWC (0–20 cm), reflecting rapid physiological adjustments to surface water fluctuations. By contrast, middle-canopy traits showed markedly weaker linkages to VWC, while lower-canopy traits displayed overall non-significant relationships with any measured soil layer, including 60–80 cm, indicating that lower leaves may depend on stored stem water or hydraulic redistribution rather than direct soil uptake. Furthermore, key ecological thresholds were identified as follows: (1) normal function: plant growth proceeded normally when VWC exceeded 3.04%, with optimal canopy WUE empirically confirmed within the measured VWC range of 10.07%–11.76%; (2) functional maintenance: as VWC declined to 1.52%–3.04%, photosynthetic activity gradually ceased in a layer-specific sequence—upper canopy<3.04%, lower canopy<2.03%, and middle canopy<1.52%; and (3) functional degradation: when VWC decreased to below 1.52%, total canopy photosynthesis ceased completely, indicating irreversible physiological decline. These findings provide an empirical basis for water management strategies, survival maintenance, and early-warning systems for degradation in C . korshinskii vegetation within arid areas.