NPP plays a constraining role on water-related ecosystem services in an alpine ecosystem of Qinghai, China

Understanding the constraint effects of ecosystem services, which can indicate thresholds and other key features, is helpful to formulate reasonable measures of ecosystem service protection. In Qinghai Plateau, exploring threshold effects and scale effects of net primary production (NPP) on multiply water-related ecosystem services and the associated influencing factors are crucial to maintain its status of "Asian Water Towers". In this study, NPP and the multiply water-related ecosystem services, including surface runoff, soil water, and subsurface runoff were evaluated by Carnegie-Ames-Stanford Approach and Community Land Model from 2000 to 2018. The constraint effects between NPP and each water-related ecosystem service were analyzed by a segmented quantile regression approach at the scales of region and functional zones. The thresholds, slopes, and constant terms of the regression equations were used to characterize the key features of the constraint lines. Finally, we analyzed the environmental factors affecting the key features of constraint effects. The results showed that the constraint lines of all paired ecosystem services were hump-shaped curves, negative lines, inverted S curves, or logarithmic curves. The shapes of the constraint lines of NPP and soil water varied greatly with scale. The soil, terrain, and other ecological contexts may shape the constraint lines, and the climate factors and vegetation coverage determined the values of the key features. The NPP thresholds generally range from 400 gC m(-2) to 800 gC m(-2) on the hump-shaped constraint lines, which indicates that too high vegetation coverage should not be pursued in the alpine ecosystem of Qinghai, but the ranges of NPP values where water-related ecosystem services decrease with increasing NPP could be narrowed with the management of surface vegetation. The slopes and constant terms on the negative constraint lines were dominated by climate factors and not easily changed by surface vegetation factors. Appropriate adjustment of surface vegetation in response to climate change could weaken the constraint effect of NPP on the water-related ecosystem services and achieve synergy of multiple ecosystem services, thereby protecting water-related ecosystem services and promoting sustainability in the alpine ecosystem.