Trade-offs and synergies of ecosystem services and their threshold effects in the largest tableland of the Loess Plateau

Trade-offs and synergies in ecosystem services (ESs) are subject to considerable uncertainty due to different spatial and temporal scales. Smooth changes in ESs at different scales may cause discontinuous changes in trade-offs and synergies, i.e., the threshold effect. Therefore, we attempted to explore thresholds of trade-offs and synergies with changes in ecosystem services. Herein, remote sensing data (30 m), observations, and survey data from 1990 to 2020 were used in our study. ESs such as soil conservation, water yield, net primary production (NPP), and crop production were simulated based on ecological and geostatistical methods. The spatio-temporal trade-offs and synergies of ESs were quantified based on the Spearman correlation analysis and spatial statistics. Finally, the threshold values of ecosystem services were extracted by using the regression discontinuity method. As shown by our results, ESs showed an increasing trend after a short period of fluctuation except for crop production. Soil conservation had the largest increase, followed by NPP. Synergies were the main relationships of ESs and increased with spatio-temporal scales. More than 65 % of the synergies have a threshold effect with the increase of ESs. Specifically, there was a significant increase in synergetic relationships among ESs when the soil conservation exceeded 260 t ha-1, the water yield was less than 155 t ha-1, NPP exceeded 315 g C m-2, and the crop production exceeded 210 kg ha-1. Conversely, synergies decreased significantly when NPP was less than 230 g C m-2, and crop production was less than 190 kg ha-1. We can attempt to contain ESs within certain thresholds and promote synergies. Then, creating sustainable ESs and achieving a virtuous cycle of progress toward creating sustainable development goals.