PS 32-33
Forest structure and advance regeneration following the mountain pine beetle epidemic in Canada’s boreal forest
Episodic mountain pine beetle (Dendroctonus ponderosae) outbreaks have occurred for millennia in temperate forests of western North America. The current outbreak began in the late 1990s and is now of unprecedented intensity and spatial extent. By 2007, the mountain pine beetle (MPB) outbreak spread north and east into Canada’s boreal forests, which have no previous history of MPB disturbances. Given the lack of historical record of MPB outbreaks in boreal forests, we have no information to evaluate their capacity to recover from this novel disturbance and continue to provide ecosystem services. In other areas, stand development following MPB outbreaks has often been strongly dependent on growth of pre-existing, understory trees. Thus, assessment of the potential contribution of understory trees (advance regeneration or seedling bank) to canopy tree replacement is an important step in evaluating the capacity of boreal forests to recover from MPB outbreaks. Our objective was to determine the density, spatial distribution, size structure, and species composition of advance regeneration in Pinus contorta-dominated stands of British Columbia’s boreal zone. Characteristics of advance regeneration were related to overstory features (composition, density, basal area and spatial distribution), soil moisture, and stand age using multivariate statistical techniques.
Results/Conclusions
MPB killed 42 to 93% of the overstory pine and reduced overstory basal area by 12 to 83% across all stands. Understory pine were rarely killed. Among stands, advance regeneration ranged from 50 to 18,820 stems/ha and it was strongly clustered within stands. Regeneration occurred in all height classes with shade-tolerant Abies lasiocarpa dominating the smallest classes and faster-growing Picea dominating the largest. The species composition of advance regeneration differed among stands. Picea mariana was predominant under nearly pure P. contorta stands, A. lasiocarpa under mixed P. contorta-Picea stands and Picea engelmanii x glauca under mixed P.contorta-Populus tremuloides stands. A. lasiocarpa dominated regeneration of dry sites and mature stands, while P. mariana regeneration was more abundant on moist sites and in young stands. Advance regeneration densities, together with remaining overstory density, suggests high potential for development of stands with pre-disturbance tree density and basal area. However, MPB outbreaks will accelerate succession, facilitate a shift to dominance by shade-tolerant species, and produce forests very different from those produced by fire, the historical disturbance in these systems. This recovery process may be modified, though, by climate change effects on tree growth, future tree establishment, and forest susceptibility to subsequent disturbances.