Variation Of Vegetation And Soil Characteristics Of Artificial Robinia Pseudoacacia Plantation In Ansai,Northern Shaanxi

In this study,the plantation of Robinia pseudoacacia planted in Ansai County of Loess Hilly and Gully region was studied with the corresponding years of natural vegetation as control.The tree growth,undergrowth vegetation and soil water and nutrient changes in Robinia pseudoacacia stands with different ages were studied,especially the Robinia pseudoacacia planted after“Grain-for-Green”.It provided the references for the healthy maintenance and sustainable management of artificial Robinia pseudoacacia forest in this area.The main findings are as follows:1)Tree height and DBH of Robinia pseudoacacia showed an increasing trend with the increase of age,but decreased with the increase of forest density.The highest cover of understory vegetation was 47.8% at 10 years,and the canopy density reached the maximum(60.3%)at 20 years.At 30 years,the forest was basically mature and the reached the highest stand density(2961 plants/hm2),while at 40 years tree height and DBH reached the maximum(9.07 m and 12.1 cm),The average tree height and DBH of the 15-year-old Robinia pseudoacacia were 7.28 m and 7.82 cm,respectively.The average cover of understory vegetation and aboveground biomass were significantly lower than those of natural vegetation.2)Arbor,shrub species appeared both in the late restoration of Robinia pseudoacacia and natural restoration plots,but distributed scatteredly,especially in the undergrowth of Robinia pseudoacacia land.In the study area the seedling renewability of the main constructive species in natural vegetation were stronger than those in Robinia pseudoacacia land,but the regeneration ability of the main speices was weak,and there were no seedlings of shrub species.It indicted that the vegetation restoration process was slow.3)Soil water content in the forest land was lower than that in the natural vegetation except 10-year plots,and the soil moisture was close to the withered coefficient at 40 years.The soil water content of the 15-year-old Robinia pseudoacacia forest land in 0~5m depth was 4.52~8.17%,which was negatively correlated with the planting density.It shows the initial afforestation density of Robinia pseudoacacia should not exceed 1670 plants / hm2.4)The content of organic matter,total nitrogen,total phosphorus,available nitrogen and available phosphorus in forest land reduced with soil deepening.Compared with natural restoration,the soil nutrients content in different soil layers in forest land was relatively higher(p <0.05).5)Tree height,DBH,canopy closure and undergrowth vegetation cover of Robinia pseudoacacia plantation after “Grain for Green” were at a low level,and the arid trend of soil moisture in forest land was severe,coupled with large-scale distribution of Robinia pseudoacacia plantation,there is a risk of sustainability for Robinia pseudoacacia plantation developent.