Prescribed Burning In Practice
Prescribed burning applied as an invasive plant management tool merges the principles of Integrated Fire Management (IFM) and Integrated Pest Management (IPM). IFM is an approach to addressing the risks and benefits of fire within the context of natural environments and socioeconomic systems in which they occur (Myers 2006).
- technical components of fire management
- ecological attributes of fire
- socioeconomic impacts of fire on society
The IFM approach supports the US Fish and Wildlife Service guiding principles of fire policy.
Prescribed burning affects whole plant communities, not just the target invasive plant. Controlling invasive plants with fire requires strategies that address invasive plant species at the population level and all plant species at the community level. In general, prescribed fire is used to (1) reduce dominance of a target invasive plant species and (2) increase dominance and diversity of desirable plant species.
Achieving these goals requires a solid understanding of how invasive and desirable vegetation within a particular ecosystem will respond to a specific fire treatment. The effect of fire on a plant species depends on the compatibility of the plant’s biological traits with the characteristics of the fire. The immediate and long-term response of plant communities to fire is also influenced by pre- and post-fire climate variables, activities of other taxa, management activities, natural and human-caused disturbances, and other environmental variables.
Diagram showing interactions between plant communities and fire regimes within the context of ecosystem variables such as climate conditions, other taxa, and other disturbances (adapted from DiTomaso and Johnson 2006).
To review, fire regimes are characterized by patterns of fuel consumption, fire spread, intensity, severity, frequency, and seasonality. Prescribed fire for invasive plant management manipulates fire behavior and patterns so that fire is applied at a time and intensity that puts the target species at a competitive disadvantage relative to the desired plant community.
For fire to damage or kill a plant, the following must occur:
Vulnerable plant structures such as perennating structures and propagules must be present and preferably actively growing.
Vulnerable plant structures must be exposed to a sufficient duration of lethal temperatures.
These circumstances are dictated by the timing and intensity of a burn. Although intensity is influenced by factors that cannot be controlled (such as slope, soil texture, humidity, and temperature), intensity can be manipulated to some extent by season of burn, pretreatments that influence fuel load, and ignition strategies (D’Antonio 2000).
The temporal patterns of fire refer to when (seasonally and diurnally) and how often (frequency) fires burn in a particular area. Seasonality also corresponds to the phenological development of plants and their potential for injury or mortality from fire exposure.
- In addition to topography, fire behavior is a function of weather and fuel characteristics, which are both influenced by season. Weather conditions that influence fire behavior such as humidity, temperature, and wind typically vary seasonally. Depending on the time of year, vegetation will have more or less moisture content, which influences a plant’s flammability and tolerance to the heat of a fire.
- Plant growth stage is also an important consideration for timing a burn treatment because there may be an ideal time during the phenological development of a target plant species when the application of fire will have its maximum effect (Thomsen et al. 1991).
- The interval between fires allows burned vegetation to recover. Multiple burns may be applied to an area to effectively control an invasive plant (Burke 1990). However, adequate time between burns should be allowed for ecosystem recovery and regrowth to replace fuel loads for subsequent burns. Care should be taken to avoid environmental problems associated with multiple burns such as increased risk of soil erosion (Brooks 2001).
Fuel load, continuity, and structure can be modified by altering vegetation with chemical methods, physical methods, and livestock grazing. Vegetation management methods can be applied or deferred to influence fire intensity, fuel consumption, and fire spread patterns across the landscape.
- Fuel loads can be enhanced or reduced to create hotter or cooler fires.
- Continuity and distribution of fuels can be altered to create patchy rather than continuous burns, or surface rather than crown fires.
- Natural or constructed fire breaks help to control the size and shape of the burn area.
Ignition strategies take advantage of existing terrain and weather conditions to create the desired fire characteristics. Fast-moving fires may be less intense than slow-moving fires because of a shorter heat exposure. Anticipating spread direction is important to avoid burning areas outside of the designated burn area.
- The position of ignition relative to wind direction, wind speed, and slope influences the direction and speed by which fire will spread.
- Single ignitions can start fires that spread naturally across an area, or multiple ignitions may be used to create a backfire or to light several fires that burn simultaneously.
- Different ignition tools may be used depending on the size and location of the planned burned area. For example, thermally reactive ping-pong balls may be airdropped into remote locations to burn a larger area, drip torches are useful for igniting a flaming front designed to burn across a smaller controlled area, or flaming torches may be used to burn individual plants.
Monitoring and evaluation of prescribed fires is an essential management activity. USFWS fire management policies require that pre- and post-burn monitoring document the short- and long-term effects of burning.
In addition, there are monitoring provisions in the National Environmental Policy Act (42 ISC 4321-4347). Establishing a consistent method to evaluate the effects of prescribed burning helps determine whether management goals/objectives are being met, to provide a basis for improving economic efficiency, to validate fire behavior and conditions, to refine future prescribed burns, and to improve information sharing between land managers.
As previously discussed, fire can have a variety of effects on invasive plant populations and their subsequent interactions with environments that are changed by fire. In addition to monitoring fire effects, land managers should design a specific monitoring program to ensure that prescribed burning treatments are meeting invasive plant management objectives. Monitoring methods should detect changes in the desired plant community, target plant populations, and the establishment and expansion of nontarget invasive plant species.
Prescribed burning can be used to realize a number of invasive plant management options and to mimic historical fire regimes that promote desirable vegetation.
