Study On Crown Characteristics And Individual Crown Model For Aerially Seeded Pinus Massoniana Plantations

The crown is the main place for photosynthesis,which has an important influence on productivity and ecological benefits.The size and growth condition of the crown can reflect the growth vitality of the trees,which are closely related to forest density.Based on the research area of Xingguo County in Jiangxi Province,this paper took the aerially seeded Pinus massoniana plantations as the research object.Four groups include group I(900-1500 trees·hm^-2),group II(1500-2100 trees·hm^-2),group III(2100-2700 trees·hm^-2),and group IV(2700-3300 trees·hm^-2)were set up according to the sample plots density.The effects of density on forest growth,crown width and crown biomass were analyzed.Furthermore,the individual crown prediction model was constructed by using mixed effect model.Based on the crown model combined with the nutritional area method and the tree crown overlap rate,the correction maximum density of different diameter forest was calculated.A reasonable operating density table was compiled by using the half-peak width formula.The results of the study can provide a reference basis for the scientific operation of the aerially seeded Pinus massoniana in the research area.The main results and conclusions were as follows:（1）The average diameter at breast height and average tree height of the stand decreased as the density increased.The order of the stand volume of density groups was as follows:density group II>density group III>density group I>density group IV.The diameter distribution of density groups of forest strains was normally biased,the peak of the forest diameter distribution gradually shifted to the left with the increase of density,and the number of path-level trees and their proportion increased with the increase of density.（2）The average crown,crown length,crown surface area and crown volume of the stand were represented as density group I>density group II>density group III>density group IV.Except for the difference between density group II and density group III,the difference of average crown width was not significant（P>0.05）.There were significant differences among other density groups（P<0.05）.The average crown length of density group I was significantly higher than that of density group III and density group IV（P<0.05）.There were no significant differences among other density groups（P>0.05）.The crown surface area and crown volume were not significantly different between density group II and density group III,density group III and density group IV（P>0.05）.There were significant differences between other density groups（P<0.05）.The crown length rate and crown productivity increased with the increase of density,and there was no significant difference in crown length rate between different density groups（P>0.05）.The crown productivity of density group I was significantly higher than that of other density groups（P<0.05）,density group II was significantly higher than that of density group IV（P<0.05）.There was no significant difference between other density groups（P>0.05）.（3）The average branch,leaf biomass and crown biomass of individual trees in each density group decreased with the increase of density group,and there were significant differences between different density groups（P<0.05）.Forest branching,leaf biomass and crown biomass showed a tendency of increase first and then decrease with the increase of density group,and density group II was the highest.Except leaf biomass was significantly lower than other density groups（P<0.05）in density group I.There was no significant differences between other density groups（P>0.05）.（4）The basic model in the form of a power function has a good fit and predict effect on the crown,and the accuracy of the generalized model with the under branch height and density groups was better than the basic model.In the meantime,the accuracy of the model can significantly improve by considering the random effect.The specific performance was as follows:generalized mixed model（R²=0.6508,RMSE=0.6246,MAE=0.4928）>basic mixed model（R²=0.6338,RMSE=0.6396,MAE=0.5028）>generalized model（R²=0.6010,RMSE=0.6676,MAE=0.5318）>basic model（R²=0.5693,RMSE=0.6981,MAE=0.5482）.The heteroscedasticity in the mixed model was best eliminated by the exponential function.Both the explanatory variables and random effect can effectively improve the accuracy of the individual crown prediction model.（5）The maximum density of forest fractions was ranged from 675 to 26119 trees·hm^-2.The density operating range which calculated by the half-peak width formula was ranged from 0.5 to 0.82.The reasonable management density for Pinus massoniana plantations with a site index of 14 was compiled,which can be consulted according to the forest average diameter at breast height and density management degrees.