樹木の肥大成長の縱斷的配分

Abstract

本論文は樹木の肥大成長の縱斷的配分に就て考慮したものである｡ 針葉樹を材料として調査したところによると幹の年輪に就て凡そ次の樣な傾向が認められた｡ 1. 樹冠内に於て幹の年輪幅は頂端より下方に向い枝の數を加うる毎に廣くなる｡ しかし伸長成長の衰えた, 新葉を生ずる事の少い枝の附近より下は年輪幅增加の傾向が少くなり, むしろ多少減少する傾向をあらわす｡ 2. 枝下の部分にては樹冠の發達狀態によつて相違がある｡ 強度の鬱閉狀態にあり, 樹冠が小さく且枝下の高いもの, 甚しい被壓狀態にあるもの等では上より下に向つて常に年輪幅が狹くなる｡ 樹冠の發達稍々良好にて多少側壓を受ける程度の成長中庸の樹木では上より下に向つて初め, 僅宛, 年輪幅を減ずるが基部に近いある點にて最小を示して夫より根際に向い急に大となる｡ 此最小點の位置は樹高の大となるに從て或程度迄高くなるが, 必ずしも, 樹高又は枝下高に比例するものでなく, 又年によつて變化がある｡ 樹冠の大なる優勢木では枝下に於ける年輪幅減少の程度が少く, 疎立狀態にあつて樹冠の發達よく, 枝下の低いものでは枝下にて殆んど年輪幅を減ぜず寧ろ增加し, 特に根際にて著しく廣くなる｡ 3. 年輪の面積に就て之を見ても略々上記の樣な關係は認められるが, 幹形の梢殺の程度によつて必ずしも一致せず, 樹冠内に於ける最大點は年輪幅の最大なる位置より稍々低いところに現れ, 枝下に於ける最小點は年輪幅の最小なる位置より高く, 樹冠に近いところになる傾向がある｡ 幹に於ける肥大成長の配分は上述の如く樹冠の發達狀態によつて相違するが, 人工的に枝又は葉の量及び分布の狀態に變化を與うれば, また肥大成長の配分にも影響が現れると考えられ, 之に關し種々の實驗を行つたが其主要なる結果は次の通りである｡ 1. 芽及びすべての枝を除去すると軸の肥大成長は停止するが, 基部にのみ僅少の成長をなすものがある｡ 2. 頂芽を殘しすべての枝を除くと頂芽の直下の附近にのみ肥大成長が繼續する｡ 3. 芽のすべてを除去すると頂部にて肥大成長減少し, 基部に向つて大となる｡ 4. 樹冠の片側の枝を除くと幹全體に亙り肥大成長もまた半減するが, 特に下方に影響が著しい｡ 5. 力枝より下位にある陰の枝は之を除去しても幹の肥大成長に殆んど影響しないが, 夫より上の枝を除くと枝を除去した部分より下へ急に成長量を減ずる｡ 但し其減退の程度は除かれた枝又は葉の量に正しく比例するわけでなく, 上位にある枝の除去程影響の程度が著しい｡ 6. 樹冠の中部の枝を除けば, 特に枝を除いた部分の肥大成長を減じる｡ 7. 樹冠の上部を除けば幹全體の肥大成長に影響するが特に頂部に減退の程度が著しい｡ 8. 輪截を施すと肥大成長は其直上部にて著しく大となり, 直下部にて減ずるから, 結局成長層は向基的に廣くなる｡ 9. 急に疎立狀態となつた樹木では肥大成長は基部に向つて大となる｡ 此傾向は特に優勢な樹木にて顯著に見られる｡ 此樣な肥大成長の縱斷的な偏りを生ずる理由に關し從來皮壓說, 營養說, 機械的刺戟說, 水分通導說, 成長素說等とも呼ぶべき諸種の學說が出されているが夫々難點がある｡ 成長の要因として近時明かにされてきた成長素の作用と, 組織の新生に要する構成物質の補給の難易によつて之を考える時は上記の諸種の場合を通じて略々說明し得るものの樣である｡ 一般に肥大成長は基部に向つて大となる傾向があるが, 之は成長素が常に向基的に移動し, 基部に向つて集積するからであると考えられ, 又頂部の芽の伸長に應じて肥大成長は頂部にも大ならんとする傾向をとるが, 之は成長素の形成が主として頂部の芽又は新葉に形成され, しかも頂部の軸の形成層輪は小であるため此部分の形成層に對し比較的豊富の成長素が作用するからであると思われる｡ 併し成長は成長素の刺戟のみによつて起るものではなく構成物質の供給を必要とする｡ 樹冠がよく發達して同化物質の生產が十分である時は各部の成長に對する要求を充たす事が出來るが, 庇陰にあつて樹冠の發達の不良な時は同化物質が缺乏し, しかも其轉流は距離の大なるに從て困難となるから如何に成長素の刺戟はあつても各部の成長は達せられない｡ 從て成長の分配は成長素の分布と構成物質の補給とによつて支配されると言う事になる｡ 此樣な考えを確かめるためには實際の成長素の分布狀態を明かにしなければならないが, 樹木に就て此方面の研究は未だ十分でない｡ 二三筆者がマツに就て測定せるところによつて, 成長素は主軸の頂芽に最も多く生產される事, 樹冠の下部の枝は上部の枝よりも少い事, 特に伸長成長の衰えた陰にある下枝にて少く, 其基部には全く成長素が見出されない事, 枝を除去すると幹の成長素の減少が現れる事, 根には地上部に比べて成長素に乏しい事等が知られている｡

