Integrated trade-offs/synergies and interactions approach to quantifying the direct and indirect effects of environmental factors on ecosystem services.

The driving mechanisms of ecosystem services (ESs) involve two aspects: the effects of environmental factors (e.g., precipitation and slope) on ESs and the effects of trade-offs/synergies on ESs. Clarifying the complex causal relationships between environmental factors and ESs is essential for decision-makers to formulate ES management. However, existing studies have focused more on identifying the main drivers of ESs without adequately exploring the direct and indirect effects of environmental factors on ESs, especially those based on the interactions between environmental factors and trade-offs/synergies on ESs. In this study, we proposed an integrated approach of trade-offs/synergies and interactions to quantify the direct and indirect effects of environmental factors on ESs by differentiating between the effects of trade-offs/synergies on ESs and the effects of environmental factors on ESs. Three typical ESs, net primary productivity (NPP), soil conservation (SC), and water yield (WY), were estimated in Sichuan Province from 2000-2020. The trade-offs/synergies between ES pairs were subsequently explored using correlation analysis and the geographically weighted regression (GWR) model. The interactions between environmental factors and ESs were verified and separated utilizing the Geodetector model and partial correlation analysis. Finally, the direct and indirect effects of environmental factors on ESs were measured through the bootstrap method. The results revealed that (1) from 2000-2020, three ESs exhibited significant spatial heterogeneity in Sichuan Province. (2) Complex trade-offs and synergies among these ESs were apparent at the provincial scale, characterized by distinct spatial heterogeneity. (3) DEM, temperature, precipitation, and relative humidity were the dominant factors affecting the spatial heterogeneity of ESs. Notably, the interactions involving environmental factors and ESs demonstrated more robust explanatory power for ESs and their trade-offs/synergies than individual drivers did. (4) DEM and temperature had significant direct and indirect effects on ESs when NPP and WY served as the mediating variables, and these mediating variables contributed significantly to the total effect. The integrated trade-offs/synergies and interactions approach deepens our understanding of ES mechanisms and provides a theoretical basis and reference for decision-making, rather than blindly pursuing the maximization of a particular service at the expense of others.