Density Control Techniques And Successive Planting Effects Of Poplar Plantations In Jianghan Plain

Jianghan Plain is an important high-yield poplar plantation basis in southern China.For a long time, however,the poplar plantations in Jianghan Plain were always managed extensively with no clear directive cultivation targets and irrational cultivation densities,so their growth potential playing unfully. At the same time,some ecological and economic problems had become more obvious which included poplar growing slowly,biomass decreasing,productivity dropping,land capability declining and pest taking place frequently resulted by pure stand managing widely and long-term successive planting. According to management situations,questions and demands of poplar plantation in Jianghan Plain,and applying to the methods of field survey,laboratory analysis,random sampling and space replacing time, density control techniques of poplar plantation were researched symatically which included biomass and productivity,felling age,cultivation period and the optimal management density under different density conditions of high density(＞1000 strains.hm^-2),middle density(1000～500 strains.hm^-2)and low density(＜500 strains.hm^-2),and successive planting effects on soil physical,chemical and biological characterristics and growth of poplar plantation were also studies initially in order to applying poplar plantation cultivation potential,maintaining soil fertility,achieving plantation sustainable management and promoting regional economy development.The main results were as follows:1.The biomass of different parts of the stand changed diversely with different stand densitities.In stands with middle,low density,it was the regular pattern that the biomass of stem＞branch＞bark＞root＞leaf.However,there was no obvious regularity in stands with high density.The biomass ratio variation extents of all parts had the same regularity of leaf(7.48%)＞bark(3.53%)＞stem(2.51%)＞root(0.97%)＞branch(0.55%)in regardless of densities.Stand densities also had obvious positive influences on the net productivities,which meaned that net productivities of stands and all parts all decreased with stand densities decreasing.The output of stem was biggest among all parts of stands, which was 5.78～15.73t.hm^-2.a^-1,5.03～5.69 t.hm^-2.a^-1and 2.58～3.74 t.hm^-2.a^-1respectively in stands of high,middle and low densities.2.For stands with the same ages,the optimal management densities decreased with site index increasing.Space between plants should be larger with less poplar remained with better stand conditions; it should be smaller with more poplars remained with general stand conditions.It was suitable for 3.0m×5.0m of space between plants of poplar plantation in most areas of Jianghan plain.3.Harvest cutting ages and quantitative maturity ages of man-made poplar forests with different stand densities showed a general trend of becoming larger with decreasing stand densities;on the other hand,the maximum annual mean net and stand stock per area showed the larger trend with increasing stand densities.The cultivating periods were 5～7 years of high density plantations for paper-making and particle boards,8～10 years of middle density plantations for core boards and furniture,and 11～12 years of low density plantations for plywoods.4.It was not suitable for cultivation of large-diameter timber at every type different site and density. At typeⅠandⅡsite,it was better combination selection to more ideally cultivate large,middle, small-diameter timbers when using categories of middle-density and low-density;however,there were not very large difference among various categories of density at typeⅢsite.The everage cultivating periods were 5～7 years for small-density timbers,7～10 years for middle-density timbers,and 10～13 years for large-density timbers.5.Under successive plantation,the soil became compacter,soil porosity and water-holding capacity became smaller,and the stabilish of soil aggregate became worse.Grey evaluate showed that the relating value of soil physical characteristics in second generation forests was 14.80%less than that in first generation forests,which meaned soil physical conditions worsen.In second generation forests macroelements such as N,P,K decreased by 2.88%～12.20%,and the soil N element suffered the most to successive plantation followed by K and P.it also accounted for that Soil microelements decreased by 3.52%～53.37%with the order of severity of Zn,Cu,Fe,Mn.The quantities of total microbe numbers decreased by 56.05%;and the activities of soil phosphataese,urease,sucrase,proteinase,catalase decreased by 1.92%～50.00%,while the activity of soil polyphenol oxidase increased by 100.00%.6.Comparied with the first generation forests,the influence of successive plantation on poplar growth was not significant in the previous four years,the DBH,height,volume and individual biomass decreased by 0.70%～12.05%,7.28%～20.41%,17.78%～27.83%and 7.26%～32.42%respectively in second generation forests from the fifth year to the ninth year.It showed that the productivity of first generation man-made poplar forest was higher than that of second generation man-made poplar forest by the mothod of AHP.