リギダマツの生育におよぼす摘葉の影響

Abstract

食葉性昆虫の食害が林木の生育におよぼす影響を知るために, 多くの摘葉試験が行われているが, 本報告は, マツ属を供試した一連の摘葉試験として行われたものである。供試材料は, 京都大学農学部附属演習林本部試験地の苗畑に育てられていたリギダマツ (P. rigida) で, 1984年に摘葉し, 1987年まで摘葉後の生育状況が調査された。摘葉試験と平行して, リギダマツ, パンクスマツ (P. banksiana) の新梢の伸長経過が調査された結果, 両種の新梢の伸長は, クロマツ (P. thunbergii) の新梢の伸長とほぼ同時期で, その間に複数の枝階を展開することが明らかになった。旧葉摘葉, 新葉摘葉, 全葉描葉の各処理を4月から10月まで, 各月末に行い, 処理後, 3年間または4年間の生育経過は次のようであった。1. 5月から10月に旧葉を摘葉した場合には, 摘葉は生育に影響しなかった。ただ, 4月の旧葉摘葉で, 処理当年の樹高成長, 直径成長ともに, いくらかの成長減退がみられた。2. 全葉摘葉では, 9月区の全処理個体が枯れ, 7月区では8個体のうちl個体が, 8月区では3個体が枯れた。その他の月の摘葉個体はすべて生き残った。3. 樹高成長におよぼす摘葉の影響は, 処理翌年に最も激しく, 全葉摘葉では, 5月処理区の成長率は, 無摘葉区の成長率の74％, 6月区で47％, 7月区で34％, 8月区で28％, 10月区で38％であった。しかし処理後3年目では, 8月区を除いて, 年間成長率はほぼ無摘葉区と同じ程度に回復していた。4, 直径成長におよぼす摘葉の影響は, 処理当年から直ちにあらわれ, さらに処理翌年も続いた。すなわち, 2年目の年間成長率は, 無摘葉区の成長率の56％ (6月区), 34％ (7月区), 24％ (8月区), 14％ (10月区) で, 摘葉翌年の直径成長の減退が目立った。新葉摘葉の場合も, 全葉摘葉ほどではないが, 成長減退は目立つた。処理後3年目においても, 生育期後半に摘葉された処理区には, まだその影響が残っていた。直径成長に対しては, 生育期の後半に摘葉するほど成長減退が目立った。5. 摘葉による幹材積の成長には, 直径成長にみられたと同様な傾向で, その影響があらわれた。すなわち, 処理当年, その翌年ともに成長減退がみられ, 摘葉区の成長率は, 3年間で無摘葉区の成長率の, 全葉摘葉で53 - 88％. 新葉摘葉で68 - 92％であった。6. 一般に, リギダマツは, アカマツ (P. densiflora), クロマツ (P. thunbergii), ストローブマツ (P. strobus) に近い摘葉の影響をうけ, テーダマツ (P. taeda) よりは, 摘葉によって顕著に成長減退があらわれることが明らかになった。

In order to know the effects of the feeding damages by herbivorous insects upon the growth of forest trees and stands, 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, Eastern white pine, Pinus strobus and Loblolly pine, Pinus taeda, the relations between its growth and the unusual decrease of the foliage during growing season have been nearly completely explained. An experiment was conducted at Kyoto to assess the effects of artificial removal of the needles of different ages at Pitch pine, Pinus rigida, upon the growth of tree-height, diameter and stem volume. Effects of artificial defoliation upon the growth of Pitch pine were investigated for four growing periods, from 1984 to 1987, in the nursery of Kyoto University forest, and these results were compared with the growth of four defoliated pines, Pinus densiflora, Pinus thunbergii, Pinus strobus and Pinus taeda, previously. Healthy Pitch pines of about 80cm tall and five years old were tagged and measured prior to the defoliation. At the end of each month from April to October, various artificial defoliations were carried out in four groups as follow; 1. removal of old (one and two-year-old) needles only, two trees each month; 2. removal of new (current year) needles only, seven trees each month; 3. removal of all (old and new) needles, seven trees on April, May and June and eight trees on July, August, September and October; 4. non-removal of needles on seven check trees. The needles were removed from the test trees with scissors remainning part of the leaf sheath. Tree height and diameter at the base of all test trees were measured at the beginning of the defoliation and tree height were measured at the end of each growing season from 1984 to 1987. Most test trees were cut down at the base on January of 1987 and other trees on February of 1988. The fresh weight of stem, branch and needlein each tree was measured and the materials for the determination of the dry-fresh weight raitio and for stem analysis were sampled. The results obtained from these investigations were as follows: The height growth of Pitch pine under the normal conditions began early on April and continued until the last week of May or the first week of June, and four internodesat new shoot were formed. In this growing period, branch shoot elongated similarly. The process of height growth of Jack pine, Pinus banksiana, also was just the same as Pitch pine and these height growth were very much the same as that of Japanese black pine (Fig. 1 and 2). Whole test Pitch pines defoliated all needles on September began to wither. In all needles removal group on July and August, one and three of eight test trees began to wither in each month, and other defoliated trees were continued the growing. Old needle defoliation from May to October had no influence upon the growth of Pitch pine. The influence of defoliation upon the growth of Pitch pine in the end of April was smaller than that of Japanese black pine, Eastern white pine and Lobloly pine. That is, the parallel growth rate (percentage of growth rate of defoliated tree tothat of nondefoliated tree in same period) in treated year was 82% in height growth, 56% in diameter and 64% in stem volume. However, in the following year and inthe third year after defoliation, these growth rate was similar to that of check trees. In the defoliated year, the height growth of the defoliated trees from May to October was similar to the check trees. However, in the following year, the influnce of the defoliation upon the Pitch pine growth was the greatest. In case of all needle defoliation, the parallel growth rate was respectively 74% (defoliation on May), 47% (June), 34% (July), 28% (August) and 38% (October). In the third year after treatment, the growth rate was the same or similar to that of the non-defoliated trees exclusive of all needle defoliation on August. The defoliation had a greatest influence upon the diameter growth both in the treated and in the following year. The parallel growth rate in the following year was 56% (defoliation on June), 34% (July), 24% (August) and 14% (October) respectively in all needle defoliation, and the parallel growth rate in new needle defoliation was 88% (June), 49% (July), 35% (August), 21% (September) and 16% (October). It seemed that the later the defoliated month was, the less the growth became in the following year after the treatment. In the third year after defoliation, the growth rate of treated tree was smaller than that of check trees in case of the defoliation in the latter half of the growing season. Artificial defoliation had the influence upon the stem volume growth as same as diameter growth, both in the treated year and the following next year. The growth rate of stem of Pitch pine continued the growing after defoliation was 53~88% at all needle defoliation and 68~92% at new needle treat as compared with that of the chack trees for three years after defoliation. Generally, the effects of artificial defoliation during the growing season upon the growth were similar among Pitch pine, Japanese red pine, Japanese black pine and Eastern white pine, and these pines suffered greater influences than Loblolly pine.