NDVI-indicated long-term vegetation dynamics in Mongolia and their response to climate change at biome scale

Based on the vegetation map of Mongolia, Global Inventory Monitoring and Modelling Studies (GIMMS) normalized difference vegetation index (NDVI) (1982-2006), the Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI (2000-2010), and temperature and precipitation data derived from 60 meteorological stations, this study has thoroughly examined vegetation dynamics in Mongolia and their responses to regional climate change at biome scale. To ensure continuity and consistency between the two NDVI datasets, the MODIS NDVI was first calibrated to the GIMMS NDVI based on the overlapping period of 2000-2006. Good calibration results with R-2 values of 0.86-0.98 between the two NDVI datasets were obtained and can detect subtle trends in the long-term vegetation dynamics of Mongolia. The results indicated that for various biomes, although NDVI changes during 1982-2010 showed great variation, vegetation greening for all biomes in Mongolia seem to have stalled or even decreased since 1991-1994, particularly for meadow steppe (0.0015 year(-1)), typical steppe (-0.0010 year(-1)), and desert steppe (-0.0008 year(-1)), which is an apparent turning point (TP) of the vegetation growth trend in Mongolia. A pronounced drying trend (from -4.399 mm year(-1) in meadow steppe since 1990 to -2.445 mm year(-1) in alpine steppe since 1993) occurred between 1990 and 1994, and persistently warming temperatures (0.015 degrees C year(-1) in alpine steppe to 0.070 degrees C year(-1) in forest and meadow steppe) until recently have likely played a major role in this NDVI trend reversal. However, the NDVI TP varied by biome, month, and climate and was not coupled exactly with climatic variables. The impact on climate of both same-time and lagged-time temperature and precipitation effects also varied strongly across biomes and months. On the whole, climate-related vegetation decline and associated potential desertification trends will likely be among the major sources of ecological pressure for each biome in Mongolia, which could intensify environmental problems like sandstorms in other East Asian regions.