【摘要】 灌丛化作为全球干旱半干旱区草原普遍发生的现象,其定义为草原生态系统中灌木/木本的生物量、密度、盖度的增加以及草本的生物量、密度、盖度的减少。草原灌丛化是气候变化和人类活动多种因素综合作用的结果。过度放牧被认为是引发草原灌丛化的主要原因之一。最新的研究结果表明过度放牧并不能导致草原灌丛化,但过度放牧后实施休牧却改变了草本与灌木的种间作用,有可能导致灌木的扩张。灌木入侵草原长期以来被认为是草原的退化,结论来源于干旱区土壤沙化的情形,在沙化的灌木林中,土壤碳库被局限于灌木株丛及其周边,使草原的碳截留和储存降低。但最近的全球性集成研究表明草原中灌木覆盖率盖度增加对生态系统可以产生积极作用,灌木可以增加土壤水分的下渗,有利于生态系统的水分储存和和养分的转化(如加强氮的矿化过程)。草原灌丛化对生态系统结构和功能影响存在景观尺度和斑块尺度上的差异。进一步研究适应灌丛化过程的管理机制,综合不断变化的气候条件因素和地域因素,采取合理的草原管理策略,对于全球草原区生产具有极其重要的意义。更多还原

【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.更多还原