Unexpected hydraulic lift occurs in different tree species in mountainous areas of Beijing

The vertical soil moisture heterogeneity in the rhizosphere affects the way vegetation absorbs water, resulting in hydraulic lift (HL). However, quantifying and resolving HL is difficult due to the unpredictability of subsurface components and complex plant-soil interactions. In this study, we investigated the HL occurrence in tree species (Acer truncatum Bunge and Pinus tabuliformis Carrière) in the mountainous areas of Beijing by pot experiments through the addition of heavy water (²H₂O) to create a soil moisture gradient. The results indicated that soil moisture differences (0.107–0.204 cm³ cm⁻³ for A. truncatum, 0.048–0.145 cm³ cm⁻³ for P. tabuliformis, respectively) led to a significant occurrence of HL in both broadleaf (A. truncatum) and conifer species (P. tabuliformis). The total HL was up to 4767 and 2735 cm³, with daily fluxes of 64–1,49 and 84–1133 cm³ d⁻¹ for A. truncatum and P. tabuliformis, respectively. The flux of HL in P. tabuliformis was lower and appeared later than that of A. truncatum. This is because P. tabuliformis will respond to drought by preferentially decreasing stomatal conductance (Gs) and transpiration rate (Tr) compared to A. truncatum. As the Tr of A. truncatum and P. tabuliformis decreased, the probability of HL occurring increased, but the fluxes gradually declined. The occurrence of HL increased the likelihood of plant water uptake from deep soils, helping to alleviate drought stress in shallow soils. This study will help to understand the water acquisition and allocation strategies of plants under drought stress.