Linking landscape dynamics to the relationship between water purification and soil retention

Abstract

Landscape pattern is vital for supplying ecosystem services, and demonstrating the interactions among landscape dynamics and multiple ecosystem services is a key scientific basis for ecosystem management. As a typical ecologically vulnerable region in China, the Three Gorges Reservoir Area (TGRA) suffers from both severe soil loss and non-point pollution. Although many ecological restoration projects have been implemented to control sediment and pollutants into the reservoir, it is still unclear whether and how reforestation improves both soil retention and water quality. Therefore, we proposed a conceptual framework to couple landscape dynamics and multiple ecosystem services, in which drivers could directly affect two ecosystem services or indirectly affect them by affecting landscape dynamics. Ecological models were applied to assess water purification and soil retention, and spatial analysis tools were adopted to demonstrate their spatial relationship. Structural equation model documented the effect values of the conceptual framework, meanwhile, relative importance analysis reported the contributions of drivers to forest cover, water purification and soil retention. Our results showed that nitrogen loss and soil loss of the TGRA both decreased from 2001 to 2015, indicating an improvement in both water purification and soil retention. However, correlation analysis revealed significant spatial heterogeneity in their relationship, which may partly be explained by the district and county differences in human activities and development policies. Structural equation model documented the correlation coefficient between nitrogen loss and soil retention was −0.71. Relative importance of forest cover to the nitrogen loss was more than 50%, indicating that water purification could mainly be explained by the effect of forest cover on nitrogen loss. Annual precipitation contributed 26.9% to soil retention, and the overall contribution of climate conditions was 52.1%, which indicated the direct and indirect effects of climate conditions are both important for soil retention. Moreover, the landscape plannings of vegetation restoration were suggested for these key ecological zones in the TGRA to synergistically improve two ecosystem services.