天然更新に関する研究 (I) : 木曽地方湿性ポドゾル地帯におけるヒノキ属の更新

Abstract

この研究は高寒地の森林において, 実際行なわれあるいは行なわれようとしている天然更新の環境と更新の実態を生態学的に調査し, その環境に適した安全な更新法を見出そうとしたものである。まず第一にわが国の代表的な天然生美林の一つである木曽のヒノキ林をとりあげた。この中特に問題になるのは不成績造林地の多い木曽谷の西南一帯の準平原地域 (王滝営林署管内, 三浦, 助六地区) にあらわれる湿性ポドゾル地帯の更新である。この湿性ポドゾル地帯は, 粗腐植が厚く堆積し, そのため特にPやCaなどの養分が表層土中から多量に容脱されたきわめてせき悪な土壌からなっている。さらに木曽谷のこの地帯の特徴は伐採跡地ばかりか天然林内にも密生したササがみられることで, 最大は平均高180cm, haあたり地上部乾重約24tonにも達した。ヒノキ属を主とした天然林の下にはほとんど稚樹は更新しない。これに反し, 小面積に伐採された若い造林地にはよくヒノキ属の稚樹が更新する。稚樹は林縁附近がもっとも多く (平均75, 000本/ha), 遠ざかるに従って減少するが, 林縁から樹高と同じ位隔たったところでもhaあたり15, 000本以上の更新稚樹が認められた。現在更新後まだ10年前後しか経過していないので, 測定地点ごとの稚樹の大きさは平均高10 - 27cm, 根系の深さの平均4 - 7cm, 平均年令4 - 8年位で, 林縁から10 - 20m離れたところがもっとも大きかった。また, 年平均伸長量は約4cmで場所の違いによる差は少なかった。天然林内には60ton/ha (乾重) 前後の粗腐植が堆積していることが多いが, 伐採後しばしば下刈りを行なって陽光を与えた林地では5 - 10年位で50％以上減少するようであった。すなわち夏季, ササのある林地の5 - 10cm深さの地中温度は20℃を越えることはなかったが, ササのない裸地ははるかに温度が高くなり, 粗腐植の分解が促進されるものと思われた。天然林内の地床の明るさは林外の地床より常に低く, 林内のササ上の明るさの1/5位であった。この光不足と上述の厚い粗腐植の堆積が林内の更新をさまたげているようであった。更新後ほぼ10年以内の稚樹がもっともよく伸長する最適の明るさは, 相対照度 (陽斑点も含めて平均) 2 - 5％の範囲で, 数10％を越えると何らかの障害が起る可能性がある。それ故木曽地方の湿性ポドゾル地帯においては, 粗腐植の分解を促した後, ササを保護植生としてコントロールすれば, 更新面を樹高のほぼ2倍にとった帯状 (群状) 皆伐あるいは群状母樹法などによって, 天然更新をより確実に行なうことができよう。

The subject of the studies is to investigate the environment and the condition of the natural reproduction ecologically that have been actually carried out in the forest of cold region, and to find out a reproduction method suitable to its environment. In the first place, this paper deals with the Hinoki (Chamaecyparis) forest of Kiso district in Nagano prefecture, one of the fine natural forests in Japan. Especially, the investigation aims at the forest regeneration of the wet podzolic zone which appears in the pene-plain area in the south-west of Kiso valley (Miure and Sukeroku area in Ootaki district forestry office). In the wet podzolic zone accumulates a raw humus thickly, and therefore there consists of extremely sterile soils where nutritions of phosphorous and calcium etc. were leached on surface layer. Another characteristic of that zone at Kiso valley is that Sasa (bumboo grass) always coveres closely not only the cut-over area but natural forest too. The maximum of the mean height amounts to about 180cm and the maximum dry weight of the above-grund parts was estimated at about 24 ton per hectare. Few have been found out the reproduced seedlings under the natural forest which mostly consists of Chamaecyparis, while many seedlings of Chamaecyparis have been found in young plantatious hewn down at small area. The greatest of mean number of seedlings amounted to 75, 000 per hectare, and really found at near forest border. The number gradually decreased according to the distance from the forest border, but still more than 15, 000 seedlings per hectare were found even in the place as far as mean height of trees. It has elapsed only about 10 years after reproducing, so the mean height of seedlings on each sampling places ranged from 10cm to 27cm, mean depth of root system 4--7cm and mean age 4--8 years-old. The biggest seedlings were seen at the distance of 10--20m from forest border. However, annual mean increment of the height was estimated at about 4cm, and little difference was recognized among sampling places. In general, the raw humus in natural forest accumulates about 60 ton per hectare at oven dryweight, but the quntity of the raw humus on ground floor where sunlight were enough because of cut-over reached only less the half after 5 ~ 10 years. Namely, it was seemed that decomposition of raw humus was promoted in summer, because of the soil temperature in 5 ~ 10cm depth from the surface at opening place became higher than 20℃, but at the place covered with Sasa never exceeded its soil temperature. The light intensity beneath the Sasa in natural forest was about one fifth of it under the forest crown, and always lower than it beneath the Sasa in cut-over area. It was considered that the reproduction in natural forest is obstructed for the insufficiency of sunlight and thickly accumulated raw humus as mentioned above. The optimum light intensity for the elongation of the seedlings less than about 10 years ranged from 2 to 5 percent of full daylight, with average including sun-fleck, however, becoming the relative light intensity over several ten percent, it was presumed that somthing inconvenience may occure. Therefore, in the wet podzolic zone of Kiso district, if we controll the Sasa as a shelter vegetation after promoting a decomposition of raw humus, and apply the strip (or group) clear cutting system or group seed-tree system that a width of reproduction area was taken at about 2 times of tree height, the natural reproduction would surely be improved.