Relative effects of climate variation and human activities on grassland dynamics in Africa from 2000 to 2015

Grassland plays a key role in socioeconomic development and environmental protection in Africa. Climate variation and human activities are two main drivers of grassland dynamics. Quantitatively assessing the contributions of these two factors and understanding the driving mechanisms are important in ecosystem adaptation and management. In this research, the spatiotemporal patterns of grassland dynamics in Africa during 2000-2015 were analyzed based on the net primary productivity (NPP), M-K test, linear regression analysis, and correlation analysis. In addition, the potential NPP (PNPP), human-induced NPP (HNPP) and actual NPP (ANPP) were employed to establish scenarios to distinguish the relative impacts of climatic and human factors on grassland dynamics. An overall grassland ANPP increase than decrease (62.91% vs 37.09%) was found during 2000-2015. 21.80% of the total grassland area showed increases in ANPP, which was influenced by climate variation, whereas 23.61% were affected by human activities. The ANPP decreases induced by climate variation, human activities and the combination of these two factors occupied 19.31%, 8.39% and 9.39% of the total grassland area, respectively. Therefore, the contributions of climatic and human factors on ANPP increase were almost consistent, while climate variation was the dominated factor on ANPP decrease. In addition, the respective roles of these two factors were quite different in five grassland types. The dynamics of ANPP in closed shrublands, non-woody grasslands, and open shrublands were mainly attributed to the climate variation. Meanwhile, the human-dominated increases in ANPP were observed in woody savannas. Further analysis demonstrated that the increases in African grassland ANPP are likely due to the mitigation of drought and reduction in land use intensity, while the decreases in ANPP were related to unbalance of local hydrothermal condition and overgrazing. This study expects to improve the understanding of the respective contributions of climatic and human factors on grassland dynamics in Africa.