The mesic southern boreal mixedwood (SBMW) forest comprises a large portion of the forested landscapes of the northern Lake States and holds significant ecological and economic importance. Mature forests of this type are dominated by trembling aspen, balsam fir, paper birch and white spruce. Despite the prevalence of this forest type, our knowledge concerning stand developmental patterns and the impacts of natural disturbance (i.e. fire, wind, pest, and pathogen) on stand composition and structure is quite limited, impeding efforts to develop natural disturbance-based forest management systems for these communities. Moreover, this lack of knowledge also presents challenges in forecasting the potential shifts in forest disturbance regimes and compositional patterns likely to accompany climate change within the region.
SBMW forests also hold tremendous potential in terms of maximizing aboveground carbon storage within forested areas. In particular, these communities commonly form stratified, mixed species stands composed of species with a high degree of complementarity in resource use. As such, there is a potential for greater levels of productivity within these communities relative to pure stands.
For this study, we have combined dendrochronological and stand-wide carbon measurements to assess forest disturbance patterns over the last century and to relate these patterns to current forest composition and stand-wide structure. In addition, we are addressing questions, including: Do stands with a greater diversity of species and age classes display high levels of productivity and resilience, and in turn, higher net carbon storage than less complex systems?
Results/Conclusions
For all sampled stands, decadal disturbance chronologies and age distributions revealed a pattern of canopy disturbance and recruitment typical of mesic, wind dominated systems where gap-phase dynamics are fueled by insect defoliation, fungal infection, and windthrow. While all sites show similar patterns of recruitment, tree species diversity varies with stands containing higher levels of overstory diversity also exhibiting the greatest levels of aboveground tree biomass. This positive relationship reaches a point of inflection where increases in productivity due to species diversity may be outweighed by the increase of coniferous species and there stifling affect on stand-level nutrient cycling. This finding confirms our a priori hypothesis that hardwood-softwood species mixtures may be beneficial, but that such relationships are likely non-linear. Ultimately, our study clarifies the relationship between disturbance and stand composition and structure to inform the design and implementation of silvicultural systems that foster such resilience.