Quantitative analysis of the contributions of climatic and human factors to grassland productivity in northern China

An accurate quantitative analysis of the contributions of climate change (CC) and human activities (HA) to grassland productivity is crucial for elucidating the relevant driving mechanisms. In this study, grassland in northern China was analyzed. We chose the net primary productivity (NPP) as an evaluation indicator of grassland productivity and identified the relative roles of climatic and human factors in NPP changes. A quantitative method based on partial derivatives was used for evaluating the contributions of climatic factors to NPP changes, and the difference between the inter-annual variation rate of NPP and climatic factors contribution was considered as the HA contribution. Then, different scenarios were designed to evaluate the relative contribution proportions of CC and HA to grassland restoration and degradation. The results revealed that the average grassland NPP over northern China showed a significant increasing trend at a rate of 1.66 g Cm-2 year(-1) from 2000 to 2015. The contributions of temperature, precipitation, and solar radiation to grassland NPP changes were 0.06, 0.50, and 0.52 g Cm-2 year(-1), respectively. Solar radiation made the greatest positive contribution among all of the climatic factors, followed by precipitation. The contributions of CC and HA to grassland NPP changes were 1.08 and 0.58 g Cm-2 year(-1), respectively. Moreover, the role of HA in both grassland restoration and degradation was larger than that of CC. Overall, the positive effect of HA on grassland productivity may be greatly offset by its negative effect, and therefore the positive contribution of HA to grassland NPP changes was less than that of CC. Thus, the effective measures and policies used to control grassland degradation should be further strengthened to protect grassland resources.