Net primary production increases in the Yangtze River Basin within the latest two decades

Abstract Human-induced land use change and climate change are two interacting factors that affect vegetation productivity. In the Yangtze River Basin (YRB), reforestation projects have been implemented in the last few decades while urbanization is rapid because of the Yangtze River economic belt. However, few studies have explored the roles of human activities and climate change on vegetation carbon change within the four reaches of the YRB, individually. In this study, we used a time series of net primary productivity (NPP), an indicator of the capacity of vegetation to sequester carbon, to analyze spatial and temporal patterns of vegetation carbon storage in the YRB. We conclude that overall, vegetation carbon has increased 0.75 g C m-2 yr-1 in the basin during this period with the most rapid change rate in the upper reach (1.76 g C m-2 yr-1). 88.70% of the basin had non-significant changes, while areas of significant decreases in vegetation carbon were near urban areas. Overall, human activities are the main reason for the significant decreases and increases in NPP during 2000-2014. However, the impacts of human land-use management and climate differ across the reaches. Climate change is the major driver for vegetation carbon increases in the headwater where 72.56% of the area shows significantly positive effects. Climate-induced effects are weaker from the upper reach to the lower reach. Human activities mainly play a positive role in vegetation carbon increases in the headwater (∼23.56%) and upper reach (∼16.04%) while the negative effects of human activities are more significant in the lower reach (∼7.48%). During 2000-2014, grassland accumulated the most carbon (∼ 0.12 Mg C) in the headwater, while forests sequestered the most carbon in the upper, middle and lower reaches, with increases of 7.10 Mg C, 5.14 Mg C and 0.80 Mg C, respectively. Forests, including newly planted forests, are the main sources for increased vegetation carbon by land use in both changed and unchanged areas. Thus, forest protection and reforestation are important activities to mitigate rapid urbanization in the basin. Our findings can improve forest management and river basin management in the various reaches of the YRB.