关键词: 灌丛化, 过度放牧, 气候变化, 植被变化

Abstract: Shrub encroachment of grassland is defined as the increase in density,cover and biomass of indigenous woody or shrubby plants in grasslands,and the phenomenon has been documented for many arid and semiarid regions in the world. The causes of woody plant encroachment are complex,among which livestock overgrazing has been considered as a major one in early studies. Recent studies indicate that the direct factor to trigger shrub encroachment is not overgrazing,but the shift of grazing intensity after long-term overgrazing because the shift changes the interaction between grass and shrub. Shrub may get established in open areas during long-term overgrazing before productive herbaceous species evolve back. Many other factors,such as changes in rainfall seasonality and intensity,altered fire regimes,as well as elevated carbon dioxide level in atmosphere,may also mediate shrub encroachment of grassland. The replacement of herbaceous plants in grassland by shrubs had been shown to decrease the grassland productivity,to increase soil erosion,and to reduce the soil carbon storage. Therefore,shrub encroachment of grassland has long been considered as a process of grassland degradation. However, recent global synthetic analyses indicate that the impact of increased shrub cover in grassland may not always be negative. In many cases,it increased upward and downward water movement as well as soil nitrogen mineralization,hence enhanced the storage of water and nutrients in ecosystem. Given the above advances and discrepant conclusions and explanation,we suggest more studies should follow to test the generality of the above conclusions. On the other hand,grassland managers should also be alerted,as appropriate plans of conservation and production may just opposite to our intuitive understanding.

Key words: shrub encroachment, overgrazing, climate change, vegetation transition