Objective This study investigates the spatiotemporal response characteristics and regulatory mechanisms of soil moisture in Populus tomentosa plantations under different thinning intensities and water gradient controls. It aims to elucidate the impact of management interventions on the water utilization and retention capacity of plantations, with the goal of providing scientific basis for precise water regulation, optimization of silvicultural measures, and continuous improvement of forest quality in artificial forests across the North China Plain.

Method The research was conducted in an 8-year-old triploid P. tomentosa S86 plantation. Two thinning intensities were established: alternate row thinning with 50% thinning (T50), and alternate row and alternate tree thinning with 75% thinning (T75). In the T75 experimental area, two treatments were set up: full irrigation (T75W20) and no irrigation (T75). In the T50 experimental area, three treatments were arranged: full irrigation (T50W20), deficit irrigation (T50W45), and no irrigation (T50). The soil volumetric water content (SVWC) was continuously monitored during the growing season, and soil water storage in the 0 – 600 cm profile was determined using the oven-drying method at the end of the season to assess the temporal and spatial variations in soil moisture and the formation of dry soil layers.

Result (1) Soil moisture in the P. tomentosa plantation showed distinct vertical and temporal variation patterns. The surface layer (0 – 30 cm) exhibited monthly fluctuations, with minimum SVWC in July and maximum in August. Soil moisture increased stepwise with depth, with pronounced variation in the 30 – 100 cm layer and relative stability below 100 cm. (2) Thinning intensity significantly influenced moisture distribution in the root zone (25 cm from trees). A 75% thinning rate altered the two-dimensional spatial distribution of soil moisture in the root zone, while a 50% thinning rate primarily enhanced soil moisture in the middle layer (30 – 100 cm). In some seasons, the root zone exhibited relatively lower water content. (3) Irrigation significantly improved surface soil moisture. The T50W20 treatment in the root zone increased the average soil moisture by 28.78% compared to T50, while the soil volumetric water content in the 10 – 70 cm root zone under the T50W45 treatment remained relatively low. Under the T50W20 treatment, SVWC in the root zone increased by 28.78% compared with the control, while the T50W45 treatment enhanced water depletion within the 10 – 70 cm layer. Without irrigation, the deep soil (100 – 600 cm) showed higher moisture in the non-root zone (100 cm from trees). (4) From 2022 to 2023, soil water storage decreased overall, but increased thinning intensity markedly enhanced water storage, especially in the 100 – 200 cm layer. The T75 treatment increased total soil water storage by 53.00% compared with T50. The T75W20 combination further improved multi-layer moisture conditions and promoted recovery in the 500 – 600 cm layer. (5) Dry soil layers were mainly distributed within 0 – 300 cm. Under 50% thinning, the dry layer reached 300 cm, whereas under 75% thinning it was reduced to 200 cm. Thinning significantly alleviated the dry soil layer, while irrigation alone failed to eliminate dryness within 0 – 200 cm.

Conclusion Soil moisture dynamics in P. tomentosa plantations exhibit significant spatiotemporal heterogeneity. Both thinning and irrigation effectively regulate vertical soil moisture distribution, and an appropriately increased thinning intensity can mitigate the development of deep dry soil layers in the North China Plain. The research results provide a quantitative basis for precision tending of plantations based on water constraints, offering important reference value for constructing site-specific thinning-water synergistic regulation models and enhancing the ecological stability and management sustainability of plantations.