Pile age and burn season influence fuelbed properties, combustion dynamics, fuel consumption, and charcoal formation when burning hand piles
Piling and burning is widely used to dispose of unmerchantable debris resulting from thinning in forests throughout the western United States. Quite often more piles are created than are burned in a given year, however, causing piles to persist, accumulate, and age on the landscape. The effects of burning piles of increasing age has not been studied. We examined the effects of time since construction (i.e., pile age, in roughly six month increments for two years) and burn season (fall and spring) on fuelbed properties, combustion dynamics, fuel consumption, and charcoal formation for hand-constructed piles in thinned ponderosa pine-dominated sites in New Mexico (n = 50 piles) and Washington (n = 49 piles). Piles compacted over time similarly for both study sites, losing approximately 15% of their height annually for the first two years following piling. Peak flame height decreased and the duration of flaming combustion increased with increasing pile age for both burn seasons in New Mexico, yet depended on burn season in Washington. Increasing fuel moisture and compaction reduced peak flame height and increased flaming duration modestly for both sites. Peak flame height was reduced 6–7 cm and flaming duration increased 0.9–2.3 min for every percentage increase in small fuel moisture. Similarly, peak flame height was reduced 4–5 cm and flaming duration increased 0.6–0.8 min for every percentage reduction in pile height. Fuel consumption was high, averaging 90% in New Mexico and 95% in Washington. Fuel consumption patterns differed between locations, however; fuel consumption decreased with age and was slightly higher for spring than fall burns in New Mexico, whereas, neither pile age nor burn season affected fuel consumption in Washington. Charcoal formation as a fraction of pre-burn pile weight averaged 2.8% in New Mexico and 1.2% in Washington, and was not affected by pile age or burn season. Fuel consumption and charcoal production were unaffected by fuel moisture or compaction levels at either site. Findings from this study will inform fuel and fire managers about the potential effects on fire behavior, fuel consumption, and charcoal formation of burning piles of different age in different seasons under different environmental conditions.