NDVI-based monitoring of long-term vegetation dynamics and responses to multi-time scales droughts in Inner Mongolia

The characteristics of vegetation and drought for different seasons
between 1982 and 2015 in Inner Mongolia were studied based on the
Normalized Difference Vegetation Index (NDVI) and the Standardized
Precipitation Evapotranspiration Index (SPEI). The response of vegetation
to drought over various time scales for different seasons and vegetation
types was investigated using the maximum Pearson correlation, allowing a
discussion about the possible causes of any changes. The results indicate
that the vegetation NDVI in Inner Mongolia showed an increasing trend in
different seasons, with spring vegetation NDVI (April to May) having the
largest significant increasing rate, followed by the growing season (April
to October), autumn (September to October), and summer (June to August).
Accordingly, the proportion of stations with decreasing SPEI was, in
descending order, summer, growing season, autumn, and spring.
Additionally, the magnitude of the SPEI decrease was greater in eastern
Inner Mongolia. NDVI and SPEI were positively correlated in most regions
of Inner Mongolia, indicating that changes in vegetation in most parts of
this region were affected by the spatial and temporal characteristics of
drought, the correlation being them being strongest in the growing season,
followed by summer, then spring and autumn. Considering the different
types of vegetation, forests were less affected by drought, with broadleaf
forests more affected than coniferous forests. The meadow steppes and
typical steppes were more affected by 12-month droughts in the growing
season and summer, 6-month droughts in spring, and 3-month droughts in
autumn, with desert steppes mainly affected by 3-month droughts. The
shrubs, sandy vegetation, and cropland were mostly affected by droughts in
summer, and show a greater response to 3-month droughts in autumn.
Finally, the water balance was found to be the most important factor
affecting the response of vegetation to drought in Inner Mongolia.