Abstract
Prescribed fires are conducted extensively in pine-dominated forests throughout the Eastern USA to reduce the risk of wildfires and maintain fire-adapted ecosystems. We asked how fire behavior and fuel consumption during prescribed fires are associated with turbulence and energy fluxes, which affect the dispersion of smoke and transport of firebrands, potentially impacting local communities and transportation corridors. We estimated fuel consumption and measured above-canopy turbulence and energy fluxes using eddy covariance during eight prescribed fires ranging in behavior from low-intensity backing fires to high-intensity head fires in pine-dominated forests of the New Jersey Pinelands, USA. Consumption was greatest for fine litter, intermediate for understory vegetation, and least for 1 + 10 hour wood, and was significantly correlated with pre-burn loading for all fuel types. Crown torching and canopy fuel consumption occurred only during high-intensity fires. Above-canopy air temperature, vertical wind velocity, and turbulent kinetic energy (TKE) in buoyant plumes above fires were enhanced up to 20.0, 3.9 and 4.1 times, respectively, compared to values measured simultaneously on control towers in unburned areas. When all prescribed fires were considered together, differences between above-canopy measurements in burn and control areas (Δ values) for maximum Δ air temperatures were significantly correlated with maximum Δ vertical wind velocities at all (10 Hz to 1 minute) integration times, and with Δ TKE. Maximum 10 minute averaged sensible heat fluxes measured above canopy were lower during low-intensity backing fires than for high-intensity head fires, averaging 1.8 MJ m−2 vs. 10.6 MJ m−2, respectively. Summed Δ sensible heat values averaged 70 ± 17%, and 112 ± 42% of convective heat flux estimated from fuel consumption for low-intensity and high-intensity fires, respectively. Surprisingly, there were only weak relationships between the consumption of surface and understory fuels and Δ air temperature, Δ wind velocities, or Δ TKE values in buoyant plumes. Overall, low-intensity fires were effective at reducing fuels on the forest floor, but less effective at consuming understory vegetation and ladder fuels, while high-intensity head fires resulted in greater consumption of ladder and canopy fuels but were also associated with large increases in turbulence and heat flux above the canopy. Our research quantifies some of the tradeoffs involved between fire behavior and turbulent transfer of smoke and firebrands during effective fuel reduction treatments and can assist wildland fire managers when planning and conducting prescribed fires.
Highlights
IntroductionPrescribed fires are conducted extensively throughout North America because they are one of the most cost-effective methods to reduce hazardous fuels and mitigate the risk of severe wildfires [1,2,3,4]
Prescribed fires are conducted extensively throughout North America because they are one of the most cost-effective methods to reduce hazardous fuels and mitigate the risk of severe wildfires [1,2,3,4].In the Eastern USA, prescribed fires are essential for the maintenance of fire-adapted ecosystems, such as those dominated by pines and oaks [5,6,7,8]
Quantitative measures of fuel consumption and above-canopy turbulence and heat fluxes reported here, along with within-canopy and near-surface measurements (e.g., [15,25]), can provide important information for the evaluation of recently-developed physics based fire behavior models targeted at simulating prescribed fires (e.g., QUIC-Fire; [46]), and smoke dispersion models which include the effects of forest canopies on turbulence regimes [14,47,48]
Summary
Prescribed fires are conducted extensively throughout North America because they are one of the most cost-effective methods to reduce hazardous fuels and mitigate the risk of severe wildfires [1,2,3,4]. In the Eastern USA, prescribed fires are essential for the maintenance of fire-adapted ecosystems, such as those dominated by pines and oaks [5,6,7,8]. Prescribed fires may enhance forest resilience to other disturbances such as insect infestations [12,13]. Because prescribed fires can result in significant emissions of particulate matter and regulated pollutants, smoke management has become an important issue when they are conducted near larger human population centers in the Eastern USA [14,15,16]. Additional constraints to the use of prescribed fire involve reducing the risk of escapes by limiting ember production and transport and controlling fire perimeters in areas with high property values
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