Biodiversity responses to harvesting coarse woody debris for bioenergy

Timber harvest operations often produce substantial quantities of woody residue. Use of logging residues as bioenergy feedstock can reduce wildfire hazards, facilitate forest regeneration, and provide economic returns to forest owners while increasing supplies of renewable energy.

Impacts on biodiversity are among the ecological concerns associated with utilization of logging residues. Although detailed information about biodiversity response to harvest residue removal has not been collected, the importance of snags and down coarse woody debris (DCWD) to cavity-nesting birds and other wildlife has long been recognized. As a result, some research efforts have experimentally manipulated levels of snags and DCWD in a way that closely mimics changes likely to occur from biomass harvests.

To assess potential responses of vertebrates to removal of snags and DCWD during biomass harvests, NCASI supported a review of the literature and a meta-analysis of experimental studies. The research was conducted by Sam Riffell of Mississippi State University, Jake Verschuyl of NCASI, Darren Miller of Weyerhaeuser Company, and Ben Wigley of NCASI. Results from the review and meta-analysis have been published as “Biofuel harvests, coarse woody debris, and biodiversity—A meta-analysis” in Forest Ecology and Management (Vol. 261, Pages 878-887). The abstract follows.

“We calculated 745 biodiversity effect sizes from 26 studies involving manipulations of CWD (i.e., removed or added downed woody debris and/or snags). Diversity and abundance of both cavity- and open-nesting birds were substantially and consistently lower in treatments with lower amounts of downed CWD and/or standing snags, as was biomass of invertebrates. However, cumulative effect sizes for other taxa were not as large, were based on fewer studies, and varied among manipulation types. Little is currently known about biodiversity response to harvest of fine woody debris. Predicting the effects of biomass harvests on forest biodiversity is uncertain at best until more is known about how operational harvests actually change fine and coarse woody debris levels over long time periods. Pilot biomass harvests report post-harvest changes in CWD levels much smaller than the experimental changes involved in the studies we analyzed. Thus, operational biomass harvests may not change CWD levels enough to appreciably influence forest biodiversity, especially when following biomass harvest guidelines that require leaving a portion of harvest residues. Multi-scale studies can help reduce this uncertainty by investigating how biodiversity responses scale from the small scale of manipulative experiments (i.e., 10-ha plots) to operational forest management and how biodiversity response to CWD levels might vary at different spatial and temporal scales and in different landscape contexts.” 

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