Effects of age and density of Pinus sylvestris var. mongolica on soil moisture in the semiarid Mu Us Dunefield, northern China

Although the planting and the desertification control of Pinus sylvestris var. mongolica have long been explored, effect of the plantation density is still unclear, on the southwestern edge of the Mu Us Dunefield in northern China. This study investigated the soil moisture at five mineral soil depth from 2017 to 2018, and analyzed the effects of the trees on the soil moisture balance in plantations with different densities and ages. We found that the plantations changed water movement and storage in the aeolian sandy soil. Both stand age and density significantly affected the soil water content, but the effect of plantation age was greater than that of density. The soil water content decreased with increasing age, the thickness of the surface dry layer increased, and soil drying increased significantly. In the 4-year-old forests, soil moisture was adequate, and seasonal rainfall could partially compensate for the soil water deficit. In the 9-year-old forests, the water deficit was a serious concern at high tree densities, and seasonal rainfall did not compensate completely for the soil water deficit at densities greater than 400 trees ha(-1), so the soil remained relatively dry at the end of the rainy season, even after more than 640 mm of rainfall. The 15- and 30-year-old forests experienced serious drought due to their drying effect on the soil. Overall, the trees promoted soil water loss, thereby creating a serious imbalance between the water supply and demand in this desert environment. High-density planting accelerated the deterioration of the water environment (i.e., soil drying) and threatened the future survival of the trees and other plants. Thus, ecological managers must reduce tree planting and test the effectiveness of reducing the density to 333 trees ha(-1) during the young stage.