The Windbreak Effect And Optimization Model On Allocation Of Shelterbelts And Natural Vegetations At An Oasis-desert Ecotone

It is distributed with large areas of shelter forests and natural vegetations at an oasis-desert transitional zone, where is the important area in reducing wind and fixing sand in the western arid zone of China. The shelter forests and natural vegetations can play a significant role in preventing desert moving and protecting the ecological safe of oasis.In this paper, according to the different habitat types of natural vegetation,8 experimental samples was set at an oasis-desert ecotone of Mosowan Region and Xiayedi Region in this Basin and chose 17 shelter forests, where structural factors of shelter forests and species of natural vegetation were investigated and wind speeds were measured and soil samples were collected. Exerting digital camera FUJIFILM FinePix S5000 and computer image disposal software Photoshop7.0, we conducted digitized measurement on shelterbelts porosity of shelter forests and established the dominant factors model via stepwise regression between shelterbelts porosity and other structural factors of shelterbelts. Analysis of the species diversity indexes, we studied species composition and distribution and windproof effect of natural vegetation in early spring in this area. Following Wasson and Nanninga's first modeling idea, we constructed a mathematical model between coverage of natural vegetation and relative wind speed and established effective vegetation coverage in different relative wind speed according to this model. We studied the influence of synergistic action of shelterbelts and its surrounding natural vegetations on wind speed reduction and air temperature and surface soil and then constructed a collaborative allocation model of shelterbelts and natural vegetations by stepwise regression. Using software programming, we obtained the optimal collaborative allocation model of shelterbelts and natural vegetations in the smallest relative wind speed and predicted the optimal collaborative allocation model of shelterbelts and natural vegetations by this program in different relative wind speeds. The main results we got in this study were as follows:(1) Because the system error and human error of testing results of shelter forests shelterbelts porosity were smaller, we elicited that using Photoshop and digital camera could realize the digitized measurement on shelterbelts porosity, which was exact and credible in the extent of 0.01-0.06. Therefore, it could be realize that we could determine shelterbelts porosity quickly and accurately. The dominant factors model between shelterbelts porosity and other structural factors of shelterbelts showed that there existed significant correlation among total shelterbelts porosity and the area of crown and the average of crown and the average of clear bole height, as well there was significant correlation between crown shelterbelts porosity and the area on breast height. The function relationship between porosity and sine value of relative wind speed at two heights is a parabolic function relationship. The optimal porosity of shelterbelts is between 0.35 and 0.37.(2) Natural vegetation of transition zone was dominated by shrub no matter from species number or coverage, and Haloxylon ammodendron acted as its construction species and there was larger difference among the index of species richness and species diversity and species evenness in different region. Linear correlation analysis was done among average height of natural vegetation, coverage of natural vegetation, relief amplitude of natural vegetation and average wind speeds at two heights (0.5m and 1.5m), which discovered that there was only most significant difference between relief amplitude and average wind speeds at two heights. However, correlation coefficients displayed that the average wind speeds at two heights decreased with average height of natural vegetation and coverage of natural vegetation increasing. The mathematical model between coverage of natural vegetation and relative wind speed invented that negative exponential function lied in both. The effective vegetation coverage was predicted in different relative wind speed following this model as well.(3) Wind performed a significant regularity when existed obstacles such as shelterbelts and natural vegetations, which was that the synergistic action of shelterbelts and natural vegetations could reduce wind speed through analysis of relative wind speed at two heights (0.5m and 1.5m). For loosened shelterbelts, the degree of wind speed reduction in three locations was internal natural vegetations> middle natural vegetations and shelterbelts> external shelterbelts; for close shelterbelts, the degree of wind speed reduction in three locations was middle natural vegetations and shelterbelts> internal natural vegetations> external shelterbelts when the wind speed was smaller. However, the trend of air temperature changing is opposite to the wind speed changing trend, which is that the air temperature is lower when wind speed is higher. Wind had significant effect on soil particles and the middle soil particle is affected most. Stepwise regression between structural factors of shelterbelts and grown indexes of natural vegetations showed that there was the most significant correlation among the crown volume of shelterbelts and breast height area of shelterbelts and spacing of shelterbelts and average height of natural vegetations and coverage of natural vegetations and relative wind speed at two heights. Using software programming, we obtained the optimal model of collaborative allocation of shelterbelts and natural vegetations in the smallest relative wind speed and in different relative wind speeds.