Dynamics of dead tree degradation and shelf-life following natural disturbances: can salvaged trees from boreal forests 'fuel' the forestry and bioenergy sectors?

Trees killed by natural disturbances have been recognized by the International Panel on Climate Change (IPCC) as a promising resource for bioenergy at the global scale. In the eastern boreal forest of Canada, there are two major types of natural disturbances that can generate large amounts of biomass for the production of bioenergy: wildfire and spruce budworm outbreak. For example, the mean burned area between 1970 and 2010 was estimated at 2900 km(2) per year. Following such disturbances, typically only trees and stands with a merchantable value, i.e., of acceptable quality for traditional forest products (lumber and pulp) are salvaged. However, adding bioenergy to the potential basket of products may both divert trees of marginal quality and profitability away from traditional products and to bioenergy facilities and lengthen the window of opportunity during which salvage operations can occur. This review shows how the dynamics of wood characteristics of trees affected by natural disturbances can be used to predict through time the basket of wood products that can be taken out of a salvaged stand and ensure the best fit between sources of fibre and types of processing facilities. The most important factors influencing fibre quality include degradation caused by fungi and insects. The most suitable time to salvage trees for the production of lumber in stands killed by either spruce budworm or fire is generally limited to 1-2 years after death. For the production of pulp and paper, trees can usually be salvaged for as long as the wood moisture content remains above the fibre saturation point, but usually is not recommended after 3 or 4 years following death. Thus, past this period, salvaged trees may yield better opportunities for the bioenergy sector. Information on wood as bioenergy feedstock (wood chips, wood pellets and liquid biofuel) highlights the importance of wood chemical components in the chemical reactivity of biofuel. This study offers background knowledge and a framework of analysis that highlights the potential of dead wood from natural disturbances for the production of forest and bioenergy products, which can be further adapted to other regions of the world, building on the Canadian experience of adapting practices to natural disturbances.