著者

梶 幹男 沢田 晴雄 斉藤 俊浩 斉藤 登 中山 勇 赤岩 朋敏 伊藤 幸也

出版者

東京大学大学院農学生命科学研究科附属演習林

雑誌

東京大学農学部演習林報告 (ISSN:03716007)

巻号頁・発行日

no.85, pp.p49-66, 1991-07

被引用文献数

1

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1989年2月25日から26日にかけて東京大学秩父演習林栃本作業所管内の森林に大きな雨氷害が発生し,人工造林地での被害は本数18,931本,材積3,711m3,面積は23.6haに及んだ。しかも被害の発生した標高域はほぼ900~1,450mの範囲に限られていた。そこで雨氷現出の原因となったと思われる要因解析を行った。まず滑沢(標高1,150m),突出峠(1,650m)両地点の自記々録を比較した結果,この頃ここに明かな気温の逆転を生じていたことが判明した。しかも同じ時間帯に少し下方の栃本観測所(標高770m)で34.5mmの降雨が記録されていた。従って,雨氷害発生の経過についてこの時のこの雨が冷たい気層の中を落下する間に十分に冷やされて過冷却状態になり,それが枝,葉に当って瞬時に氷結し,着氷量を増加させていった。そしてこの着氷の荷重によって幹や枝が損傷を受け,雨氷後の強風がその被害を一層大きくしたものと推定された。また造林樹種のうち,カラマツが本数,材積ともに最も大きな被害を受け,総被害量の76%を占めた。被害形態を樹冠部の折損,幹の傾斜・湾曲,主幹の折れ,根倒れの四つに区分し,樹種別の被害率を求めた結果,樹冠部の折損はヒノキ,幹の傾斜・湾曲はカラマツ,主幹の折れは二葉松類,根倒れはスギでそれぞれ最も高い値を示した。そこで,カラマツ,スギ,ヒノキのそれぞれについて,雨氷害と地況および林況要因との関係を明かにするため,数量化I類による多変量解析を行った。解析に用いた七つのアイテムのうち,六つには樹種間で一定の傾向は認められなかったが,標高のみで,3樹種とも1,100~1,200mの範囲を中心に高いスコア値が認められた。このことは,今回の雨氷害の中心がこの標高域にあり,そこで着氷量が最も多かったことを示唆するものである。The glaze occurred at various places in Kanto from the 25th. to the 26th. in February, 1989 gave heavy damages to the manmade forests in Tochimoto District of the Tokyo University Forests in Chichibu, totals of 18,931 in the number, of 3,711m3 in the stem volume and of 23.6ha in the area (Table 2, 3). And most of these damages were found in restricted altitude ranges from 900 to 1,450m a.s.l.. For the purpose to search out main factors having caused these glaze damages, various analyses were carried out. The comparison of the temperature records taken from Namesawa (1,150m a.s.l.) and Tsundashitouge (1,650m a.s.l.) during the period concerned obviously clarified that the invasion of temperature occurred between these two altitudes (Fig.1). On the other hand, the rainfall of 34.5mm was recorded at Tochimoto observatory (770m a.s.l.) during the same period (Fig.3). It can be supposed from these two facts that the rain water was cooled to the over-cooled condition fallen passing through cold atmospheric layer below, and consequently it was frozen and turned to ice as soon as it dropped on branches and leaves, and fixed and accumulated on them. Therefore, stems and branches were broken by the heavy load of ice, and furthermore the damages were enlarged by strong wind blown after the glaze (Table 1). Among the manmade forests of different species, stands of Japanese larch (Larix kaempferi) were nost severely damaged showing 76 per cent of the total damage both in the number and the stem volume. Having classified the damages of stand trees into four types, i.e., crown breakage, stem leaning, stem breakage and up rooting and surveyed the degrees of damage of every tree species, it was known that the percentage of crown breakage was the highest in the stands of hinoki cypress (Chamaecyparis obtusa), that of stem leaning in Japanese larch, that of stem breakage in the species of hard pines, and that of up rooting in cryptomeria (Cryptomeria japonica), respectively. For the purpose of analysis of causal relations between the degree of glaze damage and the factors of topographical and or stand conditions, several stands of Japanese larch, cryptomeria and hinoki cypress were subjected to quantification analysis Quant-1, for which five items for topographical factors and two items for stand conditions were selected (Table 6-8). Among the seven items, six did not present any obvious relation among the stands. But a category of 1,100-1,200m in the item of altitude alone was significantly related to the occurrence of glaze damage in the stands of every species. This fact suggests that the amount of ice deposition was the largest in this range of altitude which caused heavy damages.