| In this paper,the Larix olgensis plantation in Mengjiagang Forest Farm of Heilongjiang Province is used as the research object.Using 1434 knot data of 50 Larix olgensis trees,based on the poisson distribution,the glimmix module in SAS9.4 software is used to establish the optimal basic model of the number of annual rings in the three stages of the branches which was finally forming the knot,the three stages include branches from formation to cessation of growth(BC),cessation of growth to death(CD),and death to complete occlusion(DO).The best hybrid model is selected by calculating the corresponding indicators.After the optimal basic model is obtained,by introducing random effects,a hybrid model based on tree effects and a hybrid model based on plot effects are established,and the optimal hybrid model is selected by calculating the corresponding indicators.(1)The number of annual rings of the BC period of the branches shows that:The higher the knot height(KH)and the greater the knot diameter(KD),the greater the number of annual rings of the branches in the BC period.The higher the stand density,the fewer the annual rings in the BC period.There are obvious differences in the number of annual rings of the branches between different plots and between different forest individuals.The fitting effect of the mixed effect model under the two random effects is better than the basic model.The adjusted coefficient of the optimal mixed effect model based on the tree effect is 0.76,the root mean square error is 0.1494,and the average absolute error is 0.1094;The optimal mixed effect model based on the plot effect has an adjusted determination coefficient of 0.70,a root mean square error of 0.1802,and an average absolute error of 0.1367.(2)The number of annual rings in the CD period of the branches shows that:The number of annual rings in the CD period of the branches decreases as the height of the knots increases.Among the stands with more than 700 plants per hectare or less than 300 plants per hectare,the number of missing-rings is the least.,After considering the tree effect,the optimal mixed effect model has an adjusted coefficient of 0.66,a root mean square error of 0.2101,and an average absolute error of 0.1713;In considering the optimal mixed effect model of the plot effect,the adjusted coefficient of the model is 0.55,the root mean square error is 0.2541,and the average absolute error is 0.2000.The fitting effect of the two mixed effect models is better than the basic model,and the mixed effect model based on the tree effect has the best fitting effect.(3)The number of annual rings in the DO period of the branches shows that:The diameter of the knot at the upper position is larger,and the time of occlusion(DO)after branch death is positively related to the diameter of the knot,that is,the higher the height of the branch,the larger the base diameter of the branch,and the longer the occlusion takes place.At the same time,with the increase of the insertion angle of the branches,the longer the occlusion time of the branches after death.Under different forest density conditions,the annual ring number of branches in the DO period is the smallest within the range of 760-500 per hectare.There are significant differences in the number of annual rings in the DO period between different forest individuals and between different plots.The optimal mixed effect model considering the tree effect,the adjusted coefficient of the model is 0.74,the root mean square error is 0.2374,and the average absolute deviation is 0.1715;Considering the optimal mixed effect model in the case of plot effects,the adjusted coefficient of the model is 0.64,the root mean square error is 0.3307,and the average absolute deviation is 0.2595.All statistics of the mixed effect model are better than the basic model,of which the mixed model based on the tree effect has the best fit result..From the research results,it can be obtained that the annual ring number of branches in each period is more affected by the tree effect than by the plot effect,that is,the difference between individual trees is greater.The mixed effects model with random effects corrects this difference and makes the model's prediction ability effectively improved.By grasping the law of the annual ring number of branches in different growth periods,you can grasp the growth and development stages of existing branches by measuring some external attributes of the branches during the forestation process.This will provide theoretical support for artificial pruning and make the forest management more scientific and standardized.In the production and management,the time of the BC period in which the branches are located can be reduced through reasonable close planting in the early stage.After the branches stop growing,thinning and artificial pruning are carried out in time to reduce the time of the CD and DO periods of the branches,thereby reducing the volume of the knots to achieve the purpose of cultivating no knots. |
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