Evaluation Of Soil And Water Conservation Function Of Robinia Pseudoacacia Plantations With Different Densities In Loess Region Of Western Shanxi Province

The structure determines the function.Stand density is an important indicator that affects stand structure,stability and its function.In order to explore the effect of the density of Robinia pseudoacacia forests on the soil and water conservation function in the loess region of Western Shanxi Province,field investigation and laboratory experiments were carried out on Robinia pseudoacacia plantations in Caijiachuan watershed of Jixian County,Shanxi Province.The functions of water conservation,soil conservation,biodiversity conservation,runoff interception and sediment reduction of Robinia pseudoacacia forests with six different densities of 475,900,1200,1575,1825 and 2350 plants/hm2 were studied.Topsis evaluation method was used to comprehensively evaluate the soil and water conservation function of Robinia pseudoacacia forests,in order to provide scientific guidance for the optimal allocation and management of Robinia pseudoacacia forest in this area,and to promote the sustainable development of forest vegetation construction.The results are as follows:(1)Effects of stand density on water conservation function of Robinia pseudoacacia forests.The maximum canopy water capacity of Robinia pseudoacacia forests with six different densities varied from 1.62t/hm2 to 5.33t/hm2,and that of 2350 plants/hm2 was the largest,reaching 5.33t/hm2.The total litter thickness of Robinia pseudoacacia forests with six different densities varied from 28.67mm to 54.33mm,and the total accumulation varied from 2.98t/hm2 to 10.65t/hm2.With the increase of stand density,the maximum water holding capacity and effective interception capacity of litter increased first and then decreased,and there were significant differences among some stands.The water holding capacity and interception capacity of litter were the strongest in stands with density of 1575 plants/hm2,37.01 t/hm2 and 23.64t/hm2,respectively.The water holding capacity as well as water absorption rate of the litter and soaking time are logarithmic function and power function respectively:Q=alnt+b,V=ktn.The medium-low density forest has a small soil bulk density and good aeration.The soil water storage capacity and infiltration performance were the best with a density of 1575 plants/hm2,and there was a power function relationship between the infiltration rate and the infiltration time:f=at-b(2)Effects of stand density on soil conservation function of Robinia pseudoacacia forests.Soil organic matter in medium and high density stands was generally more,and 1825 plant/hm2 was the largest,which was 15.070g/kg,and there were significant differences among some plots.The fluctuation range of total nitrogen content was 0.549?0.809g/kg.The ammonia nitrogen content ranged from 14.756mg/kg to 22.260mg/kg.Nitrogen content increased and then decreased with the increase of stand density,and reached the highest at 1575 plants/hm2.Total phosphorus content had no significant difference among different soil layers,and fluctuated among different densities.900 plant/hm2 was the highest,which was 0.676g/kg.The content of available phosphorus decreased with the increase of density,and the difference between different densities did not reach significant level.(3)Effects of stand density on biodiversity conservation function of Robinia pseudoacacia forests.There are 102 species of plants under Robinia pseudoacacia forests of six densities,belonging to 47 families and 88 genera.As the stand density increases,the dominant species transited from sunny plants to mesophytes and shady plants.The richness index of shrubs and herbs under Robinia pseudoacacia forests showed no obvious change rules,and the species richness of understory plant communities under medium and high density stands remained at a higher level.The species richness of herbaceous layer in Robinia pseudoacacia forest was more abundant than that in shrub layer.The Shannon-Wiener index and Simpson index of both showed a change process of first increasing and then decreasing with the density,and the diversity index of 1825 plants/hm2 was the highest.The evenness index of both shrub layer and herb layer were the highest at 1825 plants/hm2.(4)Effects of stand density on runoff and sediment yield of Robinia pseudoacacia forests.For Robinia pseudoacacia forests with six densities,the yield of runoff and sediment decreased with the increase of stand density,showing that 2350 plants/hm2<1825 plants/hm2<1575 plants/hm2<1200 plants/hm2<900 plants/hm2<475 plants/hm2.(5)Topsis method was used to evaluate the soil and water conservation function of Robinia pseudoacacia forests with six different densities.The results showed that the comprehensive order was 1575 plants/hm2,1825 plants/hm2,1200 plants/hm2,900 plants/hm2,2350 plants/hm2 and 475 plants/hm2.It suggests that the medium density stands(1200?1825 plants/hm2)have good soil and water conservation function,and too high or too low density is not conducive to giving full play to soil and water conservation.It is recommended to regulate this density range in the future.