Response of Zanthoxylum bungeanum Transpiration to Microclimate and Soil Moisture Conditions in Different Slope Aspects of a Plateau Gorge in Subtropical Monsoon Climate Zones

Abstract

Climate change profoundly influences local microclimates, and plant transpiration processes are directly regulated by these microclimatic conditions. As a key topographic factor, mountain slope aspects shape local microclimatic characteristics through the redistribution of hydrothermal resources. Although plant growth and physiological processes are more directly influenced by local microclimates than by macroclimates, the differentiated responses of plant transpiration dynamics to environmental factors across different slope aspects and their underlying driving mechanisms remain unclear. This study investigates Zanthoxylum bungeanum forests on sunny and shady slopes at the same altitude in plateau gorges within a subtropical monsoon climate zone. Through in situ monitoring of sap flow (SF), local meteorological factors, soil water content (SWC) and stable isotope composition, we explored the effects of differences in microclimate and SWC on plant transpiration and water use strategies. The results showed that: (1) Water use patterns exhibited significant seasonal and slope-related differences. During the rainy season, plants on both slopes primarily relied on shallow soil water. In the dry season, plants on the shady slope consistently utilised deep soil water (30–60 cm), whereas plants on the sunny slope rapidly shifted to using shallow and intermediate water sources following light rainfall, facing a higher risk of water source depletion. (2) During both dry and rainy seasons, SF on both slopes was primarily driven by vapour pressure deficit (VPD) and air temperature (Ta), which exerted threshold control effects on SF. These thresholds varied with slope aspect and season. Specifically, the VPD threshold during the rainy season was sunny slope (2.7 kPa) < shady slope (3.2 kPa); during the dry season, it was sunny slope (31.9°C) > shady slope (30.1°C). The Ta threshold during the rainy season was sunny slope (34.3°C) < shady slope (38.4°C); during the dry season, it was sunny slope (31.9°C) > shady slope (30.1°C). Furthermore, the stomatal conductance of plants on the shady slope was more sensitive to changes in VPD and Ta than that of plants on the sunny slope. Therefore, under drought conditions, plants on the shady slope exhibited a more robust water use strategy, with greater stability in transpiration and stomatal regulation, significantly reducing the risk of hydraulic dysfunction and demonstrating stronger drought adaptability compared to plants on the sunny slope.