Spatiotemporal variations of Alxa national public welfare forest net primary productivity in northwest China and the response to climate change

Net primary productivity (NPP) is an important index to evaluate vegetation growth. This study used MODIS remote sensing image data during growing seasons to calculate the normalized difference vegetation index (NDVI) and surface water stress coefficient (W-epsilon) and then utilized Carnegie-Ames-Stanford-Approach (CASA) model to estimate the NPP of Alxa public welfare forest. Furthermore, the impacts of national policy, temperature and precipitation on spatiotemporal variations of NPP were also discussed. The results showed that: (1) Significant changes of NDVI and vegetation NPP of Alxa public welfare forest were found since the beginning of the monitoring in 2010. The NDVI of the forest area increased during 2010-2018 from 0.110 to 0.122. In most areas, NDVI showed a significant or insignificant increasing trend. (2) Obviously spatial differences of the distribution of vegetation NPP in Alxa public welfare forest were detected. The vegetation NPP increased from 50.41 to 58.61 gC center dot m(-2)center dot a(-1) during 2010-2018, with an average NPP of 53.95 gC center dot m(-2)center dot a(-1). (3) National policy was the dominant factor affecting the increase of vegetation NPP of Alxa public welfare forest. In addition, among the meteorological factors investigated, precipitation was the main factor affecting the spatial variation of NPP. There was a positive correlation between precipitation and NPP in 63% of the regions, whereas the spatial correlation between temperature and NPP was generally weak. This study could provide a macroscopic basis for the monitoring of vegetation growth in Alxa public welfare forest in recent 10 years.