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Fire

1.1 Suitability of Prescribed Fire for Site Preparation

1.2 Prescribed Fire for Forest Vegetation Management

1.3 Prescribed Fire, Insects and Disease

1.4 Effects of Prescribed Fire on Site Productivity

1.5 Prescribed Fire and Air Quality

1.6 Using Prescribed Fire with Other Site Preparation Methods

1 Background

Prescribed burning has a long history in British Columbia. The aboriginal people used fire thousands of years ago to create and maintain wildlife habitat, and to stimulate the production of herbs and berries. Until recently, prescribed fire has been the major tool used in B.C. for both fire hazard abatement and site preparation.

1.1 Suitability of Prescribed Fire for Site Preparation

When properly applied on appropriate sites, prescribed burning is an excellent method for achieving many site preparation objectives. Fire can be used on steep terrain, does not compact the soil, and improves access for planting, surveying, and stand tending activities. Fire impacts can also improve seedling survival and growth by reducing competing vegetation, by increasing nutrient availability, and by warming the soil. Fire can also be used to enhance wildlife habitat, range resources, and, in some areas, it can limit insects and fungi by reducing their habitat.

When implemented under the correct conditions, prescribed fire will reduce the moss layer, the upper few centimetres of the forest floor, and coarse debris. It will also eliminate much of the fine debris, thereby reducing the mechanical obstructions to planting. In the interior, it is rarely a system to be used where natural regeneration is anticipated. In the case of lodgepole pine, fire will generally destroy the residual cones and seed, and in the case of other species, there is little likelihood of exposing adequate areas of mineral soil unless the burn is unacceptably severe.

While prescribed burning has many benefits, it also has certain disadvantages, namely: it carries a risk of escape; its use can be restricted by weather; it is cause for public concern in areas sensitive to smoke; under certain conditions, it may reduce long-term soil fertility; and it can be very costly.

Further information on the beneficial and detrimental effects of fire are contained in the following publications: Feller [1982]; Ballard and Hawkes [1989]; Hawkes et al. [1990]; and Walstad et al. [1990].

1.2 Prescribed Fire for Forest Vegetation Management

On many sites, proper vegetation management is imperative for a successful regeneration program. The site preparation method chosen for a site will often significantly influence the type and abundance of subsequent vegetation. When properly planned and implemented, prescribed fire is one of the most effective and efficient tools available for manipulating vegetation.

The silviculturist may wish to manipulate vegetation for several reasons including: improving plantability; increasing availability of light, moisture, and nutrients for newly established seedlings; reducing mechanical damage to seedlings (e.g., snow press); enhancing wildlife habitat or improving forage for domestic livestock.

An excellent summary of the use of prescribed fire for forest vegetation management is provided by Haeussler [1991]. The report details fire adaptations and response for numerous forest plants of British Columbia. The report also comments on predicting post-fire succession and developing fire prescriptions for vegetation management.

1.3 Prescribed Fire, Insects and Disease

The effect of prescribed fire on insects and disease can be positive or negative depending on species and site.

Insects

Fire may affect insects directly, by consuming them, or indirectly by manipulating their environment. Fire has proven to be an effective tool for preventing the buildup of insects such as the spruce beetle (Dendroctonus rufipennis). The spruce beetle prefers downed material such as fresh windthrow, logs, stumps, or logging debris. A freshly logged spruce site provides excellent habitat for the beetle to build to epidemic levels. The beetle will then attack live trees in adjacent stands. If the logging slash is treated by prescribed fire before the spruce beetle can multiply, the population buildup can be avoided. On the other hand, if a prescribed fire escapes into standing timber, the trees that are scorched by the fire may be more susceptible to insect attack.

The black army cutworm (Actebia fennica) is directly associated with fire, and can significantly damage newly established plantations. The black army cutworm emerges as an adult moth in summer (early July to middle of September) and deposits eggs 2 to 5 weeks later, usually on slashburned sites. In spring, the larvae feed on herbaceous plants and conifer seedlings for approximately 5 to 6 weeks before pupation.

Several factors influence the severity of damage associated with a black army cutworm infestation: if the number of cutworms is relatively low and there is an ample alternate food source (e.g., herbaceous vegetation), seedling defoliation will be low; lodgepole pine is more tolerant of defoliation than white spruce; mortality levels are much higher if defoliation occurs at the time of planting than if it occurs one year after planting.

