Stability Evaluation Of Protection Forest System In Oasis Irrigation Area

The protection forest system in oasis irrigation area play an important role in improving agricultural production conditions, stabilizing and enlarging oasis area, promoting local economy development, and improving peoples living quality, and its stability is of great theoretical and applied significance towards safeguarding the ecological stability of oasis irrigation area, and improving the generation and development of protection forest system in oasis irrigation area. Based on the theory of ecosystem stability, referring to methods and results of ecosystem stability evaluation in domestic and overseas, as well as taking Jingtai oasis irrigation area as an example, the stability evaluation of protection forest system is studied, and the results are summarized as follows:l.The indicators system of stability evaluation on protection forest system in oasis irrigation area is established on the basis of four hierarchies: the goal layer (T), the criterion layer (A), the limiting layer (B) and the decision layer (C). The goal layer (T) indicates general state of the stability of protection forest system in oasis irrigation area, namely, if stable and how about the stability level. The criterions layer (A) includes two factors composed of structural rationality and functional stability. The limiting layer (B) includes six factors. Among them, forest types structure, stand structure and tree species structure are comprised in structural rationality; and ecological protection function, economic profit and social benefit are comprised in functional stability. The decision layer (C) is composed of nineteen factors. Among them, farmland shelter forest area, wind break and sand fixation forest area, the ratio of production forest area to shelter forest area, and sand-closed land area are embodied in forest types structure; ventilation degree, the planting density of wind break and sand fixation forest, the planting density of production forest, vegetation coverage in sand-closed land, and the percentage of regeneration reflect the state of stand structure; native tree species ratio, evergreen tree ratio, improved variety ratio in production forest, shrub tree ratio and the percentage of plant diseases and insect pests show the state of tree species structure; wind velocity index and evaporation index mirror the condition of ecological protection function; input-output ratio of forest and effects of crops output reflect the condition of economic profit; the percentage of forest cover indicates the state of social benefit.2.The indicators are quantified by classification method, and by means of AHP. theweight values of all factors are calculated. The results show that in criterion layer, both of structural rationality and functional stability are of the same importance which influences the stability of protection forest system in oasis irrigation area; in limiting layer, ecological protection function and tree species structure have high contribution to the stability of protection forest system in oasis irrigation area; and in decision layer, wind velocity index, effects of crop output, percentage of forest cover, evaporation index, native tree species ratio are the main factors influencing the stability of protection forest system in oasis irrigation area.3.The results of both hierarchic assessment and synthetic assessment show that the goal layer is less stable, that is to say, the general state of protection forest system is less stable; in criterion layer, functional stability is less stable, and structural rationality is less fragile; in limiting layer, stand structure and ecological protection function are stable, economic profit is less stable, forest types structure and tree species structure are fragile, and social benefit is extremely fragile; in decision layer, ventilation degree is strong stable, planting density of production forest and wind velocity index are stable, the ratio of production forest area to shelter forest area, planting density of wind break and sand fixation forest, vegetation co...