| It is well known for the Loess Plaetae that the natural conditions is very sever,the soil and water resources in this area lose heavily,the eco-environmental situations are very weak to resists the disturbances from human beings or nature.In order to improve the eco-system quality in this area,the forestry ecological engineering projects such as construction shelter forests,returning farmland to forests have been completed.The Populus hopeiensis is one kinds of typical native species,with high resistance to disturbance and high potential to reconstruct forest from root tillers,has become one of the domain specie for the local ecology re-vegetation.However,the relative lack of Populus hopeiensis community stand information and its suitable growth conditions impedes the development of using its community characteristics to reconstruct the local vegetation.In this paper,the Wuqi County was selected as research area,and the typical natural Populus hopeiensis forest was observed.The statistics analysis methods,Ripley's K function,Mann-Kendall tendency analysis,Grey correlation analysis,and combination with long-term local hydrometeorological data,typical plot samples observation data and DEM data were used to explore the Populus hopeiensis community structure characteristics and its suitable growth conditions,the main results as follows:(1)On the semi-arid region of Loess Plaetae in north of Shaanxi,there are a negative relationship between the growth characteristics of the Populus hopeiensis community and that of the community density at different slope location,slope orientation and micro-tophography.The growth characteristics including community age,height,diameter at breast height and its canopy.Compairing the cloudy slope to the semi-cloudy slope,the community growth index values are larger for the former,and the community density is smaller than latter.On the cloudy slope,the growth index increases gradually with the slope location down to the valley,and on the semi-cloudy slope:up slope>down slope>middle slope and the community density is in the reverse order.On the different micro-tophography,the growth index values are:sink hole>gully>ephemeral gully>platform>beach;on the semi-cloudy slope:platform>ephemeral gully>gully>sink hole>scarp,and the community density also in the reverse order.(2)The species composition of the community belongs to the flora of northwest china,the dominant woody plants include Armeniaca sibirica(Linn.)Lam,Populus simonii Carr,Amygdalus davidiana(Carr.)C.de Vos ex Henry and Ulmus pumila L,the shrub layer mainly are Hippophae rhamnoides Linn,Caragana korshinskii Kom,and the herb layer consists of 16 families and 37 genera with Asteraceae,Leguminosae and Gramineae.Among the herb layer,the Leymus secalinus(Georgi)Tzvel,Leymus secalinus(Georgi)Tzvel,Artemisia sacrorum Ledeb and Cyperus rotundus are on the dominant position.The growth of the Populus hopeiensis plays a depress effects on the accumulative biomass of the herb layer and a stimulation effects to that of the species diversity.On the mature stage,the average density,height,coverage,biomass and species diversity of the herb plants living under the canopy reach to stable states.(3)There are an obvious contagious distribution among the woody plants,shrub plants and Populus hopeiensis that being on different succession stages within the community.On the cloudy slope,the contagious diameter is larger than that of semi-cloudy slope,and the contagious strength also higher for the cloudy slope.The competition among the dominant species and its associate species is decrease with the spatial scale,and which becomes more active on the semi-cloudy slope.For the interspecies competition,the strength shows a decreasing tendency with the spatial scale increases.This tendency is more notorious when the spatial scale larger than 35 m and 10 m for the cloudy slope and semi-cloudy slope,respectively.(4)The growth characteristics index including plant age,height,canopy and diameter at breast height have an obvious difference for that at different elevation,slope location,slope orientation and micro-topography.Among this factors,the micro-topography has much more effects to the growth index than other factors.On the different micro-topography,the soil water content and soil nutrition plays an important role on the dominant species growth.There are a good consistence with the soil water,nutrition and the community distribution,in the general:gully>ephemeral gully>platform>sink hole>scarp.(5)On the different slope location,the Populus hopeiensis mainly breeds along the slope to the gully.It takes 10 years and 20 years for the breeding from the up slope to the down slope for the cloudy and semi-cloudy slope,respectively.The mother plants firstly appear at ephemeral gully and sink hole,then spread to the gully,platform and beach on the cloudy slope.For the semi-cloudy,the mother plants firstly appear on the ephemeral gully,then spread to the gully,scarp,platform,sink hole.(6)The community of Populus hopeiensis is suitable for growth in the area with better soil moisture conditions.In the area,The growth characteristics index of Populus hopeiensis including plant age,height,canopy and diameter at breast height will be better and the community of Populus hopeiensis will tends to vegetative growth.But in the area with worse soil moisture conditions,the population density of the Populus hopeiensis community will be larger and it will tends to reproductive growth.In different slopes,the the shady is more suitable for the Populus hopeiensis community to growth.In different aspects,the bottom of the slope is more suitable.And in different micro-topographies,the sink hole,ephemeral gully and gully is more suitable. |
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