総　説（Review Article)

無節材生産を目的とした枝打ちに関する研究

竹内　郁雄
　　　
Studies on Pruning for Knot-free Timber Production

TAKEUCHI Ikuo

要　　旨
　本研究は，スギ，ヒノキ人工林で無節材生産を目的とした枝打ちについて，枝打ち方法を解明し，この方法で枝打ちした林分の樹冠構造や林分現存量，成長量などを明らかにするとともに，より正確な枝打ち管理技術を確立することを目的として行った。先ず，枝着生部の幹直径と枝直径や着生枝数の実態を把握した。次に，普通の枝打ち作業での節解析から，枝打ち時における枝着生 部の幹直径（枝打ち時期）と，無節材および巻き込み後の年輪走行が正常になる平滑材の生産開始 幹直径との関係を求めた。スギやヒノキ林分で10.5cm角の心持ち無節柱材が生産可能な枝打ち時 期は，幹曲がりを考慮し6.0cm以下であることを示した。一方，枝打ちの欠点である材部変色の発 生原因や大きさを詳しく解析した。変色の欠点を抑制するには，心持ち無節柱材生産が可能な枝打 ちが効果的であることを示した。本研究で得られた枝打ち時期で，密度や地位の異なる林分を対象 にほぼ２年ごとに枝下直径を4.0cmにする強度で枝打ちを繰り返し，林分の樹冠構造や林分現存量， 成長量，形状比などの変化，枝打ちによる除去枝葉量などを明らかにした。また，枝下直径成長の変化から，枝打ち強度や間隔などの枝打ち方法を示した。以上の成果を用い，スギ林分とヒノキ林 分で10.5cm角の心持ち無節柱材が生産可能な枝打ち管理例を，成長予測を含めて提示した。

