Canadian Forest Service Publications
Fuel size impacts on carbon residuals and combustion dynamics in masticated woody debris. 2016. Thompson, D.K.; Schiks, T.J.; Wotton, B.M. Forest Ecology and Management 369(2016):59-65.
Issued by: Northern Forestry Centre
Catalog ID: 36794
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Mastication of standing trees to reduce crown fuel loading is an increasingly popular method of reducing wildfire hazard in the wildland–urban interface of Canada. Previous research has shown that masticated fuel beds can leave considerable pyrogenic and black carbon residuals after burning, though the impact of fuel particle size and the influence of combustion dynamics such as the rate of combustion are not understood. Replicated fuel beds of masticated boreal conifer woody debris were experimentally combusted under two moisture content levels and with and without coarser fuel particles (1–3 cm diameter). Pyrogenic carbon production in fragments greater than 1 mm was doubled with the inclusion of coarse fuel particles, and carbon content of the pyrogenic carbon increased to 80%. Conversely, combustion dynamics such as the duration and mean mass loss rates during flaming and smouldering phases were controlled exclusively by moisture content, with no effect of particle size. Pyrogenic carbon production in fragments <1 mm in size was independent of moisture or size treatment, but related to two metrics of combustion dynamics: maximum conductive heat flux and the duration of combustion. Prescribed burning of masticated fuel beds undertaken with both coarser fuel particles while under moderate moisture conditions will achieve both a higher yield of residual carbon owing to the particle size effect, as well as limiting the intensity of combustion due to the moisture content effect.
Plain Language Summary
Chipping or mulching entire trees in the boreal forest to reduce fuel in case of wildfire is an increasingly common practice in Canada, especially near human settlements, to protect them from fire. But there is no set standard for the size of mulch particles, and we do not know how mulch particle size affects the rate of combustion and the carbon left after a fire. We conducted a laboratory study to determine how mulch particle size and moisture content affect the rate of combustion as well as the quality and quantity of any remaining partially burned material. The study found that mulch containing larger particles (1 to 3 centimetres wide) burned just as completely and spent similar time flaming versus smouldering when compared with mulch with smaller particles. However, moisture content did have an effect, with wetter mulch burning slower and spending more time smouldering rather than flaming. The amount of leftover materials after fire doubled when larger mulch particles were included, and those particles had a higher carbon content, meaning they were less likely to decompose in the future. Production of “black carbon” — a long-lasting form of carbon that resists decomposing — was not affected by particle size or moisture content, but rather by how long and how hot the mulch burned. This laboratory study can guide those involved in mulching trees for forest fire protection, who want to limit combustion and increase the amount of leftover materials by using larger mulch particles with moderate moisture content.