What is the relationship between ecosystem services and urbanization? A case study of the mountainous areas in Southwest China

China is undergoing a rapid urbanization process, and urbanization will have a direct impact on regional ecosystems and affect regional ecosystem services. Considering the mountainous counties in Southwestern China as the research object, this study reveals the spatial clustering characteristics of four typical ecosystem services (food production, soil conservation, water yield and carbon sequestration) as well as the trade-offs and synergies among ecosystem services in different urbanized areas. At the same time, piecewise linear regression is used to determine the threshold of the influence of urbanization on ecosystem services. The results indicate that: 1) There are spatial autocorrelations among the four typical ecosystem services; with strong clustering characteristics, the positive correlation types are "clustered" locally; and with significant spatial heterogeneity, the negative correlation types are scattered and mainly appear in the highly urbanized area. 2) There are also remarkable differences in the relationship among various ecosystem services in different urbanized areas, and in particular, there are marked trade-offs between food production and carbon sequestration in the moderately urbanized area and the highly urbanized area. However, there are synergies between them in the lowly urbanized area. 3) With an increase in the compounded night light index (CNLI), water yield, carbon sequestration, food production and overall ecosystem services values present an increasing-decreasing trend, the soil conservation function value shows a decreasing-increasing trend. The response of water yield, carbon sequestration, food production, and overall ecosystem services to the compounded night light index (CNLI) has a threshold of 1.2642, 1.4833, 1.3388, 1.5146 and 1.2237, respectively. Based on the detected relationships between urbanization and ecosystem services, this study provides a theoretical reference for the selection of urbanization development models in key ecological functional areas.