If slashburning occurs after the moth flight has finished for the year, seedlings may be planted the next spring. Larval feeding will not occur until the seedlings have been established for one year and seedlings are more tolerant of damage. However, if burning takes place before or during moth flight, the cutworm will have the opportunity to deposit eggs on the newly burned site. In this case, planting should be delayed until the following year after larvae have finished feeding.

For more detailed information on the black army cutworm, see Maher [1988].

Disease

Fire may affect forest tree diseases directly or indirectly (i.e., by injuring a host or causing a shift in plant species). Fire does not appear to have a significant direct effect on the major root diseases such as armillaria, laminated root rot (caused by Phellinus weirii) and annosus-although there is some evidence that fire ash leachates enhance soil fungal populations that are antagonistic to some root diseases. The fungi associated with these diseases are often contained in, or reside on, pieces of wood buried deeply enough in the soil to escape lethal temperatures.

One root rot that is directly affected by fire and may severely damage newly established plantations is Rhizina undulata. Spores of this fungus lie dormant in the soil until they are stimulated by high temperatures. Fire not only stimulates the germination of dormant spores, but also sterilizes the soil, eliminating more aggressive fungi that would otherwise become established before Rhizina.

Seedlings affected by Rhizina, as with most root diseases, usually occur in groups. The disease has been found to affect many species including Douglas-fir, western hemlock, Sitka spruce, lodgepole pine, Engelmann spruce, and western redcedar. Damage associated with Rhizina is greatest the first year after burning and decreases with time. Damage on plantations established the third year after burning is usually negligible.

For further information on the effects of fire on forest diseases, see Walstad et al. [1990]. For specific information on Rhizina undulata, see Callan [1990].

1.4 Effects of Prescribed Fire on Site Productivity

Maintenance of site productivity has become a concern to forest managers. To many, maintenance of site productivity is synonymous with sustainability.

As with any site preparation treatment, prescribed fire disturbs the site to some extent. Soil disturbance can range from beneficial to detrimental depending on the nature of the site, burn severity, and interaction with other harvesting and silvicultural practices. Literature reviews detailing the effects of prescribed fire on site productivity are provided by Feller [1982] and Lindeburgh [1990].

While it is difficult to make general statements on how slash burning affects site productivity, two conclusions can be drawn [Lindeburgh 1990]:

  • fires of lower severity have a lower risk of causing site degradation than do fires of higher severity; and
  • at any given severity of fire, drier nutrient-poor sites have a higher risk of being degraded than moister nutrient-rich sites.

All areas for which burning treatments are considered must be accurately assessed to determine the overall site-specific sensitivity to fire and the potential impacts of fire on long-term soil productivity. Where available, ecosystem guide books should be used to make initial decisions, whether an area should be site prepared and if so, by what method. Site-specific sensitivity guidelines are available in some ecological guides (e.g., Klinka et al. 1984, Trowbridge et al. 1989, and Lloyd et al. 1990).

In British Columbia, two general approaches are available to evaluate the potential effects of site preparation on site productivity:

  • Slashburning guidelines for the Vancouver Forest Region (see key in Hawkes et al. 1990); and
  • Site preparation strategies to manage soil disturbance [Curran et al. 1990].

Slashburning guidelines for the Vancouver Forest Region: In the Vancouver Forest Region broadcast burning can often affect site productivity through its influence on erosion and on the nutrient status of the site. Five site sensitivity classes are recognized and can be applied for any given site through the use of a key.

Site preparation strategies to manage soil disturbance: interior sites: This guide is applicable to the interior sites of B.C. The guide considers site sensitivity in terms of five categories: forest floor displacement, compaction, mineral soil displacement, erosion, and mass wasting. Each category of sensitivity is assessed using hazard keys and information collected during a site assessment. A high sensitivity in any one category means that the desired treatment may have to be modified to prevent long-term soil degradation.

The importance of maintaining or enhancing site productivity during a prescribed burn cannot be over-stressed.

1.5 Prescribed Fire and Air Quality

The smoke emitted from prescribed fire may have several adverse side effects such as: reduced visibility, potential health hazards for fireline workers or the public, and potential conflict with other resource values such as tourism, local festivals, or harvesting of crops. Proper smoke management is now an essential component of any prescribed fire.

Smoke Management

Under the current Smoke Management Policy each Forest Region and Forest District is required to develop Smoke Management Plans. These plans restrict prescribed burning programs to periods when air quality objectives, as set out by the Canadian Environmental Protection Act, can be met.

