No abstract

Variation in canopy composition can influence ecosystem processes, such as nutrient cycling and light transmittance, even when environmental soil conditions are similar. To determine whether forest cover type influences species composition of the understory vegetation (herbs and shrubs), the composition of this layer was studied on two different surface deposits, clay and till, and under four different forest cover types dominated, respectively, by Populus tremuloïdes Michx. (aspen), Betula papyrifera Marsh. (white birch), Pinus banksiana Lamb. (jack pine), and Picea glauca (Moench) Voss -Abies balsamea (L.) Mill. (spruce-fir) over similar environmental conditions. Detrended correspondence analysis and analysis of variance performed on the ordination scores revealed that understory plant composition was highly affected by surface deposit and forest cover. The gradient observed in the correspondence analysis proceeds from aspen, white birch, spruce-fir, to jack pine. Indicator species were identified for each surface deposit and cover type, and most of them were associated with either jack pine or aspen. The richness, evenness, and diversity of the understory vegetation did not vary between cover types, but were affected by surface deposit. By controlling ecosystem processes such as light transmittance and nutrient cycling, forest cover influences understory composition.Résumé : La composition du peuplement peut affecter les processus écosystémiques, tels que le cycle des éléments nutritifs et la transmission lumineuse, malgré la présence de conditions environnementales similaires. Afin de vérifier si le couvert arborescent influence la composition de la strate de sous-bois, des relevés de la végétation herbacée et arbustive ont été effectués dans des conditions environnementales détendancées similaires, incluant deux dépôts de surface différents, till et argile, et quatre couverts forestiers dominés respectivement par les Populus tremuloïdes Michx. (peuplier faux-tremble), Betula papyrifera Marsh. (bouleau blanc), Pinus banksiana Lamb. (pin gris) et Picea glauca (Moench) Voss et Abies balsamea (L.) Mill. (épinette-sapin). L'analyse des correspondances détendancées et l'analyse de variance effectuées sur les coordonnées des axes de l'ordination révèlent un effet du dépôt de surface et du couvert arborescent.Le gradient observé en analyse des correspondances va du peuplier faux-tremble, bouleau blanc, mélange de conifères, au pin gris. Des espèces indicatrices ont été identifiées pour chacun des dépôts de surface et couverts forestiers, et la majorité d'entre-elles étaient associées soit au pin gris soit au peuplier faux-tremble. La richesse, l'équitabilité et la diversité de la strate de sous-bois ne varient pas en fonction du couvert forestier, mais sont significativement affectées par le dépôt de surface. Les différents processus écosystémiques, tels que la transmission de la lumière et le cycle des éléments nutritifs, semblent expliquer l'influence du couvert forestier sur la composition de la strate de sous-bo...

In the absence of fire in black spruce-feathermoss stands, a thick forest floor layer dominated by bryophytes and sphagnum accumulates. This layer is associated with wet, cool and nutrient-poor soil conditions conducive to the paludification process and pushing the ecosystem towards an unproductive open black spruce forest. The presence of Populus tremuloides in theses stands may halt this process because this species has a high nutrient cycling rate and a litter that represses moss cover. The main hypothesis of this study is that, despite similar abiotic conditions (slope and drainage), the presence of Populus tremuloides in a stand dominated by Picea mariana affects surface soil nutrient availability, total N, pH as well as the decomposition process. The abundance of Populus tremuloides trees was associated with higher exchangeable cations, cationic exchangeable capacity and pH of the forest floor layer on all sites. A decrease in organic matter thickness with increasing aspen presence was also found on all sites, suggesting that this species affects the decomposition process by the quality of its litter as well as by a general improvement of soil physical and chemical properties. The decomposition rate of a standard substrate as well as in vitro potential net nitrogen mineralization were positively related to Populus tremuloides on only one of the three sites, and non-significant on the other sites. Strong immobilization of added nitrogen during incubation was observed on all sites and was not related to aspen, which suggested that in these stands, the soil microbial community is uniformly and strongly nitrogen limited. The zone of influence of Populus tremuloides was evaluated in areas around the soil sampling plot ranging from 3 to 7 m. The results revealed that this zone varies with soil properties. The results suggest that the presence of Populus tremuloides accelerate nutrient cycling, which could affect stand productivity to some extent.