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It is important that build up waters protection forests in Yulin sandy area in northern Shaanxi, because its climate is droughty and rare rain.waters protection forests can reduce the harm of wind and sand,reduce vaporation and protect water resources.The research was according to the difference of four forests structure types(close,sparsity,porosity, ventilative),investigated and observed on the three types waters protection forests(bank protection forests,reservoir and pond forests,trench protection forests)of Yulin sandy area in northern Shaanxi,in the view of their wind protection function improvement,sediment discharge,evaporation,humidity, temperature etc.On the base of field survey and experiment,analyzed the data by the statistics analysis, search for the appropriate forests structure types which have best ecological benefit.It was showed that:(1)bank protection forests can reduce wind speed,vaporation,sediment discharge.The sediment discharge of the close shelter were only 4.27%-5.04%to the comparison site; the wind protection function of the close shelter can reach to 25.93%-46.2%in the site of 1H and 18.76%-25.4%in the site of 5H;the difference of wind protection function in leaves time and no leaves time was only 4.32;The evaporation of close shelter were 22.6%～30.7%to the comparison site in April and 31.4%～34.8%in May.So the close shelter are fitting to Protect bank,its protection effect were better than other types.(2)the reservoir and pond forests,in the part that join with farmland forests should plant with sparsity forests,the average wind protection function improvement and vaporation were 33.1%and 23.7%,it could increase humidity 3.4%and temperature 0.4℃in April, depressed temperature 0.3℃and increased humidity 11%in July.So sparsity forests can defend reservoir and pond.on the other hand,it can defend farmland;in the part that join with sand-break forests and the reservoir and pond forests that specially plant for protect the reservoir and pond,should plant with combination of trees,brushes and grass that have certain space and extent,space is 150-200 m,extent 5-10m.(3)In the part that join with road,contrasted with two road protection forests showed that porosity poplar shelter had good wind protection function improvement,but sparsity Pinus slyvestris var.mongolica forests had better wind protection function improvement,so trench protection forests should plant with Pinus slyvestris var.mongolica.Rest trench protection forests should plant with combination of trees,brushes and grass or sparsity shelters.
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