Soil warming alters tree water use and canopy stomatal conductance in a mixed subtropical forest

As air temperature and vapor pressure deficit (VPD) increase continuously, forests are losing more water through evapotranspiration, with large consequences for local and global hydrological cycles. In regions with high vegetation cover, soil warming can be more pronounced than warming of the air because of reduced air movement in the forest understory and high heat conduction in the soil. Hence, soil warming could further amplify climate change impacts on forest water uptake and use. Yet, the independent effects of persistent soil warming on tree transpiration remain highly uncertain, especially in sub-tropical forests where soil temperature rise has been considerable in recent years. We carried out a long-term soil warming experiment (+2 degrees C) in a mixed subtropical forest dominated by the relatively anisohydric Acacia auriculiformis and isohydric Schima superba . Throughout the dry and wet seasons, we tracked whole-tree transpiration (E L ), night-time water recharge (E L_night ), hydraulic conductivity (K), and canopy stomatal conductance (G s ) on mature trees of both species. We found that E L and G s of A. auriculiformis increased (+ 41 % and + 5 %, respectively) while it decreased in S. superba (- 5 % and - 21 %, respectively) with soil warming. However, no changes in K were found for either species. Moreover, soil warming did not change the sensitivity of E L and G s to VPD in both species, but the reference canopy stomatal conductance (G sref ) was influenced. Our results suggest that soil warming significantly alters tree canopy transpiration, independently of air temperature. In sub-tropical climates, species with anisohydric stomatal strategy, such as A. auriculiformis , could take up more water and continue to transpire under both high and low VPD, potentially leading to increased nutrient and carbon uptake, and growth. In contrast, isohydric species like S. superba seem adversely affected by soil warming, which may compromise its dominance in subtropical forests in the future.