Charcoal in North American boreal forests: implications for carbon storage and management

Abstract

Charcoal plays an important role in boreal soil function and carbon storage, and is present in most boreal forest soils as a result of naturally recurring wildfires. Charcoal management has been proposed as a potentially valuable tool for addressing long term issues of global carbon cycles and sustainability of forest management. However, a proper understanding of boreal charcoal stocks and their effect on boreal soil carbon balance is imperative for successful application. The objectives of this thesis were to 1) review the role of boreal charcoal carbon stocks on boreal soil carbon balance; 2) determine naturally occurring wildfire derived charcoal stocks in North American boreal forests and compare these to laboratory produced charcoal; and 3) determine the risk of charcoal additions priming the decomposition of boreal soil organic matter. A review of the literature of boreal soil charcoal studies found wildfires convert 0.7-2% of biomass to charcoal, with charcoal in boreal soils accounting for 1 Pg of carbon globally. Charcoal is highly aromatized and represents a form recalcitrant carbon, with half-lives one to two orders of magnitude greater than other soil organic matter, being able to remain onsite for millennia. However, due to a lack of soil mixing processes, charcoal in boreal soils is vulnerable to re-combustion in recurring fires, inhibiting the accumulation of charcoal over time, unlike in other fire prone ecosystems. Boreal charcoal stocks appear to be highly variable, depending on the intensity of fire. Increased fire intensity results in greater charcoal formation. Charcoal stocks in this thesis were 2-3 times greater than those reported for other boreal ecosystems where non-stand replacing fires are the predominant disturbance; suggesting global estimates of boreal charcoal carbon stocks may need to be revised upwards to account for greater wildfire intensity in North American boreal forests. Within boreal forests, charcoal is an important driver of soil function, stimulating microbial activity and nitrogen mineralization and decreasing phenolic concentrations. High productivity in post-fire stands has been attributed in part to the presence of active, newly produced charcoal that helps to reverse long term declines in productivity with increasing time since fire. Changes in carbon content and chemical composition, however, occur over time, especially in the first ~100 years.