Afforestation and the impacts on soil and water conservation at decadal and regional scales in Northwest China

• Artificial forest has taken up 13% of the total forest area in Northwest China.
• Shrub was superior for reducing runoff and sediment than forest and grass.
• Increases of forest coverage did not necessarily reduce runoff at regional scale.
• Ecological water requirement of restored vegetation might lead to water shortage.

Massive afforestation has been conducted in dryland regions of Northwest China since 1978. With the impending effects of climate change, it is necessary to reconsider the effects of artificial vegetation on soil and water conservation at decadal and regional scales. Using long-term official and synthesized data, the vegetation's impacts on reducing water loss and their ecological water requirement were studied in four provinces (e.g., Inner Mongolia, Gansu, Qinghai, and Xinjiang). Results showed that vegetation of the four provinces was dominated by grass, while artificial forest had taken up 13% of the total forest area. At the plot scale, vegetation could reduce runoff and sediment by 44% and 83%, respectively. At the regional scale, soil erosion areas showed a decreasing trend, especially after the year 2000. In Inner Mongolia and Gansu, both runoff coefficients and water resource amounts showed decreasing trends. As such, future large-scale afforestation might be ecologically unsustainable in these two provinces. However, the runoff coefficients and water resource amounts of Qinghai and Xinjiang showed increases, mainly linked to climate change. This study helps elucidate the paradox of vegetation restoration in arid regions, and gives some suggestions for ecological restoration in other drylands of the world.