Evaluation of semiarid grassland degradation in North China from multiple perspectives

There has been increasing interest in grassland ecosystem degradation resulting from intensive human activity and climate change, especially in arid and semiarid regions. Species composition, grassland desertification, and aboveground biomass (AGB) are used as indicators of grassland degradation. In this study, we comprehensively analyzed variations in these three indicators of grassland degradation in semiarid grassland in North China, on multiple time scales. Since 1984, species composition has become simpler and species indicative of grassland degradation, such as Potentilla acaulis and Artemisia frigida, have become dominant. These changes indicate that serious grassland degradation has occurred since 1984. Grassland degradation was also analyzed on shorter time scales. Analyses of interannual variations during 2005-2015 showed that desertification decreased and average AGB in the growing season increased over the study area, indicating that grassland was recovering. However, analyses of species composition indicated that grassland was actually degrading during this period. Analyses of spatial variations showed that the position of slightly desertified grassland shifted and formed a band in the west, where the lowest AGB in the growing season was recorded. Grassland AGB decreased in most parts of the south and east (similar to 20 g/m(2) per decade) but increased in other regions from 2005 to 2015. Climatic factors had critical effects on grassland degradation, showing that simpler species composition mainly resulted from an increase in average temperature and a decrease in average precipitation over the past 30 years. And in recent decade, increased precipitation and decreases both in temperature and potential evapotranspiration relieved desertification and increased AGB overall. This study thus revealed that ecosystem structure degenerated seriously in the long-term, but AGB increased in the short-term mainly due to increases in both indicator species for grassland degradation and species with strong resistance to drought tolerance. Thus, this study suggested there had distinct difference in grassland degradation identified by above three indicators at different time-scales, and multiple perspectives should be considered to accurately assess the state and characteristics of grassland degradation.