Effects Of Grazing On Community Characteristics In Mountain Forests-arid Valley Ecotone In The Upper Reach Of Minjiang River

As an important part of shelterbelt networks in Yangtze river basin, abundant planted forest and secondary shrub exist in Sichuan Province, where the area of shrub has been748×104hm2, occuping25.2%of nationwide area. Grazing is the most convenient and most economic way in use of vegetation, and also one of the most primary disturbances which have made shrub, grassland and planted forest degraded. The environmental effects brought by grazing in meadow or grassland have been paid enough attention, but less attention has paid to grazing in shrub and planted forest. For that reason the shrub of Berberis aggregate, Robinia pseudoacacia, mingled forest of poplar and willow in mountain forest-dry river valley ecotone in upper reach of the Minjiang River were chosen as object of study to research the impacts on characteristics of population and community of shrub and planted forest from grazing disturbance. Partition of grazing intensity is generally adopted to research grazing disturbance. Comparative analysis is done on the basis of different grazing pressure. Due to different indexes about intensity of disturbance, there is not consistent partition standard. Grazing path are the trails on which livestocks forage and trample. The characteristics and regularity of distribution of grazing path can indicate the livestocks’actions and grazing intensity. Meanwhile the conditions of plant community in heavy and long-term grazing intensity have been paid less attention. It is necessary to research the adaptability of multiple community and species to long-term grazing disturbance. Grazing path were regarded as entry point, relationship between characteristics of grazing path and livestocks’actions, effect of grazing path on showing grazing intensity as well as effect of grazing passes on features of population of shrub and herb, diversity of functional group and diversity of species were studied. Through cluster analysis and sequences of community, the intimate connections among herd, terrain and vegetation were further tested and verified. Main results were as follows.(1) Grazing path density, Grazing path pattern, herb layer heigh, shrub layer cover and index of grassland quality were quantitatively identified as comprehensive index of grazing pressure. On the whole, comprehensive index of grazing pressure could better reflect livestocks’ actions and grazing intensity. Simultaneously some indexes else should not be ignored such as the width and length of grazing path.(2) The plantage in shrub and planted forest was mainly the composite family. Species that was able to endure graze, was spinous, poisonous and harmful occupied a many part of dominant species so that vegetation became vestigial seriously under long-term graze condition. There was a certain negative correlation between population of Berberis aggregate and distribution of grazing passes. Therefore, characteristics of population and distribution pattern had been influenced by garzing, density of shrub added with decreased density of grazing passes, population structure was unreasonable and became gradually vestigial. Herbaceous plants were adapt to grazing disturbance owing to the close correlation among population of most of herbaceous plants within shrub. The differences between dominant species in herbaceous plants and grassland in shrub were significant so that species which was able to endure cloudy and wet environment increased and were of superiority.(3) Diversity of functional group of shrub and planted forest decreased, rootstock of grass disappeared, ratio of leguminous forage and nutgrass flatsedge was too small, functional group of grass as well as proportion of weeds enlarge. Richness and homogeneous degree of species in sample plot were changing with comprehensive index of grazing pressure changing, whereas a negative correlation existed between index of diversity of species and density of grazing path. It was indirectly manifested that garzing with high intensity would cut down the diversity of species. The composition herbaceous plants under forest and which under bushwood had few species, and substitution rate of species was higher.(4) The features of grazing path and height of herb together could be the indexes of grazing intensity. Grazing path were used as the geomorphic factor along with the topographical factor to sort community, the results indicated that the features of grazing path were key elements influencing the distribution of shrub and herb. Furthermore elevation and gradient could also influence characteristics of community. Multiple vegetation form should be studied from more scales in future research to comprehensively analyse influences of varied factors on distribution of grazing passes and vegetation.