Spatio-temporal variations in vegetation types based on a climatic grassland classification system during the past 30 years in Inner Mongolia, China

Abstract

As one of the most widespread vegetation types, grasslands play a key role in the terrestrial ecosystem. In China, grasslands are threatened by degradation due to both local human activities and climate change. To analyze the spatiotemporal variation in grassland vegetation under climate change conditions, this study analyzed the climate conditions over the past 30 years in Inner Mongolia, China. The changes in grassland vegetation types over the past 30 years were simulated by the Integrated Orderly Classification System of Grassland (IOCSG), which determined the grassland types by using the >0 °C annual accumulated temperature (AAT0) and moisture index (k-value). To validate the simulated results, the normalized differential vegetation index (NDVI) was used to reflect the real vegetation variations. Over the past 30 years, the AAT0 significantly increased at a rate of 109.8 d °C /decade, and the k-value showed a nonsignificant decrease at the regional scale. The simulated results by the IOCSG showed that the areas of forest and forest steppe and meadow steppe in the past 10 years (2005–2015) decreased by 14.9% and 30.8%, respectively, compared with those in the period from 1986 to 1975. However, the areas of typical steppe and desert and semidesert showed increases of 45.6% and 5.4%, respectively, over the past 10 years compared to those from 1986 to 1975. A typical steppe area of 2.46 × 104 km2 has changed to desert and semidesert, and a meadow steppe area of 10.98 × 104 km2 has changed to typical steppe in the past 30 years. In most regions where the grassland types were predicted to have changed, the annual maximum value of NDVI (amNDVI) showed the same variation trend as the results from the IOCSG. During the study period, the amNDVI significantly decreased in the regions where the vegetation types changed from forest and forest steppe to meadow steppe, while it significantly increased where the vegetation types changed from meadow steppe to forest and forest steppe. However, in some areas, for example, the midland of Ordos, the amNDVI showed the opposite trend with the predicted results, which may have been due to the effects of human activities. Overall, grassland degradation in Inner Mongolia was predicted by the IOCSG, and the results were in accordance with the NDVI variations, which is of great significance for better understanding the grassland conditions under a changing climate.