テーダマツの生育におよぼす摘葉の影響

Abstract

食葉性昆虫類の食害が林木の生育におよぼす影響を明らかにするために, 現在まで, 種々の摘葉試験, 被害林 (木) の調査が行なわれ, アカマツやクロマツの生育とその生育期における異常な葉量減少との関係はほとんど判明している。本報告は, マツ属でアカマツやクロマツと生育経過の異なるテーダマツの幼令木を用いて, 1964年に時期別の摘葉試験を行ない, その後3生育期間の生育状況を調査し, さらに, 1965年にはテーダマツ針葉の切断後の再伸長経過を調査し, アカマツやクロマツの調査結果と比較したものである。試験は京都大学農学部附属演習林上賀茂試験地の苗畑において行なわれ, 1964年4月から10月まで各月末に, 旧葉摘葉, 新葉摘葉, 全葉 (新葉+旧葉) 摘葉の3摘葉処理を行なった。処理は葉鞘の部分を残して針葉をハサミで切断した。摘葉試験は各処理区12本あて供試し, 針葉伸長調査は200 - 250針葉を測った。供試木は毎生育終了時に地際直径と樹高を測り, 1965年12月に6本, 1966年12月に残り6本の地上部の幹, 枝, 葉の重量を求めた。調査結果の概要をまとめるとつぎのようになる。1) テーダマツの新梢の伸長はアカマツやクロマツの場合と異なり4月上旬より9月中旬まで継続した。多くの個体は4回に分かれて伸長し, 早く伸長した新梢ほど長かった (図-1)。2) テーダマツの針葉は, その伸長中に切断されてもアカマツやクロマツのように葉鞘の部分が残っていれば再び伸長することがわかった。しかし, その再伸長量は同期間に伸長した正常針葉の伸長量より必ずすくなく, また, 針葉の伸長が進んでから切断したものほどすくなかった (図-2)。3) 針葉が約20％伸長した時に切断すると, その再伸長量は正常の約85％であったが, 約50％伸長した時に処理したものは約65％で, アカマツやクロマツの場合と比べて, 再伸長量は多かった (図-3)。4) テーダマツは生育期に一度全葉がなくなっても枯れず, アカマツやクロマツと大きな差があらわれた。5) 旧葉摘葉は, 4月末の処理で, その年に摘葉の影響があらわれたが, その他いずれも処理の影響は生長にあらわれなかった。6) 樹高生長には, 摘葉の影響は処理年には5 - 8月の処理に, 翌年には7 - 10月の処理にあらわれ, 前者は6月の, 後者は10月の生長が最も悪かった。すなわち, 6月全葉摘葉で処理年は無摘葉木の46％, 10月全葉摘葉で処理翌年の生長は37％で, 生長減退は大きかったが, 処理後3年目の年間生長率はすでに回復していた (図-4)。7) 直径生長にあたえる摘葉の影響は, 処理年および処理翌年は樹高生長がうけた影響とよく似ていたが, 処理後3年目までもその影響はわずかに残っていた (図-5)。8) テーダマツは生育期に一度針葉がなくなっても, その影響はその年か翌年に限られ, 3年目にはほとんど樹勢は回復していた。9) 摘葉後3年間の生長率を対照木と比較すると, 全葉摘葉でもその75 - 85％となり, アカマツやクロマツと比べて葉量減少に対して大きな抵抗力があることがわかった。

In order to know the effects of leaf-eating insects upon the growth of forest trees, many defoliation tests and damage-analysis of defoliated forest trees have been carried out, and consequently, as for Japanese red pine (Pinus densiflora) and Japanese black pine (Pinus thunbergii), the relations between its growth and unusual decrease of needles have been almost explained. In this report, the effects of artificial defoliation upon the growth of Loblolly pine (Pinus taeda), one species of pine genus, were investigated for three growing period, from 1964 to 1966, on the nursery in Kamigamo Experimental Forest Station of Kyoto University Forest, and its results were compared with Japanese red pine and Japanese black pine. In 1965, moreover, the elongation of needles was measured through its growing period. Four groups of twelve trees every month from April to Octover of 1964 were subjected to various artificial defoliation tests. The needles were removed from the test trees with scissors leaving the part of leaf-sheath in each group as follows: 1. removal of old needles only; 2. removal of new needles of the current year only; 3. removal of both the new needles of the current year and the old needles; and 4. removal of no needles on check trees. Tree height and diameter at the base of all test trees were measured at the end of growing season each year form 1964 to 1966. Six test trees were cut down at the base in December, 1965. And the fresh weight of stem, branch and needle was measured respectively and the meterials for dry-fresh weight ratio were sampled. In December of 1966, others were measured in the same way. The results obtained from these investigations were as follows: 1) Under the normal conditions in Kyoto, the height growth of Loblolly pine began gradually early in April, and continued till the middle of September. Namely, the basal internode generally was the first to expand, and accounted for most of the increase in shoot length during the four weeks in May. The second internode elongated from late in May to early in July, the third internode from early in July to early in August and the fourth internode from early in August to September. These mean length were 30.5 cm in the basal internode, 24.4cm in the second, 15.1cm in the third and 9.3 cm in the fourth (Fig. 1). 2) If the elongating needles were cut down and the part of leaf-sheath remained on trees, its needles elongated like the needles of Japanese red pine and Japanese black pine. Its length was influenced by treated period, and always was shorter than the elongation of normal needle in the same term (Fig. 2 and 3). 3) All test trees defoliated new and old needles from April to October were not withering. 4) The artificial defoliation of old needles only late in April had little influence upon the growth of Loblolly pine. That is, the growth rate was about 75% to that of check trees in diameter, tree-height and stem weight respectively. In the next year of defoliation, the growth rate was similar to check trees. 5) The defoliation had the same influences upon the growth of height and diameter both in the treated year and in the next. 6) The height growth suffered the influences by artificial defoliation from May to August in the treated year, and from July to October in the next year. Parallel growth rate in the treated year was 54% (defoliation in May), 46% (June), 81% (July) and 86% (August) to that of the normal on defoliation of all needles respectively. In the next year, it seems that the later the defoliated month is, the less the growth becomes (Fig. 4). 7) Parallel growth rate of the diameter in the treated year was 60~80% (from May to September) and in the next year was 84% (July), 70% (August), 30% (September) and 18% (October) on all needles defoliation (Fig. 5). 8) In the third year after defoliation, the growth rate of Loblolly pine was the same to that the normal. 9) Parallel growth rate of Loblolly pine which defoliated all needles was 75~85% as compared with the normal growth rate for three years after defoliation (Fig. 6). Namely, it seems that the growth of artificially defoliated Loblolly pine suffered less than that of Japanese red pine and Japanese black pine.