[Spatial-temporal evolution pattern and terrain gradient differentiation of ecosystem service value in Zhangye, Northwest China at the grid scale].

Zhangye serves as a good case for studying ecological services because of its fragile ecological environment and its ecological importance. Using land use data from 1987, 2000 and 2016 as well as the modified ecosystem service value (ESV) equivalent, the ESV was estimated and its spatial and temporal changes and gradient differentiation characteristics were analyzed by dividing the studied region into a grid of 3 km×3 km in size. The results showed that from 1987 to 2016, the ESV in Zhangye increased by 564 million Yuan, with the mountainous areas in the south and the desert areas in the north being stable. The ESV of central oasis areas with concentrated human acti-vities significantly changed. Forests contributed to most of the ESVs. Water regulation value was the strongest among all the individual ESVs. The ESV showed strong spatial dependence. Based on the spatial pattern of the two aggregated poles, it shaped three "flower arrangement" clusters, including Ganzhou District, Yanzhi Mountain, and Jingtie Mountain. The ESV distribution showed a topographic differentiation as the value decreased from the Qilian Mountains to the plain oasis, and further to the desert areas. The linear correlation of the terrain and the ESV was negative. As the increasing elevation gradient, the ESV showed a "decreased-increased-decreased" trend, while and it had a "decreased-stable-decreased" in the increasing slope gradient. Overall, the ESV presented two turning zones on the terrain gradient, indicating that both the piedmont zone and the oasis desert transition zone acted as transition zones of the ESV. Areas with the strongest ESV were distributed in the low slope and low terrain relief zones, and the development of oases in flat terrain areas were strongly constrained by water resources.