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

Hart, S.A. 2013. Charcoal in North American boreal forests: Implications for carbon storage and management. 102 pp. 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 nonstand 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. The charcoal content of 75 soil cores 10 cm in diameter was measured to a depth of 10 cm along a chronosequence of five post-fire ages from boreal forests ~ 150 km north of Thunder Bay, Ontario. Charcoal particles > 0.25 mm were counted manually from each soil core. Charcoal stocks ranged from 2.30 to 5.20 Mg ha, with an average carbon content of 53%. Charcoal carbon stocks ranged between 1.11 and 2.80 Mg C ha with a mean of 2.19 Mg C ha Young wildfire charcoal was found to be primarily aryl carbon with smaller amounts of alkyl-C, O-Alkyl, and methoxyl groups. Over the course of a 208 year chronosequence carbon density declined from 56 to 46%, with charcoal greater than 140 years old almost entirely aryl carbon. In the absence of fire, charcoal may lose many important properties and carbon content,