Bioenergy harvesting impacts on ecologically important stand structure and habitat characteristics

Background/Question/Methods

Demand for forest bioenergy fuel is increasing in the northern forest region of eastern North America and beyond, but the ecological impacts – particularly on habitat – of bioenergy harvesting remain poorly explored in the peer-reviewed literature. In particular, only a limited number of datasets are available in eastern deciduous forests specifically quantifying potential impairments to stand structure.

We empirically evaluated the impacts of bioenergy harvests on stand structure, quantifying biometrics considered indicative of important biodiversity and habitat functions. We collected stand structure data from 35 recent harvests in northern hardwood-conifer forests, pairing harvested areas with unharvested reference areas. Biometrics generated from field data were analyzed using a multi-tiered nonparametric uni- and multi-variate statistical approach. Our analysis also treated harvest volume allocated to bioenergy as a continuous variable, recognizing that bioenergy is typically one of many different products resulting from harvests in the U.S. Northeast.

Results/Conclusions

In analyses comparing harvested to reference areas, sites that had been whole-tree harvested demonstrated significant differences (relative negative contrasts; p<0.05) in snag density, large live tree density, well decayed downed coarse woody debris volume, and structural diversity index (H) values, while sites that had not been whole-tree harvested did not exhibit significant differences. Classification and regression tree (CART) analyses suggested that the strongest predictors of structural retention, as indicated by downed woody debris volumes and H index, were silvicultural treatment and equipment type rather than the proportion of harvested volume allocated to bioenergy uses.

In general, bioenergy harvesting impacts were highly variable across the 35 sites sampled. While previous research has tended to downplay the variability evident in harvesting practices, our research highlights the importance of considering bioenergy as a continuous variable in the context of harvests having multiple objectives and products. For instance, impairment of snag availability was, in our dataset, shared to some degree by all harvesting types and did not show a specific association with bioenergy harvesting. It is therefore important to evaluate proposed bioenergy harvesting guidelines within the larger context of sustainable forest management. Equally important is grounding guidelines in rigorous scientific analyses that examine a range of stand conditions, silvicultural objectives, and harvesting practices. The variability of the impacts brought to light through our analyses supports a role for bioenergy harvesting guidelines aimed at encouraging retention of ecologically important structural attributes. Consideration of equipment selection, particularly skidder type and size, is also important for minimizing potentially deleterious impacts.