マツ枯れ激害地 - 白浜試験地における天然生アカマツ, クロマツの枯損と生存木について

Abstract

マツ枯れ激害地であった京都大学農学部白浜試験地の1 - 7林斑において, アカマツ (P. densiflora), クロマツ (P. thunbergii) の枯損の経過を過去の伐倒駆除の資料より推定し, 1983年9月 - 11月に残存している個体を調査した。白浜試験地のマツ枯れ被害は, 1966年にはじめて見られ, そのピークは1972 - 1974年で, 1978年までの13年間に, 枯損本数は3051本, 枯損材積は497. 8m_3に達し, 胸高直径40cm以上の大径木はすべて枯死した。被害初期は大径木が, 次第に中 - 小径木が多く枯れ, 1974年の激害末期までに, 本数で約60％, 材積で約80％が枯損した。以後は, 枯損は徐々に減少し, 1983年には3本, 1984年6月に1本枯れたにすぎない。335本の残存木は, 外観からは, 18本がクロマツ, 317本がアカマツと判定された。針葉横断面にみられる樹脂道の位置にもとづき, その外位率によって分けると, 85 - 100％のアカマツは277本, 65 - 85％のアカマツに近いアイアカマツは31本, 35 - 65％の中間のアイノコマツは8本, 15 - 35％のクロマツに近いアイグロマツは2本, 0 - 15％のクロマツは17本となった。針葉下表皮細胞は, 角隅を除く全周で2層がみられるものは18本, 1層であったものは316本, 中間型と思われる両者がみられる針葉をもった個体がl本で, 外観によるアカマツ, クロマツの判定とよく一致した。残存木の81％は胸高直径20cm以下の小径木で, 樹幹が通直かまたはそれに近いものは1/4しかみられなかった。さらに, 枯れ枝の付いている個体が多く, マツノザイセンチュウ (Bursaphelenchus xylophilus) による部分枯れと考えられる。これらの残存木 (1984年6月に1本枯れたため生存木は334本) は, 白浜試験地におけるマツノザイセンチュウ抵抗性個体の一次候補木としてリストアップされた。

In the seacoast district of Wakayama prefecture, the greater part of pines, P. thunbergii and P. densiflora, were killed by the pine wilt caused by pine nematode, Bursaphelenchus xylopilus. This study was initiated in 1983 in order to list up surviving pines as resistant trees against the pine wilt at natural stands of 1 ~ 7-compartment in Shirahama Experiment Station located in the heaviest damaged area in Wakayama prefecture. According to data of the felling and the medical spray to withering pines in this Station, the felling of two killed pines were put on first record in 1966. The climax of the killed pines by the pine wilt was observed from 1972 to 1974, the total withering pines were counted 3, 051 trees, and were calculated 497.8m3 in stem-volume for thirteen years from 1966 to 1978. All large diameter pines over 40cm in DBH were killed. 335 pines were survived in this research area in 1983, and by external characters, 18 pines were specified P. thunbergii and the others were specified P. densiflora. If these pines were classified by the position of resin-duct in the cross-section of the secondary leaf, 277 pines showed 85 ~ 100% in external ratio of resin-duct were specified P. densiflora, 17 pines of 0 ~ 15% in ratio were specified P. thunbergii. Other 41 pines, that is, 31 pines of 65 ~ 85%, 8 pines of 35 ~ 65% and 2 pines of 15 ~ 35%, were specified P. densi-thunbergii. There were observed the hypoderm arranged one or two row of cells in the crosssection of leaf. The hypoderm of 18 pines arranged two row of cells, these of 316 pines arranged one row of cells and the leaves of other one pine had both of one or two row. The specify of P. densiflora and P. thunbergii by observations of the hypoderm considerably accorded with that by the external characters. The greater part of surviving pines have kept withering branches and twigs. In June, 1984, one pine among 335 surviving pines in 1983 withered by the pine wilt. 334 pines will be observed continuously on resistant tree or non-resistant against the pine wilt for the future.