Stand Density Indicators And Their Applications In Growth Models Of Chinese Fir Plantations

Stand density is an important index to reflect the number and crowding degree of individual trees in a unit area stand.Through stand density control,the trees grow in the best density index,and get the highest harvest and benefit.The stand density indices include number of trees per ha,stand density index SDI,Nilson stand density index SD,dominant high nutrient area ratio Z,relative density RD,and relative spacing Rs.Among the stand density indices,different indices reflect different density meanings.Therefore,it is of great significance to study the variation law of different stand density index tables and analyze the application value of stand density index in stand growth and harvest model for accurate management of stand density.Chinese fir（Cunninghamia lanceolata）is one of the most important fast-growing timber species in China.Based on the 28 years continuous observation data of the spacing trials of Chinese fir plantation in Shaowu,Fujian Province,I systematically analyzed and studied the variation law of stand density index and the application of density index in forest growth and yield model.The plots of Chinese fir were established in 1982.The random block design was adopted.There were five planting densities:A:2 m×3 m（1667 trees/hm²）,B:2 m×1.5 m（3333 trees/hm²）,C:2 m×1 m（5000 trees/hm²）,D:1 m×1.5 m（trees/hm²）,E:1 m×1 m（10000 trees/hm²）.Each density was repeated three times.The main results are listed as follows（1）In the sparse stands,the intensity of self-thinning was higher than the denser stands.The higher the planting density of forest stands is,the earlier the competition between forest trees appears,and the earlier the natural thinning of forest stands begins.Although the self-thinning started earlier in the higher planting density stands,the differences of self-thinning slopes between stands in lower and higher planting density were not significant.In addition,the variations on self-thinning were mainly due to planting density,not the site conditions.（2）The stands in the stands of plating density A1 2×3m（SI=16）and 2×1.5m B1（SI=14）did not appear the self-thinning,which indicated that in the stands with lower planting density,competition was not obvious leading to no self-thinning when the site was poor.Based on these results,it could be infered that there is a frontier planting density leading to self-thinning in a certain site.When the planting density was higher than the frontier density,the self-thinning will start.（3）With the increase of forest age,the stand density index SDI showed a rapid increasing trend.The higher the planting density was,the smaller the differences in SDI between different sites were.When the forest reached a certain age,the increase in SDI were stable.Due to strong self-thining in some forest stands,SDI decreased significantly.In terms of density indices Z,RD,and SD,under lower planting density,the site had a greater impact on Z,RD,and SD.Under higher plating density,the effects of site condition on Z,RD,and SD were small or negelctable,which indicated the variations on Z,RD and SD were mainly due to plating density.Under different planting densities and site conditions,the stand relative density RS showed the simlar change trend,that is,with the increase of the stand age,the RS decreased rapidly,and the decline rate slowed down when the stand age was 10 years.RS was different from other stand density indices,which shown the stable RS values in any stands.（4）Based on variable rate method,the annual tree growth model was developed using seven density indices respectively.Also the model without stand density index was developed for comparing.All the eight models performed well,with R² values more than 0.96.The models including the stand density indices were better than the model without stand density index.Among all the models with density indices,the model including stand density index（SDI）has the highest R² value,followed by the model with stand basal area Ba and Nilson density index.（4）The annual growth model of stand volume of Chinese fir including five different density indices respectively were developed based on the variable rate method,as well as the model without stand density index.The R² values of the models including density indices were all larger than 0.979,and the model performance was higher than that of the model without density index.Among the five models including stand density indices,the model including living number of trees per ha（N）had the largest value of R²,followed by the model with RS density.However,the two models are not feasible because of the parameter estimates of stand density index were not significant.The R² values of all models from high to low were:stand volume model including N（0.9799）,relative spacing RS（0.9799）,stand density index SDI（0.9794）,dominant high nutrient area ratio Z（0.9793）,Nilson density index model（0.9790）and without density index（0.9728）.Accounting for significance of the parameter estimate,the model with stand density index SDI performed the best in the stand volume model of Chinese fir.In addition,stand volume in stands of planting density(1667-3333 trees·hm^-2)was larger than the denser stands(5000-10000 trees·hm^-2),which indicated that the optial planting density of Chinese fir was leass than3000 t trees·hm^-2.