| Soil erosion is serious and natural conditions are bad in semiarid loess area,northern Shaanxi.Soil moisture is the main limiting factor for the construction and restoration of forests and grass benefiting soil and water conservation.Only relying on natural enclosure is difficult to achieve ideal effects in the short term.Therefore,it is necessary to take artificial afforeatation.Customarily,trees are planted equidistantly along the contours in the light of the traditional planning and design on the afforestation of soil and water conservation forest,and even though on the mixed one,diverse species of trees are commingled with regular patterns.Traditional artificial afforestation schemes ignore the heterogeneity of habitat conditions,which lead to poor afforestation survival rate,serious individual differentiation in the stand and weak overall stability.Soil moisture is poor on the arid sunny slope.Accordingly,the afforestation get rather difficult,and the restoration of forests is lack of progress.In this research,a study slope(6.25 hm2)was selected from the western slope of loess area in Wucangpu village,Wuqi County.The spatial distribution,tree height and diameter at breast height(DBH)of "Armeniaca sibirica+Populus simonii+Robinia pseudoacacai" mixed forest were measured.The distribution characteristics of tree height and DBH were analyzed using Weibull function;and the point pattern analysis method was adopted to analyze the spatial distribution pattern of A.sibirica,P.simonii,R pseudoacacia and the spatial correlation between the different populations.The results revealed the response relationship between stand structure characteristics and microtopography,and could provide scientific basic information concerning the construction of mixed forest of soil and water conservation.The results showed that:1)A.sibirica and P.simonii were the main constructive species in the study area,while R.pseudoacacia was the companion species and A.sibirica took the advantage position in the community.The differentiation of diameter class was slow,and the stratified structure of the stand was obvious.The population of P.simonii featured trees in large and middle diameter class accounted for the majority;the population of A.sibirica featured trees in middle and small diameter class accounted for the majority;the population of R.pseudoacacia featured trees in small diameter class accounted for the majority.The stand overall degree of mingling intensity was weak,the mixed degree of R.pseudoacacia was the highest,and the aggregated distribution of A.sibirica was significant.The stand overall diameter growth was in medium status,P.simonii was in the ascendancy,while R.pseudoacacia was at a disadvantage.2)The A.sibirica,P.simonii and R.pseudoacacia featured aggregated distribution within the spatial scale of 0-125 m,which reached the highest aggregation intensity when the spatial scale came to 47,65 and 104 m,respectively.The A.sibirica,P.simonii and R.pseudoacacia presented a negative correlation within the spatial scale of 0-125 m and acute interspecific competition.The characteristics of aggregated distribution of trees were closely related to microtopography.3)Microtopography in study slope was given priority to gully.With the descend of slope position,the quantity and area of microtopography were increased.Uphill featured ephemeral gully accounted for the majority,middle slope featured sink hole and scrap accounted for the majority,downhill featured gully accounted for the majority.Soil moisture was adequate in gully on the arid sunny slope,while soil moisture in scrap was in deficit.Deep soil moisture in gully was stable.The overall soil fertility level of the study slope was relatively low.Using total nitrogen,avaiable nitrogen and organic matter as the key indicators measuring soil fertility in microtopography,gully occupied first place,and platform came second.4)The area of microtopography accounted for 24.96%of the study slope,and trees distributed on which acounted for 63.31%of the total number of trees.The quantity and desity of trees on gully was significantly larger than other microtopography and undistributed slope.The better the soil moisture and soil fertility were,the more trees were distributed and the better growth status of trees on microtopography.The diameter growth of A.sibirica and P.simonii were in good status on ephemeral gully.R.pseudoacacai tended to grow vertically on each microtopography.A.sibirica tended to grow vertically on sink hole and gully.P.simonii tended to grow vertically on gully,sink hole and platform.The near-natural afforeatation pattern and arrangement of seeds should be designed according to microtopography.Trees with large quantity and good growth status on microtopography should be selected,which means A.sibirica and P.simonii may grow on gully and sink hole,P.simonii may grow on platform. |
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