天然更新に関する研究 (II) : 木曽の三浦実験林におけるヒノキの更新

Abstract

三浦実験林は木曽御岳山の南斜面, 標高1, 200 - 1, 500mの準平原にあらわれる湿性ポドゾル地帯における森林造成の指針をもとめるために, 1966年11月, 事業的規模で設定された。本研究はこの実験林で行なわれているヒノキの各種天然下種更新試験のうち, 設定後3 - 4年を経過した帯状皆伐, 皆伐母樹法, 択伐更新試験地の更新状態についてとりまとめたものである。天然生ヒノキ林の平均樹高の2倍, 約50m幅に伐採帯と保残帯を交互に組み合せた帯状皆伐更新地には, この地域に普通あらわれるササを除草剤で枯殺した帯と残した帯が作られた。ササは多いところで地上部乾重が約6ton/haもあり, その中の地床の相対照度は1 - 2％できわめて暗く, また地温もササ除草地に比較して低い。30 - 60m間隔に3 - 10本ずつ群状に母樹を残した (保残率4％) 皆伐母樹法更新地と, 約50％の単木ぬきぎりを行なった択伐更新地はすべてササが除草されている。A_o層の堆積は各試験地ともきわめて厚く, 乾重でほぼ40 - 60ton/haと推定され, 更新上の大きな障害となっている。帯状皆伐更新地における保残帯やササの成立する帯では稚樹の発生が少ないが, ササ除草を行なった伐採帯には著しく多い。しかも林縁付近が常に多く, 実験林設定後発生した2 - 4年生の稚樹が平均7本/m_2以上に達した個所もあるが, 伐採帯の中央付近になるほど少なくなる傾向が認められた。皆伐母樹法更新地ではほぼ全面にわたって稚樹がよく更新しているが, 母樹群付近とか種予の散布が重なり合うようなところに特に多い。択伐更新地はササが除草され, 林床も比較的明るくなっているのに, せっかく発生した稚樹のほとんどは年内に消失してしまうようであった。各更新面の状態別にまとめた稚樹の大きさの頻度分布や年平均生長量から判断して, 密生したササは稚樹の発生ばかりか生長に対しても悪い影響を与えていると思われた。稚樹の根元直径 (D_o) に対する稚樹高 (H) の相対生長関係はバラツキが大きく, また更新面の状態のちがいによってほとんど分離しない。そして同じ単位にしたH/D_o=100の値より小さいものが多かった。ササ除草地に更新している稚樹の各部分量間の相対生長関係からみて, 林内稚樹の同化部乾重や地上部乾重は相対的に少ない傾向が認められる。しかし葉面積あたりの葉乾重は林内外ともほとんど差がない。以上のことからヒノキ天然生林の成立する高冷地の湿性ポドゾル地帯において, 早期に稚樹の発生, 生長を期待するならば, 母樹を樹高の2倍間隔以内に残し, ササのような地床植生をできるだけ除去することがとりあえず必要であると思われた。

The Miure Experimental Forest of Nagano Regional Forest Office was established to find out a regeneration method of forest suitable at the wet podzolic zone which appears in the pene plain area (about 1, 200-1, 500 meter above the see-level) on the south-west slope of Mt. Ontake, in Nov. 1966. This paper summarizes the survival and establishment of the Hinoki (Chamaecyparis obtusa) seedlings on the regeneration areas of strip clear-cutting, seed-tree and selection system which have elapsed for 3-4 years since the various studies of natural regeneration in this Experimental Forest. On cutting area of alternate clear strip system, the natural forest which mostly consists of the Hinoki was felled on a width of about 50 meter (as twofold of tree height), and the Sasa (bumboo grass) in remaining stand and cutover strip was either remained or weeded by herbicide. The mean dry weight of the upper ground parts of the Sasa which always coveres closely the Miure district was estimated at about 6 ton per hectare (Fig. 1, Table 1). The average light intensity under the Sasa on a cutover area showed 1-2% of full daylight, with the average including sun-flecks (Fig. 4). The soil temperature in 5-10 centimeter depth from the surface at cutover opening became higher than 20℃ in summer, but at the area covered with Sasa never exceeded its soil temperature (Fig. 8). On the seed-tree system regeneration area where mother trees were remained in group from three to ten trees at intervals of 30-60 meter (about 4% remaining rate) and on selection system regeneration area where natural forest trees were thinned out at about half of stand volume, the whole Sasa was weeded by herbicide after logging was removed. The oven-dry weight of A0 layer (consisting of litter, raw humus and humus) in each regeneration area was estimated at 40-60 ton/ha (Fig. 5, 6, 7). Therefore, it was presumed that the natural regeneration in the Miure district is obstructed for thickly accumulated A0 layer and the insufficiency of sunlight under the Sasa as mentioned above. Survival seedlings were found on all regeneration area. On the strip clear-cutting system, the regeneration area under the Sasa and the remaining stand were either poorly stocked or nonstocked, but on the cut-over opening area where the Sasa was weeded, live seedlings were abundant (Fig. 10, 11). In general, most seedlings were found near the stand edge into cutover opening area, and the greatest number of seedlings amounted to 70, 000 per hectare with 2-4 year-old seedlings that regenerated since study area was established. But it seemed that the number gradually decreased according the increase of distance from seed source (Fig. 11). On the regeneration area of the seed-tree system, live seedlings were everywhere abundant, and especially numerous near the seed-tree group or on location where seed dispersal overlaped (Fig. 12). On the regeneration area of the selected system, no seedlings over 2 yaer-old were found, and only a few 1 year-old seedlings were alive. Judging from the frequency distributions and the mean increment of seedling height in each regeneration area, it was presumed that survival and growth of seedlings were obstructed by a closely grown Sasa (Fig. 15, 16, 17). The allometric relations of height to diameter at base of seedlings roughly correlated to linear relation, but they did not differ too much among each regeneration areas (Fig. 19). In regards to the allometric relations between each two parts weight of seedlings in weeded Sasa area, the oven dry weight of green parts (assimilation organ) and upper ground parts of seedlings regenerated under remaining stand tended to be relatively smaller than those of cutover opening area (Fig. 20, 21). However, the relation of oven dry weight to surface area of seedling leaves was scarcely found out to differ with those regeneration area (Fig. 22). Therefore, on the wet podzolic zone having Hinoki natural forest, it seemed that the survival and establishment of Hinoki seedlings in cutover regeneration area would surely be promoted, so far as the closed Sasa was weeded and seed trees or stands were remained at width as twofold of tree height.