Optimal Disposition Of Vegetations Based On Soil Water Carrying Capacity In Diediegou Catchment

In past decades,national key forestry ecological project such as Three-north Forest Protection Project,Natural Forest Protection Project and Grain for Green Project have been implemented in the vast arid and semi-arid areas in the northwest of China.As a result,ecological environment of the areas has been dramatically and doubtlessly improved with the increase of forests.However,as forest area and volume both increased,new problems had appeared in the most of plantations,such as uniformed vegetation type,excessive stand density,unreasonable structure,low biodiversity and so on,which in turn,restricted severely the function on soil and water conservation of forests.Meanwhile,it not only may cause such problem as soil desiccation,but also may escalate the conflict of forest and water,and threaten seriously the security of water resources and sustainable development of society and economy in the dryland areas.Thus,on the basis of deeply understanding relative scientific problems on the relationship between forest and water,how to make a reasonable spatial configuration of forest vegetation has become an essential question needed to be solved for forestry practice in the watershed.As we known,the composition,density,structure and spatial distribution of forests in a given watershed are determined by the soil water carrying capacity of vegetation,especially in the water-limited ecosystem.Therefore,making space configuration of vegetation in the watershed,based on the quantitive research on soil water carrying capacity,should be the core issue in the afforestion and ecological recovery in future.In this paper,the spatial configuration of vegetation based on soil water carrying capacity was studied in the small semi-arid watershed of Diediegou,in the northern region of the Liupan Mountains in Ningxia.First,the optimum forest coverage of the watershed was determined based on the data from both measured and collected.Then,site types in the watershed were analyzed and divided by using mathematical statistics.Then,each of them was configured with an optimal type of vegetation,and the site type distribution in the small watershed was displayed by using Arcgis technology.Finally,the reasonable density or coverage of vegetation was determined based on the calculation of soil water carrying capacity of vegetation for each divided site.Main conclusions have been made as follows:1.Aiming at the function of water conservation of forests,the optimal forest coverage was calculated as 27.96% under averaged soil saturated water capacity of main forest types in the watershed,in the light of the relationship between amount of the maximum diurnal rainfall of 20 years and soil saturated water capacity.2.Mathematical statistics were used to study and divide site types in the watershed.Principal component analysis and Correlation analysis indicated that the aspect,slope gradient and soil thickness were the three leading environmental factors to divide site types.By using the clustering analysis,site conditions in the whole watershed could be accordingly divided into seven types named as “aspect-gradient-soil thickness”,i.e.,? shady-inclined-thick,? shady-steep-moderate,? sunny-gentle-thick,? sunny-inclined-thick,? sunnyinclined-moderate,? sunny-steep-thick,? shady-gentle-thin.3.According to the principle of para configuration and the characteristics of topography and spatial pattern of existing vegetation in the watershed,the appropriate vegetation for each of site types were provided as follows: Site ? was suitable for arbor-tree community dominated as Larix principis-rupprechtii and Populus davidiana;Site ? was suitable for shrub community dominated as Corylus mandshurica and Lespedeza bicolor;Site ? was suitable for shrub community dominated as Hippophae rhamnoides and Lespedeza bicolor;Site ? was suitable for shrub community dominated as Ostryopsis davidiana;Site ? was suitable for herbaceous community dominated as Bothriochloa ischaemum;Site ? was suitable for herbaceous community dominated as Thymus mongolicus;Site ? was suitable for herbaceous community dominated as Artemisia vestita-based.4.To determine soil water carrying capacity of vegetation under different site conditions,the relationship between structure and water consumption of typical vegetation needed to be quantified firstly.We used BROOK90 model to simulate annual evapotranspiration of the three types of vegetation,i.e.,Larix principis-rupprechtii plantation,Hippophae rhamnoides community and grass community with annual maximum leaf area index(LAI),then fitted to determine the relationship between annual evapotranspiration and LAI.The results showed evapotranspiration was significantly and positively related with LAI(R2>0.99,P<0.01),however,it tended to a threshold value with increase in LAI.The threshold of LAI existed for all various type of vegetation,indicating that when LAI was less than the threshold value,evapotranspiration increased rapidly with LAI,however,when LAI was greater than the value,evapotranspiration no longer increased with LAI,but gradually stabilized.5.To quantify the relationship between soil water carrying capacity of vegetation and site factors,taking Ostryopsis davidiana community as an example,and aboveground biomass as an indicator of soil water carrying capacity of vegetation,the relation of aboveground biomass and soil thickness was analyzed here.The results indicated that there was a significant positive correlation between the two variables(R2=0.8559,P<0.01),suggesting that soil thickness was the critical factor to determine soil water carrying capacity of this shrub community.The result further confirmed that,when annual rainfall were relatively consistent in a given watershed,available soil water,which strong depended on soil thickness,was the key determinant for vegetation growth and distribution on the slope.6.To determine soil water carrying capacity of vegetation in a specific site condition,annual maximum LAI was considered as an indicator here.And annual maximum LAI was then determined when potential evapotranspiration of vegetation(annual water consumption)equaled to the maximum available soil water(annual water supply).Further,soil water carrying capacity of vegetation(LAI)under various site types was determined based on the relationship between LAI and soil thickness(T)as follows.Site ?: LAI =0.004637 T-2.4;Site ?: LAI =0.005467 T-1.55;Site ?: LAI =0.005078 T-1.55;Site ?: LAI =0.00441 T-1.55;Site ?: LAI =0.008984 T-2.066;Site ?: LAI =0.009577 T-2.066;Site ?: LAI =0.011016 T-2.066.Thus,soil water carrying capacity of vegetation in each parcel under different site conditions could be determined based on soil thickness and the relations listed above.To guide conveniently forest management in a watershed,LAI of the three typical vegetation were converted to density(N)or coverage(C)with the equations of 26.53202.1 ?=LAIN for the arbor-tree community,C=100×[1-exp(-0.497LAI)] for the shrub community,and C=100×[1-exp(-1.226 LAI)]for the grassland,respectively.Furthermore,some proposals were presented for forest manager to optimize present spatial configuration of vegetation in the watershed.