Forest stand structure and dynamics at Riding Mountain National Park, Manitoba, Canada

We used multivariate analysis to model boreal forest stand structure and dynamics at Riding Mountain National Park, Manitoba based on data from 202 sampled stands. Eight forest stand-types were recognized based on canopy composition: black spruce on peat substrates, jack pine — black spruce, bur oak, eastern deciduous (green ash — American elm — Manitoba maple), balsam fir, trembling aspen — paper birch — mountain maple, trembling aspen — balsam poplar, and white spruce. The first four stand-types occur in edaphically distinct environments, while the four remaining boreal mixedwood stand-types occur in edaphically similar environments. We found that the composition and abundance of advance regeneration were best predicted by current canopy composition (redundancy = 54.4%); this reflects both the limited dispersal of conifer seeds and the strong vegetative reproductive capacity of hardwoods. Biotically-controlled site factors such as bareground, herb and shrub cover, ungulate browsing intensity, and stand age were also reasonably good predictors of advance regeneration (redundancy = 31.7%). Edaphic variables such as soil pH, conductivity, particle size, organic horizon depth and slope proved to be poor predictors of advance regeneration, however (redundancy = 18.1%). Size-class ordination indicated that many stand-types have relatively short successional trajectories, suggesting limited change in forest canopy composition over time. There are two exceptions: in the jack pine — black spruce stand-type, black spruce will increase over time, and in the trembling aspen — paper birch — mountain maple stand-type, eastern deciduous species (green ash, American elm, Manitoba maple, and bur oak) are forecast to become increasingly dominant. We also describe a synoptic model of mixedwood boreal forest stand dynamics for the Riding Mountain area. The model includes a number of factors that we consider to be critical determinants of forest dynamics, such as seed source availability, small and large-scale disturbances, species life-history characteristics, and environmental gradients. Our succession model is more similar those described for eastern than western Canada, which may reflect the lower frequency of catastrophic fires in the Riding Mountain area compared to boreal forests further west. Our model emphasizes that successional trajectories do not converge towards a single self-perpetuating “climax”. Instead, successional vectors may diverge, converge or remain cyclical, and multiple potential pathways are possible for each stand-type. Our results also illustrate that species assemblages, and the propensity for canopy change in the absence of fire, are governed by the cumulative and synergistic effects of climate, topography, disturbance frequency, size and intensity, edaphic conditions, and the proximity of parental seed sources. Fire suppression in the southern boreal forest has resulted in a paradigm shift in disturbance regime, from large, synchronous catastrophic fires to small-scale, asynchronous gap formation. A major challenge for boreal forest ecologists is to determine the long-term consequences of this paradigm shift on the composition, structure and health of boreal forest stands and landscapes.