Spatio-temporal pattern of NPP and related analyses with terrain factors in Wuling mountainous area

Based on the MODIS NPP data, terrain data, and land cover map, spatio-temporal pattern of NPP in Wuling mountainous area during 2001-2010 and its relationships with the elevation and slope were analyzed using regression analysis and classification statistics. Results showed that the average annual NPP of the study area from 2001 to 2010 was 590.72 g C m(-2) yr(-1). The mean NPP of forest, shrub/grassland, and cropland were 596.79 g C m(-2) yr(-1), 586.98 g C m-2 yr(-1), and 563.31 g C m(-2) yr(-1), respectively. During 2001-2010, the average annual total NPP of Wuling mountainous area was 98.90 T g C yr(-1), ranging from 92.79 T g C yr(-1) to 106.99 T g C yr(-1). Besides, the spatial pattern of interannual variability of NPP in the north of our study area presented a significant increase trend while in the south it displayed an opposite tendency. According to the relationships between mean NPP and elevation as well as slope at steps of 30m and 3 degrees, respectively, NPP increased with the altitude and slope first, then decreased slowly, but when the elevation above 1500m or the slope greater than 50 degrees, the mean NPP presented large fluctuations. However, on the whole, mean NPP increased with the altitude and slope first, then decreased again. Additionally, mean NPP within elevation range of 200m-1000m and slope range of 5 degrees-25 degrees were relatively high, but it decreased one after another in the zones above 500m and had a trend of increase when the slope zones greater than 50 degrees, which reflected the erosion intensity was weakened when the slope greater than a certain threshold.