Relationship between paired ecosystem services in the grassland and agro-pastoral transitional zone of China using the constraint line method

Understanding the relationships between ecosystem services is important for promoting ecosystem service management and sustainable development. The relationships between ecosystem services have commonly been characterized as tradeoffs and synergies. Here, we report that a third type of relationship also exists, in which one ecosystem service constrains the other. Selecting the grassland and agro-pastoral transitional zone of North China (GAPTZ) as the study area, we examined the relationships between paired ecosystem services: net primary productivity (NPP), soil conservation (SC), soil erosion by wind (SL), water yield (WY), and water retention (WR). The constraint effect of one ecosystem service on the other was determined by extracting the upper constraint lines from the scatter plots of the paired ecosystem services with segmented quantile regression on the levels of landscape, class, and ecoregion. Our results revealed eight types of constraint effects between the ten paired ecosystem services: (1) positive linear, (2) negative linear, (3) logarithmic, (4) negative convex, (5) backward S-shaped, (6) hump-shaped, (7) convex-waved, and (8) concave-waved. At the landscape, class, and ecoregion levels, there was a hump-shaped constraint effect between NPP and SC. Precipitation was the main factor shaping the constraint line of the paired NPP-SC. The gradually increasing constraint effect of higher NPP on WY indicated that, in arid and semiarid areas, improving NPP decreases water yield. In farmland areas, the backward S-shaped constraint line of the paired NPP-SL indicates that crops, unlike forests and grasslands, could not protect soil from wind erosion. The constraint effects of SL on WY and WR are negative convex on the landscape level and convex-waved or concave-waved on the class and ecoregion levels. The constraint line approach enriches the understanding of linkages between ecosystem services and the potential drivers. The constraint effects of ecosystem services have important implications for sustainable land use planning to optimize landscapes services.