Forest ecosystem is a key component of the terrestrial ecosystems playing a vital role in the global carbon cycle and maintaining climate stability. Human activities and climatic change are two major factors shaping the NPP. However, they are always tangled. And yet, a more accurate quantitative analysis is lack to disentangle them, which already hindered the more targeted management of forest lands. In this study, we developed a quantitative method to disentangle the relative roles of climatic factors and human activities in explaining net primary productivity at different forest successional stages in the Yangtze River Basin. A working hypothesis is climatic factor would be a major in shaping the NPP. Firstly, the forests are divided into two categories on the basis of the existence of forest time using the two-phase forest images: forests appeared before the study period (F_unchanged), and forests appeared during the research period (F_changed). Secondly, a quantitative method based on partial derivatives was developed to disentangling the relative contributions of climatic factors and human activities to the NPP of F_old and F_changed. Then, different scenarios were simulated to evaluate the relative contribution proportions of climate and human activities to forest restoration and degradation. Our results revealed that, from 2000 to 2019, F_unchanged and F_changed of NPP over YRB showed a significant increment trend at a rate of 3.28 g C m^-2 y^-1 and 3.77 g C m^-2 y^-1, respectively. Precipitation and solar radiation were the two major positive contributors among all of the climatic factors, explaining 62.82%, and 62.41% of NPP variables, respectively. We found that, in the Funchanged, climate change and human activities contributed 9.77% and 28.33%, respectively, in explaining NPP, indicating that human activity rather than climatic factor is a major driver in shaping the NPP. This finding unsupported our initial hypothesis. Furthermore, we found that human activities dominate either restoration or degradation of the old forest. This result may due to the attenuated human disturbances and strengthened forest managements, such as ecological polies, forest tending, closing the land for reforestation, etc.. Therefore, on the basis of disentangling the two types of factors, we concluded that human activities govern the forest succession, and imply that the environment-friendly forest managements may favorite to improving the forest NPP against the impacts of climatic change. Thus, the effective measures and polices is suggested implement in controlling forest degradation in facing the climate change.

climate change, human activities, the old forest, NPP