CHEN Yong, CHEN Guo-jie, WANG Yi-qiang
All ecosystems on earth have been under the influence of human beings. It is also true for mountain ecosystem. Comparing with ecosystem on plains/low lands, mountain ecosystem is characteristic of complexity and uncertainty, fragility and sensitivity. Mountains are three-dimensional systems, of which the complexity is displayed in their verticality, non-verticality and compilation of both. Due to the complexity of mountains, it has been very hard to give mountain a well-understood definition. The uncertainty of mountains lies in their instability of internal structure and high potential energy. The fragility and sensitivity of mountain eco-environment is displayed in that mountain ecosystems usually have a low capability to resist external disturbance and it is easy for them to exceed ecological threshold or even to go to collapse under external influences. The fragility of mountain eco-environment is determined by its bio-physical features: high elevation, cold climate, thin air, thin layer of soil, low concentration of organic matters in soil and slow biochemical process. It is usually hard for soil and vegetation in mountains to rehabilitate and restore once they are destroyed. In most cases degradation of mountain environment is an irreversible process. Due to physical constraints, mountain settlements or communities have showed their characteristics of scattering, closeness and stability. Considering the interaction between population and environment in mountains, environment, on one hand, may be a constraint for human activities. Although human being is a keystone species in earth ecosystem, human activities are always constrained by environment. It is more like this in mountains where physical environment is harsh. Humans,on the other hand, are able to adapt to their environment by transhumance, physiological adaptive reaction, storage of foods and cultivation of wild plants. The man-earth relationship in mountains may take two forms: mutual deterioration or mutual amelioration in different types of land use such as farming, animal husbandry, forestry, settlement construction and mining.