キーワード：枝打ち，枝下直径，現存量，樹冠構造，成長，変色，無節材

Abstract
　　　In Sugi (Cryptomeria japonica) and Hinoki (Chamaecyparis obtusa) stands which are main plan tations in Japan, pruning has been carried out to produce boxed heart and large-size, knot-free timber, which are highly valued. However, knot-free timber is not produced in sufficient quantities for production purposes in the major pruned stands, because a pruning management system suitable for production purposes has not yet been established. In this study, pruning methods for utilizing the effect of stem diameter at the time of pruning (the stem diameter at the lowest retained branch in thepruning position) and pruning intensity were examined. Pruning was then carried out by the pruning method derived from this examination, and changes of crown structure, biomass and growth in the pruned stands were examined. The purpose of the study was to establish accurate a pruning management system based on these results.
　　　The actual conditions of branch diameter and number, which influence the horizontal distribution of knots in the trunk wood, were investigated. Branch diameter in both Sugi and Hinoki stands increased with the increase of stem diameter of the retained branch, and in lower density stands. In repeatedly pruned stands, branch diameter tended to increase with the stand age. The number of branches which developed on the stem was 30-40 per meter stem length in both species. There were no differences between stand density and growth stage. However, the number of living branches on the lower side of stem whose diameter of 4 cm decreased to around 20 per meter stem length, because the small branches died early. It was shown that the difference of branch diameter and number in the stands did not hinder the production of knot-free timber when pruning was repeated from a young age.
　　　Knot analysis was carried out using sample trees of Sugi and Hinoki which had already been pruned, in order to analyze the pruning start time and the intensity required to obtain the pruning effect. It was found that the length of branch stub and the radial thickness for annual ring recovery increased as stem diameter increased at the time of pruning, in the cases of commercially pruned Sugi and Hinoki stands. The relationship between maximum length of branch stub (L: mm) and stem diameter at the time of pruning (x: cm) is expressed by the following equation for both species:
L=-0.05x2+2.63x-6.68
Using this equation, by knowing the stem diameter at the time of pruning, the production start stem diameter for knot-free timber can be estimated. The relationship between maximum radial thickness for annual ring recovery (P: mm) and stem diameter at the time of pruning (x: cm) is expressed by the following equation for both species:
P=27.55Ln(x)-33.43
From this equation, by knowing the stem diameter at the time of pruning, the production start stem diameter of the annual ring recovering timber can be estimated. These results may be widely used to decide the stem size at the time of pruning for production purposes.
　　　Average values of crookedness per 3-m section of the stem from 0.5 to 3.5 m height above the ground were 1 to 3 cm in Sugi and Hinoki stands. Stem crookedness is a difficult problem for boxed heart with knot-free timber production. Therefore, the size of stem crookedness and length of branchstub must be considered in order to decide the pruning time. For example, in order to produce 10.5 cm boxed heart with knot-free faces, the pruning should retain stems of diameter 6.0 cm or less at the height of the lowest branch when the stem crookedness is 3 cm or less.
　　　The quality of 3-m-long 10.5 cm boxed heart from the first and second logs of sample trees taken from a pruned Sugi stand was studied. The average distance from pith to the center of the boxed heart cross section was 2.5 cm (butt end) and 1.6 cm (top end) for the first log. It was 1.3 cm(butt end) and 1.4 cm (top end) for the second log. Though the basal crook part of the butt end of the first log was cut, the distance from pith to the center was still larger compared with that of the second log. The distance from pith to the center is discussed in relation to JAS (Japanese Agricultural Standard) grade and the number of knot-free faces on the boxed heart. Basal crook and eccentric growth have a great influence on the lower part of the first log, but even the latter has little influence on the second log. From the quality of boxed heart, it was confirmed that earlier pruning time (stem diameter at the height of the lowest branch) of the first log that produces 10.5 cm boxed heart with knot-free faces was appropriate. In the second log, the distance from pith to the center was small, and pruning at the time of 7.0 cm or less was satisfactory.
　　　Wood discoloration caused by thepruning operation is a serious problem. Wood discoloration occurs when a wound in the trunk of the tree is caused by the pruning operation. Such wounds can be classified into three types: wound in wood, peel-off and wound of split-stub. The main cause of wood discoloration is wounds in wood caused by pruning tools such as hatchets, axes, sickles and saws. The development of wood discoloration is influenced by the length of the wound in wood and the peel-off. The length and the area of discoloration tended to increase as the wound length in wood increased. The radial thickness of discoloration tended to increase as the wound length in wood increased, but the radial thickness became approximately constant over a certain value of the wound in wood. Wood discoloration of mature trees was extraordinarily large in comparison with that of young trees. These results suggest that in order to avoid discoloration by pruning, the branch must be pruned carefully without making any wounds which might cause discoloration. However, commercial pruning usually cuts off the branch collar, in which case it is difficult to avoid making a wound in the trunk and resulting wood discoloration. Wood discoloration occurs only in wood that is older than the time at which the wound was inflicted. It does not appear on sawn timber the surface of which is knot-free. Therefore, the pruning time was the most critical countermeasure against wood discoloration: pruning should be executed when 10.5 cm boxed heart with knot-free faces, whether from short-rotation or long-rotation stands,can be produced.
