Frequency distributions of standardized MODIS EVI and VIP EVI2 June–July–August ((a)–(d)) anomalies in the desert biome (2000–2010) for Inner Mongolia (IM) and Outer Mongolia (OM)
Figure 4. Frequency distributions of standardized MODIS EVI and VIP EVI2 June–July–August ((a)–(d)) anomalies in the desert biome (2000–2010) for Inner Mongolia (IM) and Outer Mongolia (OM). Positively skewed drought years (2000–2001, 2005, 2009) are characterized by the majority of negative anomalies with peak values between −1.5 and −0.5 std. and are statistically different (p < 0.001) from relatively wet years (2003, 2004, 2007). VIP EVI2 is based on a longer-term mean (1981–2010) than MODIS EVI.
Climate change has led to more frequent extreme winters (aka, dzud) and summer droughts on the Mongolian Plateau during the last decade. Among these events, the 2000–2002 combined summer drought–dzud and 2010 dzud were the most severe on vegetation. We examined the vegetation response to these extremes through the past decade across the Mongolian Plateau as compared to decadal means. We first assessed the severity and extent of drought using the Tropical Rainfall Measuring Mission (TRMM) precipitation data and the Palmer drought severity index (PDSI). We then examined the effects of drought by mapping anomalies in vegetation indices (EVI, EVI2) and land surface temperature derived from MODIS and AVHRR for the period of 2000–2010. We found that the standardized anomalies of vegetation indices exhibited positively skewed frequency distributions in dry years, which were more common for the desert biome than for grasslands. For the desert biome, the dry years (2000–2001, 2005 and 2009) were characterized by negative anomalies with peak values between −1.5 and −0.5 and were statistically different (P < 0.001) from relatively wet years (2003, 2004 and 2007). Conversely, the frequency distributions of the dry years were not statistically different (p < 0.001) from those of the relatively wet years for the grassland biome, showing that they were less responsive to drought and more resilient than the desert biome. We found that the desert biome is more vulnerable to drought than the grassland biome. Spatially averaged EVI was strongly correlated with the proportion of land area affected by drought (PDSI <− 1) in Inner Mongolia (IM) and Outer Mongolia (OM), showing that droughts substantially reduced vegetation activity. The correlation was stronger for the desert biome (R2 = 65 and 60, p < 0.05) than for the IM grassland biome (R2 = 53, p < 0.05). Our results showed significant differences in the responses to extreme climatic events (summer drought and dzud) between the desert and grassland biomes on the Plateau.