Forestation as part of the Returning Farmland to Forest Project was implemented to mitigate soil erosion in southwestern China. However, whether forestation has effectively reduced soil erosion in southwestern China remains unclear, mostly because of the lack of monitoring forest cover change and soil erosion at watershed scales. We interpreted forest cover change from satellite images and simulated soil erosion changes for the period of 1986–2018 in the Chong’an River Basin with the Water and Tillage Erosion Model and Sediment Delivery Model. Our results show that the change in forest cover has the highest correlation coefficient with the sediment yield in the watershed, with an obvious inverse phase relationship between them for all the simulated years. From 2002 to 2014, large-scale forestation and frequent droughts caused the forest cover to vary, resulting in significant changes in the annual soil erosion amount. Because crevices favoring tree growth are more developed in limestone than in dolomite, the forest cover reduction on dolomite is significantly higher than that on limestone under severe droughts in karst areas. Our study implied that the function of forestation in preventing soil erosion depends on lithology in karst areas.

Climate change poses a considerable threat to the forest-steppe ecotone in arid mountain areas. However, it remains unclear how the forest-steppe ecotone responds to climate change due to the limitation of the traditional pollen assemblages, which greatly limits the understanding of the history of the forest-steppe ecotone. Here, we examined the Tien Mountains, the largest mountain system in the world’s arid regions, as a case study to explore the migration of the forest-steppe ecotone using the pollen taxa diversity, by combining modern vegetation surveys, surface pollen and two fossil pollen sequences—in the mid-elevation forest belt (Sayram Lake) and in the low-elevation desert belt (Aibi Lake). We found that the forest-steppe migration followed Holocene climate change. Specifically, the forest belt where Picea schrenkiana Fisch. & C.A.Mey. dominates has a very low pollen taxa diversity, characterized by high richness and low evenness, which plays a key role in mountainous diversity. By detecting the diversity change of the deposition sites, we found that in coping with the warm and wet middle Holocene, the forest belt expanded and widened as the observed diversities around the two lakes were very low, thus the forest-steppe ecotone moved downward accordingly. During the early and late Holocene, the forest belt and the forest-steppe ecotone moved upward under a warm and dry climate, and downward under a cold and wet climate, as there was a reduced forest belt effect on, or contribution to, the sites, and the observed diversities were high. Moisture loss may pose the greatest threat to the narrow forest-steppe ecotone, considering the climatic niche space and the limited living space for humidity-sensitive taxa. This study highlights that temperature and moisture co-influence the forest belt change, which further determines the position migration of the forest-steppe ecotone.