Aim

Impacts of longitudinal gradients on species distributions have received less attention than latitudinal gradients. In Canada, precipitation varies longitudinally, with direct effects on plant growth and fire regimes. Despite the geographical extent of Canadian boreal forests, vascular plant diversity is relatively limited, with just under 300 species. Understorey communities comprise most of this diversity and play key roles in forests, affecting succession, nutrient cycling and wildlife habitat. Our objective was to evaluate the relative impacts of local and regional environmental conditions on vascular plant community composition and diversity in the boreal forest.

Location

Sampling took place in 33 trembling aspen (Populus tremuloides) stands along a transcontinental gradient in precipitation and fire regimes in boreal Canada.

Methods

We measured community composition and vegetation diversity indices of the vascular understorey in 400-m2 plots and measured environmental variables at the local (drainage, nutrient and light availability, site heterogeneity) and regional (annual temperature and precipitation, fire weather index, landscape heterogeneity) scales. Multivariate analyses and hierarchical model selection were used to analyze patterns.

Results

Species composition varied longitudinally, with western and eastern species pools. Western indicator species tended to be fire-adapted pioneer species, while indicator species at eastern sites were fire avoiders and late-successional species. Species richness and intra-site beta diversity seemed primarily driven by local variables and did not vary across the country. Species evenness, however, was slightly higher in the western region and decreased under higher precipitation, colder temperatures, and higher landscape-level heterogeneity.

Conclusion

Our results suggested that even for similar canopy composition environmental variables play a key role in the establishment of plant communities and structure local plant assemblages by selecting or eliminating species from the regional pool, which was also controlled by fire regimes.

Dinitrogen (N2) fixation associated with moss-dwelling bacteria (diazotrophs) is a N source in boreal forests. In feather moss-dominated understories of Fennoscandia, N2 fixation rates increase with forest age due to N limitation. Whether this is applicable across different boreal ecosystems and successional pathways is not fully understood. In eastern Canada, increasing Sphagnum dominance starting about 100 years post-fire (paludification), is prevalent. Our main objectives were to determine how potential moss-associated N2 fixation and relative contributions of Sphagnum and other mosses vary with time since fire and to explore the mechanisms driving observed patterns. Sampling was conducted in eight black spruce forests (Picea mariana; 64-724 years post-fire) in northwestern Quebec, Canada, along an established post-fire successional gradient. Potential N2 fixation was measured with the acetylene reduction method in seven common moss species (Pleurozium schreberi, Hylocomium splendens, Dicranum polysetum, Sphagnum capillifolium, Sphagnum angustifolium, Sphagnum russowii, and Sphagnum magellanicum). N2 fixation rates by moss communities were low and seemed to increase with stand age, but large variability was found between sites and species. The mechanisms underlying N2 fixation rates were related to a switch in moss communities from feather moss to a Sphagnum dominance, which had the highest individual acetylene reduction rates, and to an increasing frequency of N2-fixing samples in old forests. This study provided one of the first records of moss-associated N2 fixation in eastern Canada and offers insights into how regional and cross-continental differences in moss community composition determine N cycling in boreal forests.

In the managed portion of the boreal forest of eastern North America, logging has replaced fire as the most important disturbance agent. There, a large proportion of timber is harvested in forests susceptible of accumulating a thick Sphagnum layer that decreases forest productivity, a process called paludification. In such a context, understanding how disturbance type and severity of soil disturbances may affect post-disturbance microhabitat characteristics and understory community composition is critical for forest management. Different management techniques have been used such as careful logging and clearcutting, as well as winter and summer harvests, with various impacts on soils and forest regeneration. In the current study, we used 55 study sites representing a gradient of soil disturbance severity by harvesting (winter and summer) and fire (low and high severity) to compare their impacts on understory plant communities in the Clay Belt area of eastern Canada. At each site, understory community composition (vascular plants, bryophytes, and lichens) was assessed. We found a strong response of communities to overall severity as represented by disturbance type (careful logging, clearcutting, and fire), but little impact of fine scale disturbance severity (winter vs. summer, low vs. high severity disturbance) within each type of disturbance. Differences in community composition were reflected in the abundance of the various plant functional types, with invaders being more common in harvested sites, endurers being common in all disturbances except high severity fires, and avoiders being more common in older sites. Understory communities in harvested sites (<40?years old) were similar to communities typical of old sites originating from natural wildfire disturbances (75–100?years old low severity fires or 200?years old high severity fires) in terms of composition, but also Sphagnum spp. abundance. In order to maintain long-term forest productivity and manage forests in ways that more closely reproduce post-fire conditions, logging operations should aim at increasing soil disturbances, for example by using prescribed burns, in the Clay Belt area of eastern North America.