Grazing thresholds based on mean Net Primary Productivity (NPP) and NPP trends support region-specific grazing...

Arid and semiarid grasslands support pastoral livelihoods and play an important role in regional ecosystems. However, increasing aridity and sustained grazing pressure pose major challenges to grassland productivity. Existing studies often define grazing thresholds based on productivity (NPP) levels but pay less attention to how thresholds may differ when considering long-term productivity trends. Using Xinjiang grasslands as a case study, we classified grasslands into four grazing-productivity regions based on joint changes in grazing intensity and NPP. We identified grazing thresholds from mean NPP and NPP trends using nonlinear threshold models. We further combined these analyses with field observations to examine the associations of grazing and aridity with soil moisture, soil nutrient status, root nutrient status, belowground biomass, and NPP. Results showed that the grazing increase-productivity increase (GI-PI) region accounted for the largest proportion of Xinjiang grasslands (49.7%), followed by the grazing increase-productivity decrease (GI-PD) region (24.5%), the grazing decrease-productivity increase (GD-PI) region (16.7%), and the grazing decrease-productivity decrease (GD-PD) region (9.1%). Grazing thresholds differed markedly among the four regions. Overall, grazing thresholds identified from NPP trends (0.09-1.35 SU ha^-1) showed different ranges from those derived from mean NPP (0.09-0.19 SU ha^-1), reflecting different aspects of grassland productivity responses. Field analyses further showed that grazing and aridity were associated with NPP both directly and indirectly through soil moisture, soil nutrient status, root nutrient status, and belowground biomass. These results provide ecological context for interpreting the regional productivity responses and threshold patterns identified in this study. Thresholds based on mean NPP and NPP trends therefore have different implications among regions, and their combined use provides a clearer basis for stage-specific and region-specific grazing management in dryland grasslands.