マツカレハの被害をうけた壮齢アカマツ林の生育

Abstract

アカマツの食葉性害虫のうちで, マツカレハはその被害の激しいことで最も重視されている。アカマツに対するマツカレハのように, 林木に対する食葉性害虫の食害に対する被害木のその後の生育経過を調査した報告は次第に多くなってきた。本報告は, 樹齢40年を経過したアカマツ林を調査対象とし, 過去にマツカレハの激害をうけ, 以後, 生長を回復しつつある状況について調査したものである。調査の一部は1961年 (既報) に行なわれ, その後さらに詳しい資料を1964年の11月に採取した。調査林分は京都府下田辺町に近接した石英斑岩の丘陵地に成立したアカマツ林で, 1964年には樹齢41 - 47年, 平均胸高直径9. 0cm, 平均樹高8. 1mであった。調査は, 標準地を設け, 上木のアカマツの毎木調査を, また対象林分中より, 11本を選び伐倒し, 幹, 枝, 針葉の地上部の測定を行った。さらに樹幹解析により生育経過を求め, マツカレハの被害に関する被害解析を試みた。調査結果をまとめると次のようになる。1) 調査アカマツ林の現存量は, ha当り針葉 9. 6ton (生重), 3. 9ton (乾重), 幹99. 2ton (生重), 52. 9ton (乾重), 枝15. 4ton (生重), 7. 5ton (乾重) となった。2) 新葉と旧葉の割合は, ほぼ7 : 2で, 旧葉は全葉量の23. 5%を占めていた。3) 地上部の幹, 枝, 針葉の重量配分は胸高直径によって大きな差はあらわれず, 幹-82%, 枝-11 - 12%, 針葉-6 - 7%となった。4) マツカレハが大発生した当時の生長量の低下は激しく, 調査した大部分の個体が, 年生長量の最低の1955年には前年の大きさの1%も生長していない。5) マツカレハが生息しなくなってから, 被害木は徐々に生長を回復していることがわかった。しかし, 6年経過しても大部分の個体がマツカレハ被害前の生長率の20 - 30%で, 被害の影響から脱したとはいえない。6) 本調査アカマツ針葉の平均純同化率 (a) と非同化器官の平均呼吸率 (R) はa = 3. 714 - 2. 045g/g・year R = 0. 055g/g・year となった。7) マツカレハの発生当時の被害状況を概略推定すると, 全針葉のほぼ70%近くが食害されたと思われ, このためにはha当り約210, 000頭のマツカレハが生息していたことになる。8) 気象的な災害のためと思われる生長量の低下が1962年, 1963年と認められ, 本報告では第二室戸台風 (1961年9月16日襲来) と1963年1月の異常低温がその原因と推察した。これらの原因は調査木がマツカレハの被害から回復するのをさらにおくらせたようである。

It is well-known that the leaves of forest trees have the greatest influence upon the growth of trees, and that the defoliators have a great influence upon the growth of forest trees, too. Leaf-eating insects, one group of defoliators, feed on leaves, assimilation parts, have a direct effect upon the growth of trees and enfeeble the vitality of trees. Moreover, they make trees ready to get other damages. Pine caterpillar (Dendrolimus spectabilis Butler) is a species of leaf-eating insects and the most injurious among leaf-eating insects of pine species, and frequently it gives the greatest damage to red pine forest in Japan. Sometimes, as the result of feeding-damage of pine caterpillar, red pine deprived of all needle-leaves, will wither. In this report, the author investigated growth of red pine infested formerly with pine caterpillar and the progress that red pine was recovering out of the effects of feeding-damage, and further estimated the degree of the damage. These investigations were carried out in red pine stand near Tanabe-cho in Kyoto prefecture in November, 1964. This pine stand had suffered heavily damage by pine caterpillar in 1954 and 1955. Seven sample plots were placed and all red pines in each sample area were measured at the diameter breast high in cm. Eleven red pines of various sizes were cut. Each part of stem, branch and needle-leaf was measured in fresh weight in gr, and meterials for stem analysis and dry weight were gathered. The results obtained from these investigations were as follows; 1) The investigated red pine stand was 41~47 years old. Mean diameter breast high was 9.0cm and mean tree height 8.1m. 2) This stand was nearly closing, and stand crop were 9.6 ton/ha (needle-leaf), 99.2ton/ha (stem) and 15.4 ton/ha (branch) in fresh weight, and in dry weight the first 3.6 ton/ha, the second 52.9 ton/ha and the third 7.5 ton/ha. 3) The fresh weight of old needle-leaf was about 23.5% of total weight of needle-leaf, that is, the rate between new needle and old one was about 7 to 2 in fresh weight. 4) The distribution of dry weight of stem, branch and needle-leaf make little difference even if the diameter breast high is not the same, namely, stem: 82%, branch: 11~12%, needle-leaf: 6~7%. 5) It is well-known that these red pines were infested with a great number of pine caterpillars in 1955. In consequence of stem analysis, annual stem growth in 1955 was the least. For example, the tree that had grown 2, 000cm3 in 1952 had growth of only 110cm3 in 1955. In stem growth in 1955, the greater part of investigated red pines was less than 1% of stem volume in 1954. 6) It is found out that the damaged tree by pine caterpillar is gradually recovering from the effects of feeding-damage every year after these pine caterpillars had gone away, but even six years later growth-percentage of almost all red pines showed about 20~30% on stem growth as compared with that before feeding-damage of pine caterpillar. That is, it can not be said that damaged trees have already completed to recover from the effects of feeding-damage. 7) In case of this investigated red pine, mean net-assimilation rate was 3.714~2.045 g/g･year and mean respiration rate of non-assimilation parts was 0.055 g/g･year. 8) It is estimated that the feeding of pine caterpillar was about 70%-defoliation of all needleleaves in pine stand. Judging that mean net-assimilation rate is 2.829 g/g･year, if red pine stand is infested with about 210, 000 pine caterpillars per hectare, 70% of all needles seems to be defoliated during its larval stage. 9) It is observed that the decrease of growth in 1962 and 1963 appeared to happen through the unusual atmospheric phenomena; the Typhoon Muroto II visited the investigated red pine stand on Sept. 16, 1961 and the low-temperature in January, 1963. It seems that these facts kept damaged trees slow in recovering from feeding of pine caterpillar.