Diverse chronic responses of vegetation aboveground net primary productivity to climatic changes on Three-River Headwaters region

The ecological responses of vegetation to climatic variables along a spatial gradient can be used as guidelines for chronic ecosystem response predictions under global climate change. The chronic responses of aboveground net primary productivity (ANPP) to climatic factors on the Three-river Headwaters (TRH) region were assessed for major vegetation types at the herbage developmental stages using the geographical detectors model, the Normalized Difference Vegetation Index from remote sensing images, and precipitation and air temperature datasets from 2000 to 2019. The results indicated that precipitation was the dominant driver for alpine meadows and steppes, while the impact of temperature on alpine steppes was more evident than that in alpine meadows, and alpine shrubs were more responsive to warming changes. This was possibly because shallower root systems and larger plant leaf area promote the responses of ANPP to precipitation for alpine meadows and steppes, and dormant buds higher from the ground facilitate the impacts of temperature for alpine shrubs. Grasslands also demonstrated differentiated responses to climatic factors at different developmental stages. Precipitation during the previous withering (September-October), pre-growing season (March-April), and vegetative growth (May-June) stages were more influential for ANPP variations. In contrast, ANPP during the reproductive period from July to August exhibited a weaker response to climatic controls. This was possibly attributed to the bud bank replenishment in fall and spring, the largest biomass growth rates in vegetative growth stage, and the non-structural carbohydrates transferred from mots in reproductive stage for perennial herbaceous plants. An elevated temperature, combined with no significant change in precipitation on the TRH may lead to a decrease in ANPP in alpine meadows but alpine shrub expansion, resulting in a reduced supply of palatable forage sources and bringing challenges to grassland management.