Poplar trees do not always act as a water pump: Evidence from modeling deep drainage in a low-coverage-mode shelterbelt in China

Abstract

Large-scale afforestation can combat desertification, provide ecosystem services, and mitigate climate change. However, it can also have negative effects on the hydrological cycle and water budget, and thus on water security in dryland regions depending on the afforestation strategy used. Currently, the effects of different afforestation strategies on soil moisture dynamics, evapotranspiration (ET), and water budget at the stand scale are unclear. In this study, we investigated the water budget in two poplar stands, one planted using a low-coverage-mode shelterbelt (LCMS) strategy and the other planted using a conventional strategy (control). Deep drainage (DP), as an indicator of water budget, was estimated using a Hydrus-1D model calibrated with the observed soil moisture at different soil depths (across 0–200 cm) during the growing seasons in 2016, 2017, and 2019. The average DP in the LCMS stand was 107.0–149.6 mm, accounting for 36.5–42.4% of the rainfall occurring during the growing seasons, which was slightly higher than that in the control stand containing younger trees at a higher density. However, DP was more spatially heterogenous in the LCMS stand compared with in the control stand. Specifically, in the LCMS stand, the inter-canopy DP was 195.3–237.5 mm, accounting for 66.6–69.7% of the rainfall, whereas the under-canopy DP was only 18.7–65.0 mm, accounting for 6.4–18.4% of the rainfall during the corresponding period. Structural equation modeling indicated that, on a daily scale, soil moisture had the greatest effect on DP, followed by precipitation and soil saturated hydraulic conductivity. Other climate factors (global radiation, air temperature, vapor pressure deficit and wind speed) and vegetation structure parameters (leaf area index and rooting depth) affected ET, which had a significantly negative relationship with DP. These results help explain the effects of plantation strategy on the water budget, with important implications for artificial forest management, ecological restoration, and protection of local water resources in water-limited environments.