林道切取のり面の安定性と自然植生の回復について (2) : のり面勾配と植生状況

Abstract

林道切取りの面の勾配と安定性, 植生状態を検討するため六甲・南山城 (花崗岩), 高野山・川上村 (古生層) ののり面約300箇所を調べた。いずれも建設後10年以上経過している。のり面の植生状況を示す指標として植生の量的指標と多様性指標を決め検討した。その結果は以下の通りである。1, 現時点の各路線ののり面の安定性は六甲を除き程度の差はあるが勾配が急なほど高い傾向があり, 特に高野山において著しかった。これは林道又は山岳道路では地形的・工費的制約により, 多くののり面が均一な急勾配で施工され, 不定安ののり面が緩勾配化作用を起すためである。岩質が軟かく不定安性が高いのり面はその定安勾配に達するまで土砂移動を続ける。時間的にみれば岩質が硬く安定勾配が急なのり面ほど早く安定化する。そのためある時点で勾配と安定性の関係をみると急勾配ほど安定しているものが多い結果となる。六甲の場合は植生状況の良好なのり面を選んで調査したので緩勾配で安定性の高いものが多かった。2, のり面の植生回復にはそののり面の安定性が確保されていることが大きい前提条件である。のり面が安定している場合には勾配が急なほど生育条件がきびしく植生の生育可能量は少なくなる。(推定によれば50％ののり面が良好な状態になりうる植生量を得ることのできる勾配は35° - 40°となる。) しかし1でのべたような現時点で緩勾配ほど不安定性が高いという状況にある路線をみると不安定な緩勾配のり面では植生侵入が困難であるため, 勾配と植生の量的指標の関係はある勾配でピーク値を持つ形となる。3, のり面に侵入する植生の多様性は植生の量が少ない間はそれほど差がない。緩勾配部分が安定してくると生育条件がよいため生長の早い草本種が優占し多様性は低い (川上村など)。しかし植生遷移が進むと低勾配部の多様性は増大する (六甲)。

The authors carried out the surveys to find the relations between the gradients of cutting slopes and the vegetative conditions on them. The surveying roads were Rokko, Yamashiro (belonged in the granit zone) and Kohya-san, Kawakami (Paleozoic era zone). About 300 surveying slopes were selected on these roads. It had elapsed more than ten years after the construction of them. Two indexes (the volume index and the variety index) to represent vegetative conditions on the slopes were decided. The volume index showed the quantity of plants to grow on the slope and the variety index showed the variety of plant species. The results of investgations were as follows. 1. It had the tendency that the steeper slope was, the more stable it was, except Rokko road. This tendency was marked on Kohya-san road. Most of cutting slopes on the forest roads were cut with a nearly equal steep guadient regadless of their stability, because the mountain slopes that the road constructed, were mostly steep, and the constructive costs were short. Therefore the unstable slopes were eroded and their gradients were moderated more gently until they changed the stable ones. The slopes constituted with the hard rock became stable more rapidly with steep gradient. So it resulted in that the steeper slopes were, the more stable they were. 2. It was essential to keep the stability of slope for the recovery of vegetative growth on these slopes. At the stage that most of slopes were stable, the quantity of plants to able to grow on them were decreased as the slopes were steeper. But at the stage that the gentle slopes were unstable yet, the line to show the relation between the vegetative volume index and the slope gradient shaped a salient. 3. The Variety of plant species to make a raid into the slopes were few different in each other on the slopes with poor plant growth. At the stage that the gentle slopes were stable, the variety index on them decreased. Because the grasses to able to grow early had the priority (Kawakami), but according to advance of vegetative succession, the variety index increased (Rokko).