Abstract: In this paper, the author reviewed the concept of the vulnerable ecotone in theoretical and practical aspects. The vulnerable ecotone has been defined a s special ecotone as it was more sensitively affected by environmental factors' change and ecological processes fluctuation than the original concept of ecotone. Thus, the characteristics of material, structure, and energy flows in the vulnerable ecotone close depend on spatial scales which are divided by researchers, because the spatial area determines the complexity and stability of the vulnerable ecotone. For example, the agri animal husbandry ecotone in north China includes several provinces and more than a hundred counties, concerns different ecological, climatic and geographical systems. It is a large scale concept. Its universal character is a vulnerable ecotone in which agriculture and animal husbandry regime interlaces each other, which is considered as a homogeneous region in this scale. But if a small scale is used to analyze the above mentioned region, there are different geographical units that include Horqin, Erdos, and Bashang regions. They have same characteristics in comparison with the agri animal husbandry ecotone of north China. The complexity and stability of agri animal husban dry ecotone of China are higher than that in Horqin region under same disturbance process. In addition, the composition elements (Holon) of the vulnerable ecotone vary from small area to big area in constant spatial scale, because agricultural and grazing regime and climatic change extremely affect ecotonal composition in different spatial and temporal scales. The large scale vulnerable ecotone has higher heterogeneity and buffer ability than small scale's, so big vulnerable ecotone changes less than small in same disturbance regime. The variation rate of the element increases with the element area decrease, otherwise element composit ion complexity decreases with element area decrease. According to the above mention, it is considered that the vulnerable ecotone has hierarchical structure except its physical characters: the larger unit area is, the higher hierarchy is. So there are several key points which must be paid more attention to in future study concerning the vulnerable ecotone. First, how large spatial and temporal scales will be used to analyze and describe the vulnerable ecotone, and what kinds hierarchical structure is it. Second, how to explain and manipulate the research results are very important for extrapolating and predicting the traits of different spatial and temporal scales. Third, the change rate of composition element studied in the paper can not be used over study scale, be cause the variation rate of composition element in the vulnerable ecotone depend s on scale.

Key words: Vulnerable ecotone, Spatial temporal scale, Hierarchy theory