Species Composition and Structure of Plantation Forest Understory Modified by Slope Protection – Establishment of Terrestrial Pteridophytes and Factors Promoting Their Growth

27 Mar Species Composition and Structure of Plantation Forest Understory Modified by Slope Protection – Establishment of Terrestrial Pteridophytes and Factors Promoting Their Growth

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Abstract

法面保護のために編柵工が施工された植栽林（施工植栽林）と編柵工が施工されなかった植栽林（無施工植栽林）の下層植生を対象に,シダ植物に着目して調査・解析を行った。シダ植物の植被率,平均種数は無施工植栽林より施工植栽林で有意に大きかった。シダ植物の総種数および組成比も施工植栽林で大きい傾向が認められた。施工植栽林における単位面積当たりのシダ植物の種数は近隣の二次林のそれと比較しても大きかった。数種のシダ植物については出現頻度も施工植栽林で有意に大きかった。施工植栽林に出現したシダ植物には陽地生の種も含まれていたが樹林生の種も多く認められた。このようなシダ植物の傾向と比較して,種子植物の種組成と構造については施工植栽林と無施工植栽林であまり大きな違いは認められなかった。一部の緑化・園芸樹木に由来する樹種,先駆樹種,外来樹種,路傍生や林縁生の総本種については施工植栽林と無施工植栽林の両方に高頻度で出現した。これらの結果から,施工植栽林は,エッジ効果を受けているにも関わらず,樹林生の種を含め多くのシダ植物の生育立地として機能していることがわかった。施工植栽林におけるシダ植物の定着要因としては,編柵工の施工による土壌水分の保持能力の高さと階段状の斜面形態が考えられた。これらの知見をもとに,都市部やその近郊におけるシダ植物の保全について考察した。

English Abstract

We investigated the species composition and structure of plantation forest understory with and without slope protection, with particular focus on the establishment of terrestrial pteridophytes. Both pteridophyte coverage and the number of pteridophyte species were higher in the understory with slope protection than in that without slope protection. The frequency of occurrence of several pteridophyte species was also higher with slope protection. Furthermore, the number of pteridophyte species per unit area was considerably higher in understory with slope protection than in that of neighboring secondary forests. Pteridophyte species included not only plants growing in open, sunny sites but also those growing in the forest interior. In contrast to pteridophytes, the species composition and structure of seed plants did not markedly differ between understory with slope protection and that without slope protection. Both types of understory contained high abundances of several species of greening/garden trees, alien tree species, and high-light herbaceous plants. These results suggest that forest understory with slope protection serves as suitable habitat for many pteridophytes, even in areas with edge effects. The potential factors promoting the establishment of pteridophytes in protected-slope understory include the improvement of soil water mainte nce and the stair-like slopes formed as part of the protective function.