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九州大学農学部附属北海道演習林において, 斜面の方位や斜面上の位置の違いが植生構造に与える影響を明らかにするため, カラマツ人工林の下層落葉広葉樹植生と落葉広葉樹天然林植生について, 方位の異なった斜面の尾根部・中腹部・谷部に調査区を設置し, 植生調査を行った。カラマツ人工林内の下層広葉樹の胸高断面積合計の割合は3.8-46.8%であり, カラマツが北斜面や沢で胸高断面積が大きいのに対して, 下層...広葉樹の断面積合計は南斜面で北斜面よりも大きかった。天然林の広葉樹の断面積合計ではこうした斜面位置に対する傾向はみられなかった。人工林及び天然林双方において, 落葉広葉樹の種構成は南斜面ではミズナラのようなストレスに強い種が, 沢にはハレニレ, ヤチダモやハシドイなどの湿生種が優占し, 北斜面にはミズナラやヤチダモ, オオバボダイジュ, アサダ, サワシバ, アオダモ, カツラ, ハクウンボク, エゾイタヤなど湿生種から乾性種まで様々な種が優占していた。いずれの調査区でも, 南斜面より北斜面において種多様度が高く, 人工林の下層落葉広葉樹植生では北斜面上部で, 天然林では沢で多様度がもっとも高かった。北海道演習林におけるカラマツ人工林の下層落葉広葉樹植生は, 天然林の斜面方位や斜面上の位置の違いによる植生構造の違いに対応しており, こうした分布特性や樹種の多様性は土壌環境と関係が深いことが推察された。
We investigated the difference in forest structures among slope directions and slope positions in the larch (Larix kaempferi) plantations and in the deciduous broad-leaved natural forests of Ashoro Research Forest (ARF). The ratio of broad-leaved understory trees to larch in basal area was ranged from 3.8 to 46.8% in two plantations. Basal area of larch was higher in north-slopes than in south-slopes, whereas basal area of understory vegetation was lower in north-slopes than in south-slopes in the plantations. In the natural forests, total basal areas were not different among slope directions and positions. Both in natural and plantation stand, Quercus crispula was dominant in the south-slopes; Ulmus davidiana var. japonica, Fraxinus lanuginosa, and Syringa reticulate were dominant in the valley. There were various dominant species in the north-slopes as Quercus crispula, Fraxinus mandshurica, Tilia maximowicziana, Ostrya japonica, Carpinus cordata, Fraxinus lanuginosa, Cercidiphyllum japonicum, Styrax obassia and Acer pictum subsp. mono by forest types and slope positions. Both in natural forests and in understory vegetation in plantation, diversity index (Shannon-Weighner’s H’) was higher in north-slopes than in south-slopes. Deversity index was the highest in the upper of north-slopes of the plantation forests and the valley of the natural forest. The distribution and diversity of understory broad-leaved species in the larch plantation was corresponded to the differences in the slope direction and positions on the slope as natural forest in ARF, suggesting that the vegetation and diversity of broad-leaved trees in ARF may relate with soil fertility.show more
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