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.

Changes in the light availability in forests generated by diversified retention patterns (e.g., clear cut, partial harvest) have been shown to strongly filter the plant species present. Modified soil microsite conditions due to post-harvest site preparation (e.g., mechanical site preparation, prescribed fire) might also be an important determinant of plant diversity. The objective here was to detect how retention pattern and post-harvest site preparation act as filters that explain the understory functional diversity in boreal forests. We also assessed whether these effects were dependent on forest attributes (stand type, time since fire, and time since harvest). We retrieved data from seven different studies within 101 sites in boreal forests in Eastern Canada. Our data included forests harvested with two retention patterns: careful logging and clear cut, plus unharvested control forests. Three post-harvest site preparation techniques were applied: plow or disk trenching after careful logging, and prescribed fire after clear cut. We collected trait data (10 traits) representing plant morphology, regeneration strategy, or resource utilization for common species. Our results demonstrated significant variation in functional diversity after harvest. The combined effect of retention pattern and site preparation was the most important factor explaining understory diversity compared to retention pattern only and forest attributes. According to RLQ analysis, harvested forests with site preparation favored traits reflecting resistance or resilience ability after disturbance (clonal guerilla species, geophytes, and species with higher seed weight). Yet harvested forests without site preparation mainly affected understory plant species via their light requirements. Forest attributes did not play significant roles in affecting the relationship between site preparation and functional diversity or traits. Our results indicated the importance of the compounding effects of light variation and soil disturbance in filtering understory diversity and composition in boreal forests. Whether these results are also valid for other ecosystems still needs to be demonstrated.