In cooperation with the Ministry of Environment, Lands and Parks, Waste Management Branch, local health officials and other agencies, "smoke sensitive areas" have been designated. Included in this designation are areas where smoke accumulations might cause a safety hazard or public health hazard, or where smoke might unreasonably deny aesthetic enjoyment to the public. Within or adjacent to these "smoke sensitive areas," "modified burning zones" are established. In order to maintain air quality in the smoke sensitive areas, specialized burning techniques and other special treatments are required in the modified burning zones.

Ninety percent of the total mass emitted from wildland fires is water and carbon dioxide [Walstad et al. 1990]. Other emissions include carbon monoxide, particulate matter and volatile organic compounds. Particulate matter is the most important category of pollutant to come from prescribed fire. It is the major cause of reduced visibility.

The extent to which particulate matter affects visibility is determined by the quantity of particulate emitted and the degree of dispersion of particulates in the atmosphere. Particulate emissions from fire can be reduced by using alternatives to burning, by decreasing the amount of fuel consumed by fire, or by increasing the combustion efficiency of the fire (i.e., clean fires with less smouldering).

The following list includes techniques for reducing emissions:

  • Removing more fibre during harvest or using more of the residues following harvesting;
  • Promoting types of fire that minimize smouldering (i.e., a backing fire produces less particulate than a heading fire, also a convection fire tends to reduce smouldering [Hawkes et al. 1990]);
  • Aggressive mopup;
  • Minimizing smouldering; and
  • Rescheduling broadcast burns to more appropriate periods (e.g., from a dry fall period to a wetter spring period when the drought code is lower).

Dispersion of particulates through the atmosphere is influenced by the presence or absence of a convection column above the fire, by atmospheric stability, by wind velocity, and by wind direction. Wind velocity and atmospheric stability have been combined to produce the Ventilation Index [Nikleva 1972]. The greater the wind velocity and the greater the depth of the air layer in which smoke can mix, the greater the Ventilation Index and the greater the dispersion of particulates emitted from a fire.

Energy release rate is also important to smoke dispersion. This is influenced by the size of the fire, ignition rate, and combustion rate. For example, a small number of burning piles have much lower energy release and potential for dispersion than a rapidly ignited broadcast burn of 40 hectares.

For a more detailed discussion of smoke management strategies, including particulate emission and dispersion, see Walstad et al. [1990], chapter 16 and Hawkes et al. [1990], chapter 11.

Prescribed Fire, Carbon Dioxide and the Greenhouse Effect

Human activities are causing increases in the concentration of greenhouse gases, such as carbon dioxide and methane, in the earth's atmosphere. The major sources of carbon dioxide are activities such as the burning of fossil fuels and tropical deforestation. Hence the relationship between carbon dioxide and prescribed fire must be closely examined.

Forests play an important role in the earth's carbon cycle. Trees take carbon dioxide from the atmosphere and release oxygen during the process of photosynthesis. The carbon taken in by trees is stored in the forest.

When a forest is harvested, some of the carbon in the forest is removed in the logs taken from the site. The "slash" left behind after harvest slowly decays, releasing carbon dioxide to the atmosphere.

When trees and slash burn, carbon dioxide is released into the atmosphere. However, it must be noted that while slashburning accelerates the release of carbon dioxide, it should have little long-term effect on the total carbon dioxide released into the atmosphere. If an area is not burned after timber harvesting, much of the residual material (i.e., branches, twigs, rotten logs) decays naturally, slowly releasing a similar amount of carbon dioxide into the atmosphere.

The prompt establishment of a new forest is essential for keeping the net release of carbon dioxide to a minimum. Reforesting of harvested lands helps to recapture carbon removed from the site by harvesting or released from the site after harvest.

For more detailed information on the relationship between prescribed fire and carbon dioxide, see British Columbia forests, forestry practices, and carbon dioxide (B.C. Ministry of Forests, Research Branch).

1.6 Using Prescribed Fire with Other Site Preparation Methods

Prescribed fire may be used alone, or in combination with other treatments. Combinations of treatments include piling, bunching, or windrowing and burning; and, spraying and burning (brown and burn). The effects of each treatment combination differs and must be considered along with site characteristics and management objectives. For example, although burning piles of slash reduces the land area affected by the burn, the area burned under piles may experience a more severe fire, possibly leaving a legacy of long-term deterioration in soil quality. Use caution as windrowing may reduce coarse woody debris to unacceptable levels.


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Copyright 1999 Province of British Columbia
Forest Practices Branch
BC Ministry of Forests
This page was last updated December 1993

Comments to: Tim Ebata <Tim.Ebata@gems8.gov.bc.ca>