Study On The Change Of Site Productivity Of Pinus Tabulaeformis Forest At Different Developmental Stages In Pingquan,Hebei Province

In order to explore the maintenance of site productivity of Pinus tabulaeformis forest,the chronosequence approach was applied to select Plantations of Pinus tabulaeformis at different developmental stages(middle age forest of 32 years,near-mature forest of 40 years and mature forest of 53 years)and natural regeneration forests at different ages after clearcutting(5 years,10 years and 24 years after clearcutting)and wasteland in Pingquan area of Hebei Province as research objects,the conditions of so1l physical,soil chemical properties,soil enzyme activities,forest growth,stand volume,litter storage,litter water-holding capacity and understory plant diversity were investigated to explore the variation of soil productivity in the process of plant community succession before and after clearcutting of Pinus tabulaeformis plantation.This study aims to clarify the situation of soil productivity in Pinus tabulaeformis forest under the existing business model,and to provide a basis for the establishment of a rational forest culitivation system for Pinus tabulaeformis plantation,it has important theoretical and practical significance for the maintenance of long-term productivity and sustainable forest management.The main results and conclusions of the study are as follows:(1)The soil physical properties,nutrient content and enzyme activity of the forest land from the middle-aged forest to the mature forest stage were significantly improved.The soil quality of the forest land has sustainable improvement in the early stage after clearcutting,and the soil physical properties,nutrient content and enzyme activity of forest land declined to a certain extent from 5 to 24 years after clearcutting due to the heavy consumption of nutrients by individual competition.However,the soil quality of natural regeneration forest of 24 years after clearcutting was still better than that of middle-aged forest and near-mature forest owing to the nutrient accumulation in early age of clearcutting.Compared with abandoned land,the soil quality of natural regeneration forest was significantly improved,which indicated that the natural vegetation restoration of Pinus tabulaeformis plantation after clearcutting was beneficial to soil quality as a whole.(2)The maximum water-holding rate of litter layer in forests at different developmental stages was not much different.The litter from middle-aged forest to mature forest gradually accumulated,and the water-holding ability of litter in forest land increased gradually.The accumulation amount of litter of mature forest litter layer reached 5.02 t·hm-2,and the maximum water-holding capacity reached 10.80 t·hm-2.However,the biomass of litter in the forest land reduced remarkable after clearcutting,therefore,the water-holding capacity of litter decreased significantly.In 24 years after clearcutting,the surface litter re-accumulated,the accumulation reached 4.14 t·hm-2,nearly 32 years of plantation,the water-holding capacity of litter layer also increased to 9.18 t·hm-2.(3)Compositae and Rosaceae were the main understory plants in Pinus tabulaeformis plantation and natural regeneration after clearcutting.There were 21 shrubs and 66 herbs in 7 plots.From middle-aged forests to mature forests,the understory species diversity increased gradually.The number of understory species in the early stage after clearcutting was obviously lower than that of mature forests,and plant species rised gradually by age,and the diversity index reached the maximum value in 10 years after cutting,then the subsequent decline were associated with interspecific and intraspecific individual competition.Natural vegetation restoration after clearcutting was conducive to the improvement of biodiversity,and good soil quality and development of understory vegetation can promote each other.(4)There was no significant difference in the growth process between Pinus tabulaeformis plantation and natural regeneration forest after clearcutting in 0-20 years.The tree growth of natural regeneration forest at the age of 20 years was slightly higher than that of the plantation.The stand volume per hectare of 32-year,40-year and 53-year-old plantations were 114.30 m3 ·hm-2,113.06 m3·hm-2 and 130.27 m3·hm-2,respectively.However,the volume of natural regeneration forests in the 24 years after cutting was only 46.28 m3·hm-2.From the perspective of maintaining the long-term productivity of the stand,it was suggested that artificial replant should be implemented after natural thinning,and regeneration seedlings of Pinus tabulaeformis should be thinned artificially to reduce density in time.(5)The vegetation restoration by natural regeneration after clearcutting has achieved self-sustainment of soil productivity.Natural thinning began to occur in the forest due to excessive regeneration density after 10 years of clearcutting,it would be suggested that artificial tending should be carried out in time,some natural regeneration should be removed,and Pinus tabolaeformis and broad-leaved tree species should be replanted in the over-sparse stand to improve soil quality,the stability of forest structure and maintain the long-term productivity of the plantation.