| The area of Bashang in Fengning of Hebei Province,a poverty-stricken county in China,is a semi-arid area with fragile ecological environment and underdeveloped economy.In this area,to build ecological and economical vegetation is one of the ways to protect ecological environment and develop economy.However,the development of economic forest and fruit industry at Bashang is restricted due to the limitation of local climatic conditions,and yet there is potential for the seedling industry development needed by ecological forestry.Pinus sylvestris var.mongolica is the main afforestation tree in the north of China.Over the years,the afforestation of Pinus sylvestris var.mongolica has achieved remarkable effects in both scientific research and engineering construction,besides the demand of seedlings is increasing.It is a new method to realize the win-win of ecological and economic benefits of Pinus sylvestris var.mongolica forest at Bashang to adopt dynamic adjustment on planting density with the tree growth,which could both realize forest function for windbreak and sand fixation and economic benefits.Therefore,a survey in this paper was conducted on the 9-age Pinus sylvestris var.mongolica forest of five densities(D1-1665 plants/hm2,D2-4995 plants/hm2,D3-7980 plants/hm2,D4-9990 plants/hm2,D5-19980 plants/hm2)and the surrounding unforested area which was control treatment(CK).Then we compared and analyzed the forest growth,understory species diversity,soil physical and chemical properties,and economic benefit among different treatments,which was applied to comprehensively evaluate the Pinus sylvestris var.mongolica forest ecological and economic benefits of different densities and then identified the optimum planting density for the area.This could provide technical supports for the artificial afforestation and sustainable management of Pinus sylvestris var.mongolica forest at Bashang.The results are as follows:(1)The 9-age Pinus sylvestris var.mongolica forest had a significant influence on the specific composition and distribution of understory plants.The dominant species of D1,D2,D3,D4 and D5 were Ixeris polycephala Cass.,Ixeris polycephala Cass.,Polygonum divaricatum L.,Equisetum arvense L.and Agastache rugosa(Fisch.et Mey.)O.and the dominant species of CK were Arthraxon hispidus(Thunb.)Makino.In Pinus sylvestris var.mongolica forest plots,the Simpson's diversity index(D)and Pielou evenness index(J)increased with the increased afforestation density,and the D5 was the highest.The Shannon-Wiener diversity index(H)increased firstly,then decreased and finally increased again with the increase of density,among which the D3 was the highest.Margalef richness index(DMG)showed no obvious change with the changing density of afforestation,and D5 was the highest.CK species diversity index was all higher than the afforestation sample plot,and afforestation reduced the diversity of understory herbage plant species.Compared with those afforestation plots,the diversity indexes of CK were higher,so it could be concluded that the afforestation decreased the species diversity of underneath herbage species.(2)Considering both the ecological and economic benefits,the optimum density for the growth of the height,ground diameter and canopy of the 9-age Pinus sylvestris var.mongolica forest was 9990 plants/hm2(D4).When the density was lower,the growth was poor;when the density was higher and reached 19980 plants/hm2,the growth was also restrained.Therefore,it is essential to adjust density and consider nursery stock market.(3)The soil bulk density of all plots showed no obvious trend with the change of density,while increased with the increase of soil depth.The average soil bulk density was 1.2-1.4 g/cm3.The average total soil porosity increased gradually with the increase of afforestation density,and all the sample plots reached more than 40%.The average total soil porosity of D2 was the largest,and up to 45.71%.The maximum average natural moisture content of 0-60 cm soil layer was D2,and up to 21.48%.The maximum water capacity increased with the increase of afforestation density,and the maximum water capacity of D5 was the highest,up to 547.21 t/hm2.The capillary water capacity showed no obvious change with the change of density and soil depth.In general,different afforestation densities of Pinus sylvestris var.mongolica had certain improvement on soil pore structure,water characteristics and other physical properties,and the improvement effect of D2 sample plots was relatively better.(4)9-age Pinus sylvestris var.mongolica forest had a certain reduction effect on soil pH,and pH was significantly correlated with ground diameter,canopy and dry biomass.But the difference between different densities was not significant(P>0.05).As the density of afforestation increased,the content of soil organic matter first increased and then decreased,and the D2 was the largest.Except the total potassium content of soil,all kinds of nutrient elements showed an increasing trend first and then decreased with the increase of afforestation density.In general,the Pinus sylvestris var.mongolica forest with different afforestation densities had improving effects on soil nutrients,and D2 had the best effect.(5)With the increased density of 9-age Pinus sylvestris var.mongolica forest,total forestation cost,gross output,total profit and average annual profit of afforestation increased,among which the ratio of production to investment and capital utilization rate of D4?D5 were the highest.The ratio of production to investment and capital utilization rate could reach up to 2.46 and 146.38 yuan/100 yuan,respectively.Dynamic economic benefit trend was consistent with static economic benefit.Therefore,in the early stage of afforestation,moderate planting density and the seedlings sale should be taken into account together to obtain economic benefits. |
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