Can variable-retention harvesting help reduce fuel loads and restore mixed-pine forest ecosystems?

Background/Question/Methods

Fire suppression and legacies of past management have led to changes in the structure and composition of mixed-pine forest ecosystems of the northern Lake States. Changes include declines in natural regeneration of red pine (Pinus resinosa Ait.) and eastern white pine (P. strobus L.), accumulation of fuels outside of the natural range of variation, and an increase in dominance of short-lived and fire-sensitive species. In 2010, we implemented a variable-retention harvest aimed at regenerating red pine and eastern white pine while also reducing live and downed fuels, by reducing stand basal areas by up to 70% in either an aggregate or uniform pattern. In 2011, we inventoried seedling densities and fuels to examine initial responses to the restoration treatments compared with unharvested controls. We also examined treatment effects on leaf litter decomposition rates and C: N ratio over different time periods (1, 30, 230, 300, 365 days).  Litter decomposition was measured by both the amount of biomass lost and C: N ratios for red pine and mixed-hardwood species in harvested and control plots. Initial (1 yr) measurements were analyzed using ANOVA, and relationships among seedling densities and fuels, canopy cover and overstory species composition examined using canonical correspondence analysis (CCA).

Results/Conclusions

Initial densities of red pine seedlings were higher in the uniform stands (p=0.036), and eastern white pine seedlings higher in the aggregate stands (p<0.001), compared to the unharvested control. All pine seedlings were likely advanced regeneration, not newly recruited seedlings following treatments. High densities of red maple (Acer rubrum L.) seedlings were recorded across all treatments and are likely to be a potential competitor with red pine and eastern white pine seedlings in our treated stands. CCA of seedling densities and environmental variables suggests that fuels (litter depth, duff depth and fuelbed depth) and overstory species composition were the factors most strongly influencing composition of the seedling layer. 30-day leaf litter decomposition data indicates significant biomass losses in the aggregate treated stands for both red pine (p=0.011) and mixed-hardwood (p=0.004) litter, and no significant differences between the two treatment patterns. We plan to continue examining the effects of these treatments on the regeneration layer and fuels. These initial seedling and litter decomposition results, however, suggest variable-retention harvesting may be a useful tool to improve regeneration of red pine and eastern white pine, reduce fuel loadings, and direct the successional trajectory of these mixed-pine forest ecosystems toward the desired reference conditions.