| The initial planting density is the quantitative basis of forest structure and the main factor controlled artificially in the process of forest cultivation,it directly affects the productivity of the plantation.Based on the 36-year positioning observation data of Chinese fir?Cunninghamia lanceolata?Lamb?.Hook.?plantations with five planting densities and four site index classes in Daqingshan Mountains,Guangxi,this paper systematically analyzed the growth and development of main stand factors and timber assortment structure of Chinese fir plantation affected by density,site and age during a complete rotation period at three levels of stand,diameter class and individual tree for the first time.The suitable site quality,initial planting density and cultivation period of different timber grade were clarified.It provided a reference for the theory and practice of directional cultivation of Chinese fir plantation.Main conclusions were drawn as follows:1.The natural thinning intensity of Chinese fir stands increased with the increasing stand age,density and site index;the larger the planting density and site index were,the earlier the beginning and peak age of natural thinning of Chinese fir stands were;the thinning ratio was positively correlated with the planting density and site index,and was affected greatly by the planting density.It is found that age of 11 to 16 and 30 to 36 showed the most concentrated periods of natural thinning of Chinese fir stands in the south subtropical area,and its strong self-organization and self-adaptability.2.Mean stand height,diameter at breast height?DBH?,crown width,maximum diameter of volume distribution and individual volume for Chinese fir stand were positively correlated with site index and stand age,and negatively correlated with planting density;the under branch height of stand was positively correlated with planting density,site index and stand age;the ratio of height to diameter was negatively correlated with planting density and positively correlated with site index,presenting a dynamic change that it decreased firstly and then increased.The breast height form factor ranged from 0.50 to 0.68,which was positively correlated with the planting density and negatively correlated with the site index.The influence of the site index on the breast height form factor was greater than that of planting density,indicating a change that it decreased sharply at first and then slowly with the increase of stand age.Dominant height of stand mainly depended on site indices,was not significantly affected by planting densities.3.The stand spacing index and crown diameter/DBH ratio decreased while the stand density index?SDI?and degree of stocking increased with the increasing stand age,density and site index,and increased rapidly at the young age;the density of stocking increased with increasing density and site index.Crown diameter/plant spacing ratio decreased with the growth of stand age,and increased with the increase of site index and density at young age.The stocking density of stands with relatively low A and B densities increased with stand age,while that of stands with relatively high C,D and E densities decreased with stand age.The crown competition factor?CCF?was positively correlated with density.The ratio of dominant height to nutrient area was up to the peak firstly and then had a tendency to decrease with the stand age;the greater the density was,the earlier the stand age reached the peak,and the higher the peak value became.Stand growth space index?GSI?and growth space competition index?GSCI?showed a slow decline trend with the growth of stand age,the higher the density was,the smaller the GSI and GSCI were,and the less affected by the site index.4.Stand basal area?SBA?was positively correlated with stand age,planting density and site index.When stand age was over age of 20,the SBA tended to be stable.At the same site index class,the difference between stands gradually reduced with age and tended to be consistent.Both site and density had obvious effects on SBA,its current annual increment?CAI?,mean annual increment?MAI?,growth rate and quantitative maturity age.The quantitative maturity age of SBA came earlier with the increasing planting density and site index.The site had more influence on the SBA with lower planting densities.The change law of the basal area of mortalities was consistent with the law of self-thinning.The fast-growing period of SBA was also the period with large basal area of mortalities.The higher the stand density in the young age was,the larger the basal area of mortalities would be.5.The proportion of wood yield of dead standing trees to gross wood yield increased with the increasing planting density and site index.The total volume of the stands at the middle and young stages was nearly linearly positively correlated with stand age,but increased slowly at the later stage;the higher the planting density was,the larger the total volume was.With the maximum volume growth as a cultivation objective,Density C(5,000 trees·hm-2)in the intermediate planting density at the 16-index site was the most suitable afforestation density.Site quality obviously affected the volume growth rate of young and middle stages and the increment of nearly mature and mature stages.The stand with higher site index had faster early growth rate and later total increment,and the site effect was affected by the planting density.Site quality had a greater effect on the stand with the lower planting density,indicating that stand growth had obvious interaction between the sites and planting densities.6.Both planting density and site had the same effect on the volume of living trees as the total volume of the stand.Volume of living trees increased rapidly before age of 15,but slowed down after that.The greater the planting density was,the more the increment of stand volume was,especially at an early stage;the greater the site index was,the greater the increment of stand volume was.The peak of CAI of the stand came earlier with the density increasing.The first thinning should be completed before age of 10,but not more than age of 11 at the latest,and the last thinning should be completed before age of 14.The main cutting age of Chinese fir Plantation in the south subtropical area should not be earlier than age of 19,and the lower the afforestation density,the later the main cutting age of Chinese fir plantation.For example,the main cutting age of the stand at 3,333 trees per hectare against 1,667 trees per hectare could be prolonged to age of 25,and the Chinese fir Plantation in the south subtropical area have an earlier cutting age compared with the ones in the mid-subtropical areas.7.The wood assortment in the same diameter class showed a proportion of commercial timber>slash?waste?>firewood.The stand's ratio of merchantable volume by timber assortment could increase with planting density decreasing and site index increasing,while the merchantable volume increased with the increasing planting density and site index.Both less planting density and greater site index were the more favorable for the formation of medium and large diameter timber,and each timber assortment could reach the peak of its volume percentage earlier against that of its tree number percentage.Site conditions played a decisive role in the formation of wood assortment.There was an obvious interaction between the site and planting density in the formation of timber assortment structure.For the Chinese fir plantation in the southern area,the short-term management of the 10-index site is only suitable for cultivating small-diameter timber and small bar timber,but the prolonged management period is for cultivating small-diameter timber;the 14–index site is suitable for cultivating medium-and small-diameter timber.The planting density should be lower when medium-diameter timber is cultivated as the first wood assortment,while the planting density is slightly greater when small-diameter timber is cultivated as the first assortment.The 16-index site is appropriate for cultivating medium-diameter timber,and there is a greater proportion of medium-diameter timber in medium-low density stands.The stand at the lowest A-density can produce about 20%large diameter timber in long-term management;the 18-index site is suitable for cultivating large diameter timber,but the planting density should be controlled at A-density,and the volume ratio of large diameter timber can be close to 35%when the rotation period is prolonged to about age of 36.The site quality has a decisive effect on the formation of timber assortment,while the planting density can also be a key limiting factor in cultivating high-level wood assortment. |
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