| The loess hilly area,as one of the most fragile ecological environment areas in my country,has significantly improved the vegetation coverage and species richness since the implementation of the project of returning farmland to forest(grass).However,due to the lack of scientific management and planning,the existing plantation forests have problems such as high community density,single stand,and inability to self-renew.Therefore,this study took the Robinia pseudoacacia plantation in the loess hilly semi-arid area as the research object.Through field investigation and indoor analysis,the composition of the Robinia pseudoacacia plantation(tree height,diameter at breast height,canopy cover,crown width,plant diversity)in the middle-aged forest stage(16～25a)was analyzed,plant functional traits(specific leaf area,leaf water content,leaf total nitrogen,leaf total phosphorus,leaf organic carbon,leaf dry matter content)and soil physicochemical properties(soil water content,soil bulk density,soil total nitrogen,soil total phosphorus,soil available phosphorus,soil available potassium,soil organic carbon content and soil p H)with low density700～925 plants/hm~2,medium density 1200～1525 plants/hm~2,high density 1750～2125 plants/hm~2 and the changing law of short-term tending years 0-5a,medium-term tending years 5-10a,long-term tending years>10a;through principal component analysis and correlation analysis and other mathematical methods,the internal mechanism of plant-soil interaction was analyzed,to provide theoretical basis and technical support for vegetation restoration aiming at improving ecosystem service function and system stability.The results of the study are as follows:(1)With the increase of density,tree height and canopy density showed an upward trend,and reached the peak at high density;DBH and crown width showed a decreasing trend and reached the minimum value at high density;tree height,canopy density,diameter at breast height and crown width were at there is a significant difference between the densities.Twenty-four species of plants were found in the density plot,belonging to 11 families and 22 genera.The dominant herbs and shrubs were Melica scabrosa and Prinsepia utilis.The specific leaf area showed an upward trend with the increase of density,and reached the peak at high density;the leaf water content first decreased and then increased,and reached the minimum value at medium density;the contents of total nitrogen,total phosphorus,organic carbon and dry matter of leaves increased first and then increased.A downward trend,peaking at medium densities.The soil water content first decreased and then increased with the increase of density,and reached the peak at high density.The soil bulk density of 0 cm-40 cm showed a decreasing trend with the increase of density,and reached the minimum value at high density.Soil total nitrogen,total phosphorus,available phosphorus,available potassium,organic carbon content and soil p H increased with density in different soil layers,and reached the peak at high density.(2)With the increase of tending years,tree height and canopy closure showed a trend of rising first and then decreasing,reaching a peak in the middle-term tending years;DBH and crown width showed an upward trend,reaching their peaks in the long-term tending years.Thirty plant species were found in the breeding years plot,belonging to 13 families and 28 genera.The dominant herbs and shrubs were mainly crabgrass(Digitaria sanguinalis)and flat nuclear wood(Prinsepia utilis).The specific leaf area,leaf water content,leaf total nitrogen and total phosphorus content increased first and then decreased with the increase of tending years,and peaked in the mid-term tending years;leaf organic carbon showed an upward trend and reached a peak in the long-term tending years;leaf dry matter the content showed a trend of first decreasing and then increasing,and reached a peak in the short-term nurturing years.Soil moisture and bulk density increased first and then decreased with the increase of tending years,and reached the peak in the middle tending years;soil total nitrogen,organic carbon and soil available phosphorus from 0 cm to 10 cm showed an upward trend,and soil total phosphorus,available phosphorus and p H increased.The total nitrogen,organic carbon,total phosphorus and available potassium in the 10 cm-40cm soil showed a unimodal curve trend,the soil p H first decreased and then increased,and the available phosphorus content increased.(3)Stepwise regression analysis showed that soil total phosphorus,available potassium,total nitrogen and organic carbon contents were the main soil factors affecting leaf functional traits at different densities.Correlation analysis showed that soil available phosphorus was the main soil factor affecting the diversity index of herb layer;soil p H,soil total phosphorus,soil available potassium,soil total nitrogen and organic carbon content all affected the shrub layer plant diversity.Principal component analysis showed that medium density(0.72),lower density(-0.76)and high density(0.04)were more conducive to the restoration and improvement of understory vegetation,leaf functional traits and soil nutrients.Stepwise regression analysis showed that soil p H,organic carbon and available phosphorus were the main soil factors affecting leaf functional traits in different tending years.Correlation analysis showed that soil available phosphorus and available potassium were the main soil factors affecting the diversity index of herb layer;soil p H,total nitrogen,total phosphorus,available phosphorus,available potassium and organic carbon content all affected the diversity index of shrub layer.Principal component analysis showed that long-term tending years(0.59)were more conducive to the restoration and improvement of understory vegetation,leaf functional traits and soil nutrients than medium-term tending years(0.24)and short-term tending years(-0.83).In the future afforestation in the loess hilly area,the medium density of1200～1525 plants/hm~2 should be selected for the planting of Robinia pseudoacacia plantation,and the long-term tending years(>10a)will have the best tending effect. |
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