Growth of Abies veitchii and A. mariesii Advance Growths after the Clear-Cutting of Overstory Trees in Relationship to Microtopographic Factors

Abstract

亜高山帯針葉樹天然林の伐採跡地で，シラベ，アオモリトドマツを主とした前生稚樹による天然更新の進行状況を調べ，微地形その他との関係を検討した。調査地は秩父山系西寄りの大弛峠付近にあり，9～14年前に上木が皆伐され前生稚樹による天然更新にまかされているが，更新状況は場所による差が大きい。現在生育している針葉樹稚樹のほとんどは，上木伐採当時の樹高が40～50cm以下の前生稚樹である。更新状況の指標として，ここでは樹冠被覆度を用いたが，樹冠投影面積合計，稚樹密度，稚樹葉量と高い正の相関があり、有効な指標とみとめられた。空中写真と地形図からえた樹冠被覆度と微地形その他の要素について，現地で検定したのち，両者の関係を数量化I類を用いて解析した。樹冠被覆度に対して標高，斜面方位，斜面形状の各要素の影響がつよかった。更新成績は北斜面，凸地形，高い場所でよいばあいが多く，南斜面，凹地形，低い場所ではわるいことが多い。こうした違いがおこる理由として，伐採作業時における地表面の人為的攪乱と，土壌の乾燥の程度の微地形による差を考えた。A series of investigations have been carried on through the mesh-sampling on topographical maps on 1/5000 and field surveys in an area of subalpine coniferous forest, mainly composed of Abies veitchii, A. mariesii and Tsuga diversifolia, in the central Honshu, in order to know whether or not this area can been regenerated by advance growths of these conifers after the clear-cutting of overstory trees and how microtopography or land features affect it. The natural forests in this area (ca. 75ha) were clearly cut in 1965 to 1970 with the expectation of regeneration by abundant advance growths of Abies. The progress of natural regeneration 9 to 14 years after the clear-cutting varies with land features. The saplings of Abies growing on the cut-over area seem to be exclusively survivors from the advance growths which were from 20 to 40 cm in height at the time of clear-cutting. The crown coverage, expressed as a ratio of the area overcast by crowns of conifer saplings in a quadrat area and easily estimated from aerial photographs, is closely correlated with sapling density, total overcast area of crown projection and total leaf weight, and seems useful as a parameter of the progress of natural regeneration. The analyses of correlation between the crown coverage and the micro-topographic factors estimated from topographic maps make obvious that the crown coverage is larger on N and NW facing convex slopes at higher elevations than on W to S facing concave slopes at lower elevations. This result suggests that the conditions of soil surface disturbed by logging and skidding and the soil moisture conditions are the most important among many factors affecting the growth of advance growths and the progress of natural regeneration