Grassland change in west Northeast China plain from SPOT- VEGETATION time series data analysis

摘要

The grassland ecosystems in west Northeast China plain are sensitive indicators of environmental impacts from both climate change and direct human activities. NDVI imagery over each ten-day period in the past ten years was calculated based on the VGT sensor. The Maximum Value Composites method, annual average value, monthly average value and difference value of two indices were used to analyze the inter-annual changes of vegetation cover. Linear trends could be detected in the NDVI time series, that differ by season. We pointed that grassland cover in this area decreased with an obvious undulating trend during 1998-2007. The yearly maximum NDVI mean over the study area decreased from 0.644 in 1998 to 0.615 in 2007 indicated the decreasing trend of grassland activity. There was a significant decrease of monthly maximum NDVI in spring and early summer months, while positive NDVI trends were described for autumn. The degraded vegetation area occupied about 20.29% of the whole study area, while vegetation in some area was increase taking up 16.03%. Precipitation and temperature had different effect on yearly maximum NDVI of grassland in this semi-arid region. Grassland degradation may be explained by the duration and severity of drought.