ストローブ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, 種々の摘葉試験, 被害林における被害解析が行なわれ, マツ属では, アカマツ, クロマツ, テーダ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明らかになっている。本報告は, ストローブマツの幼令木を用いて, 1970年に京都大学農学部附属演習林本部苗畑で, 摘葉試験を行ない, 以後5生育期の生育状況について, さらに摘葉後に展開した針葉の長さと重さの関係を調査した。摘葉処理は, 旧葉摘葉, 新葉摘葉, 全葉摘葉の3処理を, 4月から10月に, 各月末にハサミで針葉をその基部で切断した。供試木は各処理区5本で, 1970年には毎月末, 以後は毎生育終了時に地際直径と樹高を測り, 1974年12月に地上部の幹, 枝, 葉の重量を求め, 樹幹解析を行なった。調査結果の概要はつぎのようになった。1. ストローブマツは3月下旬頃から新梢を伸長させ, 6月上旬には生長を終る。アカマツ, グロマツの樹高生長と似た生育をする。2. 摘葉木で枯れたものは, 全葉摘葉8, 9月区では全個体で, 全葉摘葉7, 10月区, 新葉摘葉9, 10月区で, 5個体のうち1または2個体であった。3. 新葉の摘葉は, 針葉が伸長中であれば, 葉鞘に包まれた基部から針葉が再伸長して以後の樹体を維持した。4. 旧葉摘葉では4月摘葉区で処理年に生長減退がみられ, 2年目においてもすこし摘葉の影響がみられたが, 3年目以後は樹勢を回復していた。5 - 10月の旧葉摘葉は生長には影響をあたえなかった。5. 5 - 10月の新葉, 全葉摘葉区における樹高生長は, ストローブマツが5月下旬には年間の樹高生長の大部分を終るため, 処理年には影響はみられず, 2年目に大きな生長減退がみられた。これに反し, 直径生長には, 処理当年, 翌年ともに摘葉の影響があらわれ, 生育期後半の処理ほど生長減退は大きかった。さらに, 摘葉後3年目においても8 - 10月摘葉区で摘葉の影響が残っていた。6. 3年間の生長は, 新葉摘葉で対照木の生長率の56 - 87％ (樹高), 65 - 79％ (直径), 66 - 84％ (材積) となった。7. 摘葉後4年目, 5年目では, 4年目に全葉摘葉7月区でまだすこし摘葉の影響が残っていたが, 他は生長率が対照木と同じか大きく, 樹勢を回復していた。すなわち, ストローブマツはテーダマツより摘葉に強い影響をうけ, アカマツ, クロマツと似たような結果となった。8. 摘葉後伸長した新梢に展開した針葉は, 正常な針葉より短いものが多くあらわれ, さらに細い傾向がみられた。しかし, 樹勢の回復とともに針葉の形態も正常となった。

In order to know the effects of herbivorous insects upon the growth of forest stands and trees, many defoliation tests and damage-analysis of defoliated trees have been carried out. Consequently, concerning Japanese red pine (Pinus densiflora), Japanese black pine (Pinus thunbergii) and Loblolly pine (Pinus taeda), the relations between its growth and the unusual decrease of foliage have been nearly completely explained. An experiment was conducted at Kyoto to assess the effects of the artificial removal of foliage of different ages from Eastern white pine (Pinus strobus) upon height, diameter and stem-volume growth. Effects of artificial defoliation upon the growth of Eastern white pine were investigated for five growing periods, from 1970 to 1974, in the nursery of Kyoto University Forest, and these results were compared with the growth of Japanese red pine, Japanese black pine and Loblolly pine. Seventy-five healthly Eastern white pine, about 40cm tall and four years old were tagged and measured prior to defoliation. Every month from April to October of 1970, four groups of five trees were subjected to various artificial defoliation tests. The foliage was removed from the test trees with scissors. This was carried out in each group as follows: 1. removal of old (one-year-old) needles only; 2. removal of new (current year) needles only; 3. removal of both the new needles of the current year and the old needles; 4. removal of no needles on check trees. Artificial defoliation of the test trees was not repeated after 1970. Tree height and diameter at the base of all test trees were remeasured at the end of the growing season each year from 1970 to 1974. In late July of 1972 and 1973, the twenty or thirty needles were sampled in each test tree, and the length and weight were subsequently measured. All test trees were cut down at the base in December of 1974. The fresh weight of stem, branch and foliage was measured and the materials for the determination of the dry-fresh weight ratio and for stem analysis were sampled. The results obtained from these investigations were as followş: 1. Under normal conditions in Kyoto, the height growth of Eastern white pine began late in March or early in April, and continued until the last week of May or the first week of June. Just at Japanese red pine and Japanese black pine, the greater part of the height growth of Eastern white, pine was over late in May. 2. All of the test white pines that defoliated all needles in August and September began to wither. Among the removal groups of all foliage in July and October, and the removal groups of new foliage in September and October, one or two of the five test trees began to wither. 3. If the elongating needles of Eastern white pine were cut down and part of the leaf-sheath remained on shoot, the needles elongated. 4. Late in April the artificial defoliation of only the old needles had a greater influence upon the growth of Eastern white pine than upon Japanese black pine and Lobloly pine. That is, the growth rate was 35% in height growth, 44% in diameter and 47% in stem volume. In the following year of defoliation, this growth rate was less than that of the check trees, and in the third year the growth rate was similar to that of the check trees. However, old needle defoliation from May to October had no influence upon the growth of Eastern white pine. 5. The height growth of the defoliated trees from May to October was similar to the normal trees in the treated year. However, in the following year, defoliation had the greatest influence upon the pine growth. In the case of the defoliation of all needles, the parallel growth rate in the following year was 32% (defoliation in May), 16% (June), 10% (July) and 13% (October) respectively, compared to the normal growth. 6. Artificial defoliation had the greatest influence upon the diameter growth both in the treated year and in the following year. It seemed that the later the defoliated month was, the less the growth became. Parallel growth rate in the following year was 78% (defoliation in May), 63% (June), 19% (July) and 11% (October) respectively on the removal of all needles, and in the test trees which new needles were removed, growth rate was 88% in May and 15% in October. 7. In the third year after defoliation, the growth of Eastern white pine, which defoliated all new needles from August to October, was smaller than that of the non-defoliated trees. 8. The parallel growth rate of Eastern white pine, which defoliated new needles during the growing season, was 56 ~ 87% as compared with the normal height growth rate for three years, 65 ~ 79% in diameter growth, and 66 ~ 84% in stem volume. 9. In the fourth and fifth years after defoliation, the growth rate of white pine was similar to that of the non-defoliated trees. 10. Generally, the effects of artificial defoliation upon the growth were similar among Eastern white pine, Japanese red pine and Japanese black pine, however Eastern white pine suffered greater influences than Loblolly pine. 11. The needles which elongated on the new shoots of the defoliated white pine were shorter and slenderer than the normal needles.