Climate change has driven plant species shifting their geographic distributions to higher latitudinal and altitudinal range, which would result in great compositional changes of vegetation. The alpine tundra of the Changbai Mountains is one of the two rare alpine tundra distributions in China, which has experienced significant climate warming in the past decades. To date, herbaceous plants represented by Deyeuxia angustifolia(Komarov) Y. L. Chang, a narrow-leaf small reed, has encroached upon the alpine landscape and threatened tundra biota. However, there was little knowledge on the characteristics of habitats the herbs expanded towards and the spreading rate of herb patches during the past 30 years. The NDVI (Normalized Difference Vegetation Index) data of peak growing season (Middle July) and late growing season (Late September) of 1980s, 1990s, and 2000s was derived from Landsat TM/ETM+ images in alpine tundra. Phonological differences, in terms of NDVI differences (NDVI of peak growing season minus NDVI of late growing season), between herbaceous plants and evergreen shrubs was used to quantify the area of herb patches. Topographical factors such as slope, which were derived from DEM (Digital Elevation Model) data, was applied to quantify the habitat characteristics of herb patches.
Results/Conclusions
In the past 30 years, the expanded area by herbaceous plants was 612.18 hm2 with mean annual increasing rate of 20.406 hm2/yr. Major changes of herb patches occurred in altitudinal range of 2060-2240 m located in wet regions at south related aspects, indicating that herbaceous plants favored wet and sunny habitats. There was a gradual replacement of evergreen shrubs by herbaceous plants. Among the expanding herbs, D. angustifolia made the most contribution to the expansion. Climate warming was responsible for the expansion of herbaceous plants, which provided sufficient energy for herbaceous plants to survive in the harsh environment of alpine tundra. Besides, the reducing low temperature events were no longer the constraints for the growth of herbaceous plants from low elevations. Our results indicated that global climate warming was changing the alpine vegetation structure. Herbaceous plant species was gradually climbing up higher elevations and replacing alpine shrubs. The important strategy of these upward shift herbaceous plants was to occupy favored habitats and expand continuously.