Influence of aspen on forest floor properties in black spruce-dominated stands.
Sonia Légaré, David Paré, Yves Bergeron.
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.