The breadth of the annual ring is not always equal in all parts of the tree. Even in a stem it varies considerably at different levels. Such things are by no means new, but the reason is not yet fully explained. The studies described in the present paper have been carried on with experimental work on this problem. The general tendencies in the vertical distribution of wood increments measured on a vast number of coniferous trees are as follows: 1. Inside the crown, the annual ring breadth of the stem increases from the apex to the densely foliaged part of the crown with each branching, and then, downwards, it is almost equal or decreases slightly. 2. In the branchless portion the growth distribution takes different ways according to the condition of the crown: In trees whose crown developed in full daylight and occupied the large part of the stem the ring breadth rather steadily increases to the base. In trees with those crown which developed in suppressed state and limited to the uppermost small part of the stem the ring breadth diminishes downwards, till scarcely measurable in the extreme case. In trees with the crown of medium size the ring breadth is almost the same or gradually falls off downwards to some height, which varies individually and yearly. From this point it turns again to increase to the ground level. 3. In most trees, besides suppressed ones with the small shaded crown, the abrupt increase of ring breadth is seen at the basal part of the stem. 4. The distribution of ring area takes the similar trend as ring breadth, but there is no strict parallelism between them, since the circumference of the stem becomes greater from the top downwards with different rates. Generally speaking, The maximum point of ring area in the crown is seen at somewhat lower level than that of ring breadth and the minimum point of ring area below the living crown appear at far higher level than that of ring breadth. As said above, the longitudinal distribution of radial growth was affected by the form of the crown; so it will be expected that the deformation of the crown also influences the growth distribution in the stem. The experiments were made to determine this relation. The main results are described below: 1. The removal of terminal buds and all of the side branches stopped the radial growth of all parts of the stem without basal portion, in which frequently a few new wood elements occured after the operation. 2. When all branches were pruned leaving the terminal, the radial growth of the stem continued merely at the parts below the terminal and near the base for some time. 3. The excision of all buds was followed by the returded growth, especially at the upper portion of the stem. 4. The removal of branches at one side of the crown reduced about half the growth of the stem throughout its length. 5. The removal of branches below the widest and densest part of the crown showed almost no effect upon the growth of the stem; while the pruning above such point was followed by the reduction in radial growth below the pruned part. 6. Topping, the removal of upper part of the crown, resulted in the relatively severe fall of the growth in upper level, considering the amount of lost branches. 7. In the case of girdling, the radial growth conspicously increased immediately above the girdle and dropped beneath it. Consequently, the wood increment inclined basipetally in both upper and lower portions of the girdled zone. 8. When trees grown in a close stand were brought in open condition the radial growth remarkably increased, especially towards the base. To explain such phenomena a number of hypothesis have been presented by various investigators. For example, the difference in the bark pressure, the distribution of food substances, the difference in the duration of growth period, the longitudinal pressure and tension- stimulus, the requirement for water conduction, the effect of growth hormones, the supply of soil nutrients and others were counted as the factor which determines growth rate. Among other things, the effect of growth hormone seems to be most meaningfull. In the vertical distribution of growth, there are two principal tendencies: one is to grow basipetally, and the other to inverse direction. It may be considered that the former is due to the downward movement of auxin and its accumulation at the basal part, and the latter to the relative richness of auxin in the neighbourhood of the terminal which regarded as the main source of it. But, our present knowledge of the auxin distribution in trees is not yet enough to make sure of this. From some observations in pine the following facts were known: a. The largest amount of auxin was detected in the terminal among all the buds in the same tree. b. The auxin content, indicated by the amount to cambial ring, was larger in the stem than in the branches at corresponding annual shoot. c. Upper branches in the crown contained relatively larger amount of auxin than lower branches. d. In the basal part of suppressed lower branches auxin diminished till it disappeared. e. The removal of branches was followed by the reduction of auxin content of the stem below the part of prunning. At any rate, it must be remembered that cambial growth is controlled not only by auxin but by food substances too. If the food to build new cells failed to supply, growth could not take place at all the parts, however stimulated by auxin; therefore, it may be concluded that the distribution of growth follows, first, the grade of auxin stimulation, and next, the amount of food supply. Moreover, there is some evidence assumed as cambial stimulus, such as the mechanical action of wind, but this will be thought as rather indirect factor, which supposedly affects the movement of food substances or other processes.