The creation of unnatural fire regimes can favor nonnative species (Brooks et al. 2004, Cowling 1987). Although ecosystems with natural fire regimes are certainly not immune to invasion by undesirable plant species, restoring or maintaining natural fire regimes may help to prevent invasions by stimulating desirable competitive plants.
Invasive plants may be suppressed by fire if burning treatments are effective in depleting carbohydrate reserves, depleting the seed bank, and stimulating desirable plant species that can successfully compete with invasive plant populations (Rice 2005). Multiple burning treatments are often required to sufficiently deplete carbohydrate reserves for perennial species (Burke 1990). Repeated disturbance of fire may weaken desirable plants.
Containing invasive plant populations requires preventing the transport of invasive plant propagules from the infestation to new sites. Burning invasive plants at the appropriate growth stage can reduce or prevent flower and seed production (Thomsen et al. 1991), thereby reducing the risk of infestation expansion by seed. Existing seedbanks in the soil, litter layer, or inflorescence must also be contained to prevent spread to new sites. Fire temperatures are typically not hot enough to destroy seeds and propagules near the soil surface (Daubenmire 1968). However, seeds remaining in plant inflorescence may be exposed to lethal temperature during fires (Renney and Hughes 1969).
To achieve complete control of an invasive plant with prescribed burning, all perennating and reproductive structures must be destroyed or eliminated by the fire. Though some invasive plants may be eliminated by burning, fires can create conditions that favor invasion and establishment by another invasive species.
Fire alone will rarely provide adequate control of most invasive plant species. However, preparatory or follow-up treatments can facilitate and enhance control with other methods.
Prescribed Burning and Physical Methods
- Prescribed burning can be used after mechanical methods to remove the dead biomass (Bell 1997, Dudley 2003) and prevent resprouting from cut and piled debris (Taylor and McDaniel 1998).
- Surface fires may improve access to a site so that invasive plants may be located for mechanical treatments (Eckardt 1987).
Prescribed Burning and Chemical Methods
- Burning can stimulate seed germination, depleting the seedbank and exposing seedlings for control with herbicides (Everest et al. 1991, Torell et al. 1961, Biedenbender et al. 1995).
- Burning perennials can improve the effect of foliar herbicides by removing biomass, thatch, and older plant tissues (Klinger and Brenton 2000, Washburn et al. 1999, Brender 1961, Everest et al. 1991, Moorhead and Johnson 2002).
- Removal of litter by burning or other means can improve visibility for follow-up herbicide treatment (Egan 1999, Friederici 1995) and can improve interception of preemergence herbicides with the soil surface (Winter 1993, McKell et al. 1962, Torell et al. 1961). In densely infested sites, prescribed burning may be used to remove restrictive canopy and improve access for treatment with herbicides (Taylor and McDaniel 1998).
- An herbicide pretreatment may be used to enhance the fuel load to carry a burn. Treated plants may themselves become fuel (Rolston and Talbot 1980), or pretreatment may produce fuel indirectly by enhancing fine fuels like grasses (DiTomaso 2001).
- Fire can be used after herbicide treatments to remove dead biomass and stimulate recovery of the infested site (Bell 1997, Dudley 2003).
- Topgrowth vegetation can be killed by herbicides and a subsequent burn can produce intense fires hot enough to kill the root system, or reduce regeneration and seed germination (Wolters et al. 1994, Clark 1998).
Prescribed Burning and Biological Control
- Prescribed burning can kill biological control agents if applied when insects are active. In areas where biological control populations are established, fire treatments should be timed so that insects are not harmed by fire.
- If applied when insects are not active, burning treatments may enhance establishment of biocontrol populations. Fellows and Newton (1999) attributed the change in leafy spurge (Euphorbia esula) flea beetle populations (Aphthona nigriscutis) to increased insect colonization in bareground of burned plots.
- Burning may cause an initial decline in biocontrol populations. Briese (1996) observed a decline in biocontrol insect populations (Chrysolina quadrigemina) following a burn. The beetle populations rapidly recolonized the burned area from adjacent St. Johnswort (Hypericum perforatum) infestations and became well established, perhaps benefiting from increased nitrogen uptake by recovering St. Johnswort plants.
Prescribed Burning and Prescribed Grazing
- Grazing prior to burning can create a cooler burn (D’Antonio et al. 2003), whereas deferring or restricting grazing prior to the burn can increase fuel accumulation and fire intensity (George 1992).
- Fire can be used to stimulate regrowth and may increase the palatability of some invasive plants. However, grazing too soon following fire can eliminate or greatly reduce recovery of desirable vegetation (Brown 2000).
Prescribed Burning and Restoration/Revegetation
- Fire can suppress invasive plant species that compete with desirable species (Goodrich and Rooks 1999), or stimulate desirable species that will compete with invasive plants (Masters 1992, Masters and Nissen 1998, DiTomaso et al. 1999).
- Fire opens up the soil surface and can encourage recovery of more desirable species (Taylor and McDaniel 1998). For example, in sites dominated by cool season grasses, thatch removal can increase soil heating and stimulate warm season grasses (Ehrenreich 1959).
- In areas prone to erosion, reseeding following fire may be necessary to prevent soil erosion during the rainy season.
The current trend of prescribing beneficial fires follows an era of fire suppression that was fueled by the perception that fire is a detrimental occurrence. Prescribed burning is now a commonplace tool applied to achieve cost-effective and ecologically beneficial results in vegetation management.
Several land management agencies apply prescribed burning as a primary management method to reduce encroaching and invading plant species and improve general vegetation conditions. There are over 235 published articles and abstracts related to the management of invasive plants using fire (Rice 2005).