洪水によつて砂土の堆積をうけた竹林の回復に関する研究

Abstract

本研究は洪水によつて竹林内に多量の砂土が流入し, 堆積した場合について, その後にみられる竹林の更生状態を明らかにしようとしたものである｡ 調査を行なつた竹林は滋賀県下の安曇川沿岸にある水害防備マダケ林の一部で, 昭和28年9月におそつた第13号台風によつて, 林内に多量の礫や砂土が流入し堆積した竹林である｡ 洪水後5年間にみられた結果を要約すると次のとおりである｡ 1. 竹林が洪水によつてうけた損傷は, 流水面に近い竹の一部分が折損した程度で被害はきわめてかるい｡ 2. 竹林内に流入堆積した砂土の厚さと年々の新竹発生状態については, a 砂土が5 ~ 25cm堆積した Plot (A, B, D) では, 新竹の直径はいくらか細くなり, 材積をあらわす束数もいくぶん減少する傾向がみられた｡ この原因は砂土の堆積のみによるものではなく, つよい伐りすかしによる影響もうけているように思われる｡ b 礫および砂が75 ~ 80cm堆積した Plot (C) では, 新竹の発生本数においては大差ないが, その直径はいくらか細くなり, 束数も約30%の減少がみられた｡ しかし, 洪水後に10a当り約200本, 約20束の新竹を毎年発生しており, その堆積量がきわめて多いのにその影響が比較的かるいことに注目される｡ 3. 洪水後における地下茎の生長状態については a 5 ~ 10cmの厚さの砂土堆積地における地下茎の生長をみると, 堆積前の状態とかわらず, 砂土の堆積による生長低下が認められない｡ b 多量の礫や砂が堆積 (75 ~ 80cm) し, 地下茎が地中に深く埋もれた場合は, 地下茎は枯死することなく, 堆積後徐々に地表に向つて新たに地下茎を伸ばし, 堆積後4年目には堆積前とほぼ同じ地表下の深さ (20cm前後) に達した｡ (Fig 8) その伸長量をみると, 堆積後1 ~ 2年はきわめてすくないが, その後は次第に良好となつていた｡ 4. 洪水によつて竹林内に砂土や礫が堆積した場合, マダケの異常繁殖によつてウキスダケが発生する｡ すなわち, 竹稈の根元から地下茎を長く伸ばさずに竹稈を生じclump状となる｡ ウキスダケの発生程度はその堆積量によつてちがい, その量が多いほど長年にわたつて多く発生し, その量がすくないほどその発生量はすくない｡ 5. 砂土の堆積によつて林床植物のほとんどは埋没したが, 枯死することなくやがて更生したものが多い｡ しかしその堆積量が多いほど植生の回復はおくれる傾向がある｡ すなわち, 堆積量が厚さ5 ~ 25cmのPlot (A, B, D) における林床植物は砂土の堆積後3 ~ 4年で完全に堆積前の状態に復している｡ しかし礫や砂が75 ~ 80cmの厚さに堆積した Plot (C) では, 林床植物のすべてがふかく土中に埋没したため, その幹から新しい幹や根をつくりながら伸びて, 数年後にようやく地表にあらわれている｡ その間に, 地表の日当りの良いところには陽性の草本類がわずかながら侵入している｡ 6. 竹林の生育状態と林床植物の種類との関係については, うつぺいしている部分には営養繁殖をする陰地性草本の類が多く, 細竹の多い部分ではやや疎開しているので陽地性の草本類が繁茂している｡

The present studies were carried out to make clear the recovery state of bamboo grove which has been flooded and covered with gravel and sand of great quantity. The investigated grove was a part of Madake (Phyllostachys reticulata) groves established for flood control on the bank of Ado River in Shiga Prefecture, and into this grove gravel and sand of great quantity by the flood caused by Thphoon No. 13 in 1953. The results recognized during the 5 years after the flood are as floows: 1. The Damage of this grove due to the flood was small degree that only part of the standing culms near the flowing side were broken. 2. As for the yearly state of new culms growth, a) In the plot (A, B and D) where sand and gravel accumulated 5-25cm thick, there was a tendency that the diameter of new culms became rather slender and the volume (soku) of culms was a little decreased. It seems that the cause was not only the accumulation sand, but also hard cuttings for harvest. b) In the plot (c) where sand and gravel accumulated 75-80cm thick, the number of new culms that sprouted was recognized to be unchanged, but their diameters were rather slender and their volumes decreased about 30%. However, the new culms reached 200 in the number and 20sokus in the volume per 10 a. after the accumulation. It is worthy of our notice that the influence on the growth of new culms was very small despite that a great deal of gravel and sand accumulated. 3. As for the growth of rhizomes after the flood, a) When the accumulation was 5-10cm thick, rhizome growth was not depressed in comparison with the growth before the flood. b) When the accumlation was 75-80cm thick, rhizomes were not killed but gradually produced new rhizomes towards the surface of the accumulated sand, and new rhizomes reached 20cm below the surface within the 4 years after the accumulation. Their elongation was very small in the first 2 years, but improved in the last 2 years. 4. When gravel and sand of great quantity is accumulated in bamboo groves by a flood, an abnormal propagation of Ph. reticulata, the so-called Ukisudake, occurs. Ukisudake is a group of very slender culms growing in a clump form in which new culms develop at their basal part. The degree of its occurrence varies with the quantity of accumulation, and the greater the quantity in the more years it keeps occurring. 5. Most of the floor plants of this grove buried in gravel and sand were not killed but recovered in several years. And the greater the accumulation of gravel and sand, the slower the recovery of vegetation tended to be. Namely, the vegetation restored its pre-flood state completely in 2-3 years after the flood in the plot (A, B and D) where the accumulation was 5-25cm thick. On the other hand, the vegetation in the plot (C) where the accumulation was 75-80cm thick was buried deep under, the ground, and was still in an inferior state 4-5 years after the flood. 6. As for the relation between the growth state of bamboos and the species of floor plants found in this grove, many negative herbs grow in the parts of high degree of the crown cover and herbs of sun plants grow in the open parts.