Characteristics Of Fires

Wildland fires are increasing in many parts of the world due to increased human pressure and an apparent increase in the severity of climatic conditions leading to large catastrophic fires. Although attribution is difficult, the observed warming of the planet during the past two decades has been coincident with an increase in fires in ecosystems ranging from the tropics to the boreal forests during the last two decades (e.g., Agee, 1993; Prins and Menzel, 1994; Larsen, 1996). Partly as a result of these natural disasters, policy makers and scientists are gaining appreciation for the values at risk from fire as well as the ecological importance of fire. The use of prescribed fire to sustain ecosystems, prevent catastrophes, and manage natural resources is on the increase in many countries of the world. Also, of course, the use of fire to clear forests for conversion to agriculture, especially in the tropics, continues at an alarming rate.

We use the term "fires" to be inclusive of "prescribed biomass fires" (i.e., those that are intentionally used to accomplish resource and landuse management objectives) and "wildland fires" (i.e., all fires that are unintentional). The term "wildland fuelbeds" excludes agricultural fuelbeds, but includes all of the live and dead biomass between the mineral soil and the top of the dominant vegetation canopy of ecosystems. Fires vary widely in their intensity (heat release per unit time) and severity (heat release per unit area) because of differences in the physical characteristics of wildland fuelbeds, the condition of the fuel elements (especially fuel moisture), the current weather, and the nature of ignition.

Fire intensity, which in part controls combustion efficiency and plume rise, varies over several orders of magnitude according to natural and managed variability in fuel condition, weather conditions, and ignition pattern. At one end of the spectrum are prescribed fires used for landuse conversion or wildfires during periods of optimum conditions, with heat release rates sufficient to loft plumes high into the troposphere. At the other end of the spectrum are smouldering ground fires or fires in very light fuels, with heat release rates so low that plumes rarely exceed surface boundary layer heights. Intensity is likely to vary dramatically with diurnal winds and humidity if fires burn more than several hours. Freely spreading fires also vary in intensity from minute-to-minute as wind and other burn conditions change.

Differences in fire severity cause ranges in fuel consumption from about 1,000 to 500,000 kg per hectare. Fires used for land clearing in converting forests to agriculture, grazing, or urban development are intentionally high intensity and high severity with as much as 70% to 90% fuel consumption. The highest consumption rates occur in dry fuels that are densely distributed or piled then ignited almost instantaneously. If burned early in a dry season and soon after logging, however, less than 25% of the fuels may be consumed because high fuel moistures reduce combustion. Fires in the understory of forests or woodlands may consume less than 10% of accumulated biomass as fuels are sparsely scattered and ignition is gradual or spotty. In the same biome, however, if fires involve the dominant vegetation of connected canopies, as much as 60% of total above-ground biomass may be consumed. Fire severity can be controlled by mechanically manipulating the fuel bed, and/or scheduling intentional fires and controlling the ignition pattern. Other policy options that can limit biomass consumption include preventing wildfires and prohibiting prescribed fires when high severity is expected.

Fire duration, typically ranging from an hour to several weeks, is another important variable. Emissions and biomass consumption can be minimised by nearly instantaneous ignition, creating a short-duration convection column that collapses soon after flaming stops, followed by very little smouldering. Fires that last for many days promote smouldering combustion, higher emission factors (i.e., mass of emissions per mass of biomass consumed), and serve as an ignition source for wildfires. Generally speaking, management practices that promote fires of shorter duration are favoured for reducing emissions and for limiting the impact of non-buoyant plumes.

Despite the desire to reduce emissions by reducing fire duration, long-duration fires are becoming increasingly common as land managers promote low intensity, meandering fires that remove fine fuels but do not damage large trees. This is causing the diurnal cycle to play an increasingly significant role in emission rates and related impacts. During the night, emission rates usually decrease. At the same time, however, threats to human health increase. During the day, when emission rates usually increase, greater dispersion allows lower surface concentrations but contributions to regional haze and its impact on visibility and radiative flux become pronounced. Figure 1 shows the light-scattering coefficient, which is proportional to particle mass, from a nephelometer that was placed approximately 4km down-valley from a 360 hectare, prescribed understory burn in northeastern Oregon. Ignition began at 1100 Pacific Daylight Time (PDT) on 13 May 1997 and flaming was complete by approximately 1600 PDT the same day. Smoke entered the valley from the smouldering fire as soon as radiative cooling at night diminished lofting.

Figure 1. Hourly light-scattering coefficient from measurements taken approximately 4 km down-valley from a 360-hectare smouldering under-story burn in northeastern Oregon.

Emissions continued for several days, barely noticeable during the day when emissions were lofted away from the valley, but each night smoke settled into the valley as smouldering from rotten logs and old stumps continued. Total particulate emissions from this fire were estimated to be nearly Less than one half of this total was emitted during the first few hours of ignition when heat release rates were relatively high and buoyant emissions were dispersed widely by upper-level winds. The remaining smoke, with over 10 x 106 kg of particulate matter, was emitted in the next several days after ignition while the fuel smouldered independently. The weakly buoyant emissions lingered close to the ground surface where they were transported by topographically controlled thermal winds. An estimate of impact based on total emissions would have missed the diurnal variability in emission rates and concentrations.

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