Study On Structure Arragement And Their Eco-effect Of Windbreak And Sand-fixation Forest System In Hexi Corridor

The windbreak system in arid areas is generally consisted of 3 belts:belt enclosed for restoration of natural vegetation,belt of sand-fixation trees and the forest network inside the oasis.This paper,applying the method of obscure mathematic,comprehensively assessed the existing 7 types of windbreaks that are established for 20 years in Gaotai desert area through analyzing the 4 indexes:growth situation of plants,water balance between supply and demand,sand-fixation effect and mechanical components of soils. The result reveals that the following 3 windbreaks installation:the mixture belt of Headysarum scoparium×Caragana korshinskii×Haloxylon ammondendron(6×3m),the mixture belt of Haloxylon ammondendron×Headysarum scoparium(4.5×3m)and the pure belt of Caragana korshinskii are the optimum ones that are ecologically healthy and economically valuable.Thay can be applied and extended in Gaotai desert area.In Gaotai desert area,wind erosion mainly occurs in April and May when there is strong wind with high frequency and big wind velocity.However,during this period, most of the plants just begin to emerge and the vegetation coverage is low.As the main purpose of windbreaks is to prevent wind erosion in spring and early summer, the fact that the trees without leaves will influence the protective effect of windbreaks must be taken into consideration when designing windbreak system.When the mobile sands are fixed,those natural vegetation that are adaptable to the moving sands will be replaced by those that are adaptable to the fixed sand dunes due to the changes of such ecological factors as soil and water.The natural vegetation in Gaotai desert area has changed greatly within 25 years.The original community of Tamalix ramosissima,and Headysarum scoparium has involved into the community of Tamalix ramosissima,Artemisia ordosica and Headysarum scoparium,with the vegetation coverage of 42%;the community of Calligonum mongolicum,Tamalix ramosissima and Artemisia ordosica has involved into the community of Tamalix ramosissima,Apocynum venetum and Artemisia ordosica,with the vegetation coverage of 39%;the community of Headysarum scoparium,Artemisia arenaria, Tamalix ramosissima and Alhagi sparsifolia has involved into the community of Tamalix ramosissima,Apocynum venetum,Headysarum scoparium,Artemisia ordosica and Alhagi sparsifolia,with the vegetation coverage of 65%;the community of Calligonum mongolicum and Reamuria soongorica has involved into the community of Tamalix ramosissima and Alhagi sparsifolia.In Gaotai desert area,the coefficient of wind velocity reduction of different types of windbreak(measured at the height of 2m above ground)from big to small are:pure forest of Haloxyllon ammodendron(6m×9m),pure forest of Caragana korshinskii, Haloxyllon ammodendron×Headysarum scoparium,pure forest of Haloxyllon ammodendron(3m×3m),Headysarum scoparium×Caragana korshinskii×Haloxyllon ammodendron,then Haloxyllon ammodendron×Apocynum venetum,then Headysarum scoparium×Tamalix ramosissima;and the coefficient of wind velocity reduction(measured at the height of 0.5m above ground)from big to small are: Headysarum scoparium×Caragana korshinskii×Haloxyllon ammodendron, Haloxyllon ammodendron×Headysarum scoparium,then Headysarum scoparium×Tamalix ramosissima,then Haloxyllon ammodendron×Apocynum venetum,then pure forest of Caragana korshinskii,then pure forest of Haloxyllon ammodendron(3m×3m),then pure forest of Haloxyllon ammodendron(6m×9m).This paper,in view of the fact that in Minqin desert area,the natural vegetation and artificial windbreak begin to decay at a large scale,the fixed sand dunes are moving again,and the windbreak system is seriously destroyed,analyzed the relationship between species selection and ecological effect of the existing windbreak system in this area in the aspects of afforestation density,the dynamic of water and soil.The result shows that Haloxyllon ammodendron and Calligonum scoparium are the best species for afforestation on the sand dunes in Minqin,the optimum installation is 2m×3m individual mixture and 2m×3m belt mixture,the secondary is the pure species forest.The pure Haloxyllon ammodendron forest need to be thinned after 5-8 years,keeping the density of 525-600/h m~2.For afforestation in the depressions among sand dunes,the optimum species are Calligonum scoparium(2×5m)on the sands that are easily infiltrating water and Headysarum scoparium(2×5m) on the area whose top is covered by sand.An'xi desert area is rich in natural vegetation as the water table here is relatively high.There is the famous nature reserve at national level,with an area of 0.8 million h m~2,for protecting extreme xerophyte.Priority should be given to the natural vegetation restoration through enclosure when establishing windbreak system here.As for the sand mouths that have not been fixed,the optimum species for stopping mobile sands is Haloxyllon ammodendron,which can effectively reduce wind velocity with the installation of 1m×1.5m;and for the depressions among sand dunes,the optimum species are Tamalix ramosissima and Headysarum scoparium,with the belt mixture of 1m×1.5m.Research result indicates that:after establishing windbreak in Gaotai desert area, comparing with that on the mobile sand dunes,wind velocity was reduced by 64.5%, the quantity of transformed sands reduced by 82.7%,the quantity of sand particles that are smaller than 0.25mm are increased by 72%,the organic matter content increased by 23.03%,the salt content is decreased by 9.6%,PH value is 0.39 lower. And with the installation of windbreak in Minqin desert area,comparing with that on the mobile sand dunes,wind velocity was reduced by 45.4%,the organic matter content increased by 6.65%,nitrogen content increased by 25.1%,the quantity of sand particles that are smaller than 0.1mm are increased by 263.3%.Comparing with the areas without windbreak,in the areas with farmland windbreak in Minqin desert area, wind velocity reduced by 44.55%,the temperature is 0.8℃lower,relative humidity is increased by 6.3%,evaporation in the field reduced by 9.7%.In the windbreak in An'xi desert area,comparing with that on the mobile sand dunes,the evaporation is decreased by 6.57%,the absolute humidity is increased by 15.76%,average temperature is 1.49℃higher,water content in sand dunes is increased by 1.57%.