草地灌丛化研究进展

Progress of research on shrub encroachment in grassland

草地灌丛化(Shrub encroachment)会导致生态系统结构和功能改变,甚至可能导致干旱区土地退化和荒漠化.本文从降雨、气温、土壤、生物以及人为活动等方面综合分析灌丛化的成因,总结灌丛化对生态系统的影响以及控制灌丛化的有效措施.降水增加及其分配格局的改变通常会促进草地灌丛化;温度升高能够减少灌木冻害,降低灌木死亡率,有利于灌木扩张;由于灌木适应能力较强,土壤退化有利于灌木入侵;过度放牧促进草地灌丛化,而适当的火烧会抑制灌丛的扩展.草地灌丛化会导致草地可用性降低,改变土壤养分与水分分布格局,但对植物多样性及生产力的影响表现出不同的结果.降低人为干扰强度、适当火烧、人工去除灌木是控制草地灌丛化的有效措施,但在不同区域应采用不同组合措施以提高成效.目前灌丛化研究结果多基于对野外自然过程的观察和分析,缺乏大尺度、长时期野外控制实验证据的支持;有关灌丛化草地是否能恢复,哪种条件和措施下可以恢复的全球性认识还不一致;我国草地灌丛化发生时间较长,但对其研究尚处起步阶段,应加强国际对比研究,有利于在更长的时间尺度上阐释草地灌丛化的发生过程和机制.

Shrub encroachment can change the ecosystem structure and function, even lead to the degradation or desertification of arid or semiarid grassland. This review comprehensively analyzed the impacts of precipitation, air temperature, soil character, biotic factor and human activities on shrub encroachment, and summarized the influences of shrub encroachment on ecosystem as well as the effective control measures against shrub encroachment. The increasing amount and distribution pattern change of precipitation generally facilitate the shrub encroachment. The global warming can alleviate the freeze injury to the shrub, and then decrease its mortality rate, which is favorable to the expansion of the shrub. Due to high adaption ability of shrub species, the land degradation and over-grazing can also facilitate shrub encroachment. Appropriate frequency of fire can suppress the shrub encroachment. Shrub encroachment is often associated with lower pasture value of the grassland, as well as changes of soil moisture and nutrient pattern, but has different impacts on grassland biodiversity and biomass. There are some effective control measures on shrub encroachment including reducing anthropogenic disturbance, applying adequate fire and artificially removing shrub. To improve the effectiveness of shrub encroachment control, a combination of several measures should be applied according to the specific environment. Presently, the study results of shrub encroachment were mostly based on observation in field study without supporting data from big-scale and long-term field experiments. Therefore, further study on the process of shrub encroachment is needed. Besides, there are no consistent conclusions on whether the shrub encroachment process can be reversed as well as how it can be reversed. Although research on shrub encroachment just started in China, the phenomenon has happened long time ago in the grassland of this country. Thus, comparative studies between China and other regions can help clarifying the formation mechanisms and process of shrub encroachment.