Studies On The Effect Of Planting Density And Artificial Pruning On Young And Middle Aged Betula Alnoides Plantation

With the development of modern society and economics, the living standard and life taste of people have been greatly improved. Green home has been the focus in daily life. High-quality wood furniture and wood products with its unique ecological and cultural functions have become a major component of green home and an important symbol of life taste. This leads to a great market demand on high-quality large-sized clear wood. For producing high quality timber efficiently and fulfilling the domestic timber market, a density and a pruning experimental forests were established on the basis of a kind of superior indigenous broad leaved tree species – Betula alnoides Buch-Ham. ex D. Don. We investigated and explored the effects of planting density and pruning on tree growth, stem form index, the effect of pruning intensity on the attack of a trunk border- Arbela(Metarbelidae). Linear mixed effect models were built to explore the effect of planting density on branch development, branch occlusion and knot attributes, compare the differences of branch occlusion, knot attribiutes and their main influencing factors between artificial pruning and natural shedding. The main results of the present study were as follows:1. Growth performance and stem form of 11-year-old Betula alnoides were investigated in the density experimental forests. The results showed that the mean tree height, height to crown base, ratio of crown and stem diameter and the ratio of tree height and stem diameter did not vary significantly under the five planting density treatments(3333, 1667, 1111, 833 and 500 stems per hectare), but high planting density could significantly reduce the mean diameter at breast height, crown diameter, crown length, crown ratio and the individual volume. Furthermore, trees with small diameter class(10 cm ? DBH < 15cm) increased and trees with large diameter class(20 cm ? DBH < 25 cm) decreased significantly with the increasing planting density. However, the stand volume increased with the increment of planting density and reach the maximum at the planting density of 1667 sph, then the stand volume decreased. The artificial form factor and artificial form quotient had no significant correlation with planting density. The mean artificial form factor is about 0.45, the mean artificial form quotient is about 0.70. For the stem taper, though no significant difference was found among the five planting density treatments, there was an decreasing trend with the increasing planting density.2. Stem analysis was used to assess the effect of planting density on growth process of Betula alnoides. The results indicated that the growth processes of diameter at breast height(DBH), tree height and individual volume were similar. They all showed a rise and then descending trend after planted. However, the fast growing period of DBH(annual increment >1.5 cm) and individual volume(annual increment>0.015 m3) trended to increase with decreasement of planting density, and after the annual increment reached the maximum, the decreasing trends were more quickly in higher planting density treatments. The DBH growth commenced to decrease significantly since the sixth year after planted under the 3333 sph treatment, while it decreased mostly since the eighth year under other treatments, these indicated that thinning should be conducted at the sixth year for 3333 treatment, and at the eighth year for 1667 and 1111 sph treatments, while for other treatments, it was delayed with the decrease of density. The current and mean annual increment curves of tree height all crossed when 5-6 years old, and did not reach quantity mature when 11 years old for all density treatments, while remarkable differences of the current annual increment of individual tree volume were seen among treatments.3. Branches in crown were closely correlated with planting density, the mean diameter of all branches, live branches, the maximum live branch and the proportion of large branches(>30 mm) all tended to decrease significantly with the increment of planting density. Though no significant difference was found among the five treatments on the mean diameter of dead branches and the maximum dead branch, they also decreased with the increasing planting density. High planting density had no significant influence on the number of branches and branch insertion angle. The number of all branches in crwon was about 50 and the number of live and dead branches was almost equal. The branch insertion angle of all, dead and live However, the insertion angle of dead branches were larger than that of live branches. In addition, the branch density increased significantly with the increasing planting density and extreme high(3333 sph) or low(500 sph) planting density treatments would also increase the branch mortality.4. Knot dissection and linear mixed effect models were used to explore the effect of planting density on branch occlusion and knot attributes, find out the main factors influencing branch occlusion and knot attributes and predict them. The results indicated that the mean branch occlusion time under different planting density treatments was about 2 years. But it tended to increase significantly with the decrement of planting density. The mean occluded branch diameter, radius of knots and branch insertion angle also decreased significantly with increasing planting density. Planting with high densities significantly reduced the frequency of thick occluded branches(diameter ? 20 mm) while increasing the frequency of small ones(diameter < 10 mm). The branch insertion angle maily distributed at the range from 40° to 70° and no significant difference was found between treatments for each branch angle level. The results of linear mixed models showed that branch diameter was the major factor influencing branch occlusion time, radius of dead portion of knot, total radius of knot and branch insertion angle. Branch occlusion time, radius of knot increased significantly with the increasing branch diameter. Branch insertion angle decreased with the increasing branch diameter, but the decreasing trend became slow when the branch diameter was larger than 20 mm. In addition, branch occlusion time was also significantly negatively correlated with stem diameter growth rate during branch occlusion.5. A pruning trail was conducted with series of pruning heights in five-year-old plantations of Betula alnoides, so as to assess the effect of pruning heights on growth performances, stem form and attack of trunk border on Betula alnoide. The results indicated that the increment of tree height and volume did not vary significantly under different treatments of pruning heights in three years after pruning. While DBH growth decreased apparently with increasement of pruning height and significant difference was seen between the control and other pruning treatments late in the first year after pruning, and there was no remarkable difference between them in the third year. In line with planting density, pruning also had no significant effect on artificial form quotient and stem taper index. Although the pruning height had no remarkable influence on percentage of trees attacked by trunk borer and number of insect hole in the damaged trees at stand level. The percentage of damaged pruned sections and number of insect hole in the damaged pruned section was significantly lower than those of the corresponding section of the control. Reasonable pruning heights such as 6 m and 7 m could lift the position of borer holes on the stem more than 2 m. Although pruning could not reduce remarkably the percentage of trees attacked by trunk borer in the stand level, pruning with reasonable heights could significantly reduce the damage of trunk border on pruning section.6. Knot dissection and linear mixed effect models were carried out on knots of artificial pruning sections and the corresponding sections of natural shedding to compare the difference of branch occlusion, knot attributes as well as the influencing factors. The results showed that artificial pruning is useful for shortening branch occlusion time, reducing the size of knots. While no significant difference was found for occluded branch diameter, branch insertion angle and the length of timber discoloration and decay at knot position between artificial pruning and naural shedding. The main factor influencing branch occlusion and knot attributes after artificial pruning is branch diameter, while the dead branch stub length for natural shedding,the dead branch stub length was only correlated with branch diameter, it increased with the increament of branch diameter. Additionally, branch occlusion time for both treatments was also positively correlated to stem diameter growth rate. The length of discoloration and decay at knot position significantly increased with the increase of branch occlusion time. Therefore, controlling branch diameter and increasing stem diameter growth rate were the most foundemental measures for efficiently producing large sized clear wood.Based on the above results and factors of economicial benefits, we suggested two kinds of silvicultural measures for large diameter clear wood cultivation. The first one was combining high initial planting density to reduce the size of branches and thinning as well as tending to accelerate stem diameter growth rate and branch occlusion at late period. The second one was the combination of low planting density and artificial pruning to achieve large-sized clear wood cultivation. The present study could contribute a better understanding and provide an important theoretical and pratical instructions for high quality timber production of valuable timber tree species Betula alnoides in China.