Effects of topography and human activity on the net primary productivity (NPP) of alpine grassland in northern Tibet from 1981 to 2004

Using the Carnegie-Ames-Stanford Approach (CASA) model on remote-sensing (RS), climatic, and other related data from 1981 to 2004, the researchers estimated the net primary productivity (NPP) of alpine grassland in northern Tibet. Geographical information system (GIS) techniques were used to analyse the spatial pattern of change in the NPP of alpine grassland and its response to the intensity of human activity. The researchers found that the mean values of NPP on flat (slope gradients <1 degrees) and sunny slopes were relatively lower. Between 1981 and 2004, the NPP of alpine grassland in northern Tibet tended to decrease, but with relatively large annual fluctuations. In northern Tibet, the alpine grassland NPP for high-elevation regions has a greater proportion of area (over 26%) showing a decreasing trend. The change is more significant in areas where the slope is 15-30 degrees and aspect has little influence on the extent of the change. The negative effects of local residential areas on the rate of change of alpine grassland NPP are smaller than those of roads.