　　　In order to clarify stand conditions suitable for the pruning, the effect of the difference between site quality and stand density on the growth after pruning was examined. In low density and low site quality stands, the recovery of leaf and branch biomass which was reduced by pruning was slow, and the reduction of growth was larger than that of high density, good site quality stands. Therefore, stands with high-density and good site quality are suitable for pruning. The relative value of stem growth for two growing seasons after pruning may be roughly estimated from the ratio of leaf removed, the difference of stand density or its site quality, though the value was not clear. In accordance with the increase of pruning intensity, the distribution ratios of leaf and branch became larger, while those of stem and root became smaller. The reduction of net primary production of whole portions was not as large as that of stem under the influence of pruning.
　　　Pruning was carried out at pruning time (stem diameter at the height of the lowest branch is 6.0 cm or so) for producing 10.5 cm boxed heart with knot-free faces clarified in this study. Regarding the degree of pruning, branches are removed to the height of the stem diameter of 4.0 cm, and this is repeated 4 to 5 times every 2 years. Changes of crown structure, biomass and growth were investigated in repeated pruning of Sugi and Hinoki stands.
　　　The largest difference between the control and pruned stands was 1.9 - 3.7 m in mean clear length, 3.0 - 6.7 cm in mean stem diameter at the upper end of clear length, and 2.0 - 4.8 m in mean crown length. The largest difference of the high density stand was smaller than that of the low density stand. The stand ages which showed the largest difference were almost the same as those in which the mean stem diameter at the upper end of clear length and mean crown length of control stands showed the greatest values. Afterwards, these differences did not increase by repeating the pruning.
　　　The ratio of leaf biomass of Hinoki pruned stands just after the time of first pruning to that of the control stand was 26 - 88%, and the ratio was as high as that of the high-density, young stand. The ratio of leaf biomass of Hinoki pruned stands remained almost constant when the leaf biomass of control stands reached more than half of the leaf biomass on the closed control stand. For example, the ratio in Hinoki pruned stands which stand at a density of 2300 - 2700 per ha, was 30 - 35% just after pruning, and around 65% just before pruning. The ratio of leaf biomass of Sugi pruned stand just after the time of first pruning to that of the control stand was 42%. After reaching 13 years old, the ratio of leaf biomass was 27% just after pruning, and about 60% just before pruning.
　　　In accordance with the increase of total pruned stem length, the removal amounts of leaf and branch became larger in Sugi and Hinoki pruned stands. The removal amounts of leaf and branch in Hinoki pruned stands were 9 - 15 ton per ha and 5.5 - 13 ton per ha respectively, in the case of pruning to a height of 3.5 m for the first log. In the case of pruning to a height of 6.5 m for the first and second logs, removal amounts of leaf and branch were 19 - 33 ton per ha and 15 - 26 ton per ha respectively. These removal amounts increased in high density stands. In a Sugi pruned stand of density 3,770 per ha, the removal amounts of leaf and branch were 16 ton per ha and 4 ton per ha respectively, in the case of pruning to a height of 3.5 m, and 46 ton per ha and 12 ton per ha respectively, in the case of pruning to a height of 6.5 m.
　　　In the case of pruning to a height of 3.5 m for the first log, the largest difference of growth between control stands and pruned stands appeared after 2 years from the last pruning. The largest difference was 1.2 - 1.5 cm in mean stem diameter at breast height, 0.3 - 0.8 m in mean tree height in
Hinoki stands of stand density of 4,000 or less per ha. In the case ofpruning to a height of 6.5m in these stands, the largest difference was 1.9 - 2.8 cm in mean stem diameter at breast height, 1.1 - 2.3 m in mean tree height in Hinoki stands. Pruning for the production of 3-m-long 10.5 cm boxed heart with knot-free faces from the first log was estimated to retard the harvest time of the pruned stand from that of the control stand by 2 - 3 years. For the production of second and first logs, the harvest time was estimated to be retarded by 5 - 6 years in comparison with that of the control stand. The reduction of growth by pruning was smaller in high density and good site quality stands. Especially, stands of low site quality will not be suitable for pruning, since the growth decrease caused by pruning was remarkable.
　　　In both the pruned and control stands, the larger D (stem diameter at breast height) was, the smaller the H/D ratio, and the relationship between them was expressed by a reciprocal expression of each stand age. The mean H/D ratios of every stand had been declining with height growth in the early stages. The ratios then touched bottom except in both the pruned and control Hinoki stands of lowest site quality. The stand age which showed minimum mean H/D ratio, and the value of its ratio, were almost the same in pruned and control stands. After the mean H/D ratios showed the minimum value, the ratios of pruned stands became higher than those of control stands. The ratios of pruned stands rose most after 2 - 3 years from the last pruning. That time ratios were higher than 5 - 10 compared with the control stands, then the ratios approached that of the control stands. From the changes of mean H/D ratios in the pruned stands, it was considered that the resistance to meteorological damage of pruned stands will not be drastically lower.
　　　The growth of stem diameter at the height of the lowest branch (Db) for two growing seasons after pruning increased, as the dominant tree, for younger stand age and higher site quality. In order to produce 10.5 cm boxed heart with knot-free faces and knot-free, large-size timber,pruning should be repeated every 2 years as follows. In the first log, so that Db remains 6 cm or less, Db at the pruned height should be 3.0 - 3.5 cm for first pruning and 3.5 - 4.0 cm for the second pruning and beyond. In the second log, so that Db remains 7 cm or less, Db at the pruned height should be 4.0 -4.5 cm for first or second pruning and 4.5 - 5.0 cm for the third pruning and beyond. The pruning is the management of Db which controls the horizontal distribution of the knot. Therefore, the intensity of pruning should be proportional to the growth of individual trees.
　　　In this study, pruning time for producing knot-free timber and stand conditions suitable for the pruning were clarified. Changes of crown structure, growth and H/D ratios, etc. in repeatedly pruned stands were clarified. Using these results, pruning management examples for producing 10.5 cm boxed heart, knot-free timber in Sugi and Hinoki stands were presented.

Key words: biomass, crown structure, discoloration, growth, knot-free timber, pruning

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