Spatiotemporal Patterns of Soil Moisture Drought Across Different Soil Layers and Their Relationships with Ecosystem Water Use Efficiency and Resilience in the Three-North Shelterbelt Forest Program Region

Ecosystem water use efficiency (WUE) is a key indicator of carbon–water coupling in ecosystems under climate change. While WUE responses to meteorological droughts are well-documented, the influence of different soil layers on WUE across vegetation types in water-limited regions remains unclear. Based on ERA5-Land soil moisture and MODIS vegetation products, this study investigates the spatiotemporal variations in soil moisture indices (SSMI) across three soil layers and their synchronous, lagged, and cumulative associations with WUE and resilience in the Three-North Shelterbelt Forest Program (TNSFP) region from 2001 to 2022. Our results showed that while the shallow and middle layers exhibited general wetting trends, the deep soil layer (100–289 cm) underwent continuous depletion in 56.01% of the study area. WUE showed weak synchronous responses to soil moisture drought across all layers, with no significant threshold effect. However, the lag and cumulative associations increased with soil depth, from 3–6 months in shallow layers to 9–12 months in deep layers, suggesting that deep-layer soil moisture may reflect longer-term ecohydrological stress and delayed ecosystem responses. Ecological resilience (Rd) differed by vegetation type. Grasslands were resilient to shallow drought but vulnerable to deep moisture deficits, while forests maintained stability under deep stress. These findings emphasize the importance of aligning vegetation configuration with soil water availability in restoration efforts, providing critical insights for sustainable water resource management and forest rehabilitation.