Site Classification And Initial Planting Density Analysis Of Middle And Large Diameter Timber Of Eucalyptus Urophylla × E.grandis

Eucalyptus urophylla×E.grandis has become a strategic afforestation strain in South China and which has achieved remarkable results in alleviating the contradiction between wood supply and demand in China,due to its rapid growth,excellent wood properties and diversified uses.Site conditions can significantly affect stand productivity,so the purpose of suitable land and trees can be realized by site classification and quality evaluation.However,there are few studies on site classification of Eucalyptus urophylla×E.grandis cultivation areas,and the analysis of initial planting density of middle and large diameter timber based on site classification results has not been reported at present.The planting of middle and large diameter timber of Eucalyptus urophylla×E.grandis is lacking of scientific basis,and the site selection,stand productivity and wood quality are uneven.In this study,Eucalyptus urophylla×E.grandis plantations in 7.5 years old were selected as the experimental sites.Growth data,climatic and topographic data were collected,and soil chemical and physical properties were analyzed.The relationships between site factors and tree growth were analyzed by principal component analysis and quantitative theory?.The productivity levels and site types were classified by cluster analysis.Based on the results of site classification,Eucalyptus urophylla×E.grandis plantation in 16 years old with three initial planting densities of 1 250,883 and 667tree·hm^-2 within site type of high yield group was selected as an example to analyze tree growth and yield of different diameter timber between different initial planting density treatments.The results could provide a scientific basis for site selection,especially for the cultivation of middle and large diameter timber of Eucalyptus urophylla×E.grandis in Guangxi、Guangdong and Fujian province（or region）.The main results were summarized as follows:（1）The multiple correlation coefficient of quantity model fitted by the selected eight site factors including slope direction,slope,slope position,altitude,soil nutrient grades,soil texture,soil density and annual sunshine hours was 0.956,indicating a highly significant correlation （P<0.01）,so the quantity model could be used to effectively predict site productivity potential.In the model,annual sunshine hours,altitude and soil texture were highly significant partial correlation with stand dominant height（t>2.457）,and the total score of the three factors accounted for 62.75%,so annual sunshine hours,altitude and soil texture were the main factors affecting the dominant height of Eucalyptus urophylla×E.grandis.（2）Combining the quantitative regression model with the site factors of the survey sites,the predicted values of dominant height of Eucalyptus urophylla×E.grandis were obtained and site productivity levels were divided into three groups,and ten site types were divided based on annual sunshine hours,altitude and soil texture.The dominant height ranked from high yield to low yield group were 27.16 m,24.19～25.45 m and 21.77～23.40 m,respectively.（3）The contents of soil organic matter,total P,available Cu,available Mg and clay in high yield group were significantly higher than those in middle and low yield groups,and the content of available Mn in high yield group was significantly higher than that in low yield group.Additionally,the contents of total P,available Cu,available Mn,available Mg,organic matter and clay in soil were significantly positive correlation with dominant height of Eucalyptus urophylla×E.grandis,respectively（P<0.05）.（4）Within the area of site type of high yield group,the density 883 tree·hm^-2 had the largest middle diameter timber yield and the largest average annual growth of middle diameter timber during the growth period of 7～16 years.Middle diameter timber yield of density 883tree·hm^-2 reached the maximum at 16 year,which was 331.71 m³·hm^-2.The average annual growth of middle diameter timber yield of density 883 tree·hm^-2 reached the maximum at 9year,which was 25.49 m³·hm^-2·a^-1.The quantitative mature age of density 883 tree·hm^-2 was 9years and the large diameter timber of which appeared at 16 year.