Microsite effects on first-year establishment and overwinter survival of white spruce in aspen-dominated boreal mixedwoods

First-summer establishment and overwinter survival of planted and seeded white spruce (Picea glauca (Moench) Voss) germinants were assessed in five microsites in the understory of trembling aspen (Populus tremuloidesMichx.) dominated boreal mixedwood stands. Mineral soil mounds, rotten logs, large exposed patches of mineral soil, normal LFH forest floor, and small screefed patches of mineral soil were examined. The mounds and rotten logs had the highest soil temperature, lowest soil strength, and accumulated the least amount of deciduous leaf litter whereas the small screefs were at the other extreme. First-summer establishment of planted seedlings was greatest on the rotten logs, mounds, and large exposed patches; the large exposed patches had the highest establishment of seeded trees. Over 85% of the planted seedlings that were in good condition in September survived the winter. Overwinter survival of seeded trees was least on small screefed patches, but survival of these trees was increased by covering them with small plastic cups. Low soil temperature, high seedbed strength, and high vegetation cover were associated with reduced seedling establishment whereas high litter accumulation was associated with reduced overwinter survival. Resume : L'etablissement au cours de la premiere saison estivale et la survie apres une saison hivernale de semis d'epinette blanche (Picea glauca (Moench) Voss), plantes ou ensemences, ont ete evalues dans cinq microsites en sous-etage de peuplements melanges domines par le peuplier faux-tremble (Populus tremuloidesMichx.) et situes en foret boreale. Des buttes de sol mineral, des billes en decomposition, de larges plaques de sol mineral expose, le parterre forestier normalement recouvert de litiere et de petites plaques de sol mineral denude ont ete examines. Les buttes et les billes en decomposition etaient les microsites ou la temperature du sol etait la plus elevee et la cohesion du sol la moins elevee. Ces microsites accumulaient la plus faible quantite de litiere de feuilles de feuillus tandis que les petites plaques denudees se situaient a l'autre extreme. L'etablissement des semis plantes au cours de la premiere saison estivale etait le plus eleve sur les billes en decomposition, sur les buttes et les grandes plaques de sol mineral expose. Dans le cas des semis ensemences, l'etablissement etait le plus eleve sur les grandes plaques de sol mineral expose. Plus de 85% des semis plantes qui etaient en bon etat en septembre ont survecu pendant l'hiver. La survie des semis ensemences apres une saison hivernale etait la plus faible sur les petites plaques denudees mais la survie de ces abres a ete amelioree en les recouvrant de petits verres en plastique. Une temperature du sol basse, la cohesion elevee du lit de germination et une couverture de vegetation haute etaient les facteurs associes a un plus faible etablissement des semis tandis qu'une forte accumulation de litiere etait associee a une survie hivernale reduite. (Traduit par la Redaction)