Prescribed burning is the process of planning and applying fire to a predetermined area. It is referred to as prescribed burn because they follow a prescription with a number of conditions that need to be met which include a knowledgeable manner to wildland fuels on a specific land area under selected weather conditions.
CONDITIONS
Prescribed burning involves careful planning, consultation, and monitoring. To plan and execute the burning properly, a general understanding of the effects of weather elements on fire behavior is necessary. Wind, humidity, temperature, rainfall, and air stability are essential element to considered. All these factors influence fuel moisture which is the most important factor for a burn to succeed.
For the wind, the preferred range for windspeed is 2-5 km/hr. However, enough wind should be presented to lead the fire direction and to keep the heat from rising directly into tree crowns. Prescribed burn will behave in a more predictable manner when windspeed and direction are steady. Fuel and topography affect the wind conditions.
Relative humidity is the amount of moisture in the air compared with the total amount of the air which is capable of holding at that temperature and pressure. The preferred humidity for prescribed burn varies from 30 to 55 percent. When the humidity falls below 30 percent, the burning will become dangerous because fires will be more intense. On the other hand, when the humidity is higher than 55 percent, fires will not burn enough to accomplish the goal.
The temperature recommended for prescribed burn is above 25 c. Temperature strongly affects moisture changes in forest fuels. High temperatures help dry fuels faster. When fuels are exposed to direct radiation, they will become much warmer than the air. Moisture will be moved from the warmer fuel to the air. Nevertheless, lower temperature will reduce fire intensity because additional heat is required.
Rainfall affects both fuel moisture and soil moisture. Burning should be stopped during the periods of prolong drought and resume after raining. Rain has a greater impact on fuel moisture in cleared areas; however, fuels also dry much faster in cleared areas as well due to the increased sunlight, and higher windspeed. This different drying can be used to advantage by burning a cleared area several days after a hard rain. The fuels in the surrounding forest are still damp assuring good soil moisture and helps prevent the fire spreading beyond the desired boundaries.
Fuel moisture is influenced by rainfall, humidity, and temperature. The preferred range of fuel moisture is from 10 to 20 percent. If the fuel moisture below six percent is burning, the result will be a damage to plant roots and the soil. However, if the fuel moisture reaches more than 20 percent, fires will burn slowly and irregularly leading to incomplete burns.
If the atmosphere is unstable, the hot combustion products from burning will rise rapidly due to the large temperature gap between the smoke and the surrounding air. This will increase fire intensity. On the other hand, when the atmosphere is stable, its temperature decreases slowly. The stable air will hold the combustion products closer to the ground.
Types of vegetations need to be burnt and need to be left unburnt are also considered. When prescribed burn is created, the desired fire intensity and the season are also taken into consideration.
The history and variety of past fire events over the landscape and the knowledge about fire behavior are also essential. Monitoring and forecasting the right weather conditions for igniting a burn and determining a specified pattern and sequence will achieve the desirable outcomes and prevent fires from escaping.
Monitoring the results of a prescribed burn to determine how successful the operation met the goal set for the burn and also using the results to improve the future prescribed burning operations.
TECHNIQUES
Various fire techniques can be used to accomplish the operations. The techniques must be chosen related with fuels topography and weather factors to ensure the objectives are successful while preventing the damage to forests.
A backing fire is started along a baseline such as a road, or other barrier and allowed to back into the wind. Backing fire is the easiest and safest type of prescribed fire to use if the windspeed and direction are steady. The disadvantages of this prescribed burn are the slow progress of fires and the increased potential for root damage of the fuel is not moist enough.
When a large area is to be burnt, it must be divided into smaller blocks. All blocks must be ignited at about the same time to complete the burn. These conditions prevent heat from rising directly into tree crowns.
When the humidity is low, a steady wind is blowing and fuels are continuous, a burn can be expected once the fire backs away from the control line. So, extra care must be taken to ensure the initial fire does not spot across the line.
In strip-head burning, a series of lines of fire are set progressively upwind of a fire-break that no line of fire can develop to a high energy level before it reaches either a firebreak or another line of fire. The distance between ignition lines is determined by the desires flame length. The distance can be varied within a fire to adjust for slight changes in topography, stand density, weather, or the fuel.
This type of prescribe fires allow quick ignition and provide for smoke separation under conditions. An effective method of reducing fire intensity is to use a series of spots instead of a solid line of fire. However, higher intensity will occur if the lines of fire burn together. Occasionally, a fire may be allowed to move over the entire area without stripping to accomplish the objectives. The only caution is to be sure the fire will not escape control.
This technique consists of treating an area with lines of fire set directly into the wind. It requires considerable knowledge of fire behavior, particular if used by itself.
The technique is used to secure the flanks of strip-heading fire or backing fire as it progresses. It is also used to supplement a backing fire in areas of light fuel or under more humid weather conditions. It is useful on a small area and to facilitate burning a large area in a short time when a fire line is too intense.
This method requires expert crew coordination and timing for the wind since it cannot stand variation in wind direction. For safety, all lines of flank fire should be ignited at the same time and all people should keep alongside one another.
When the fires become too intense, the fires will switch from strip-heading fires to point source fires. Timing and spacing of the individual ignition spots are keys to the successful operations.
A line backing fire is ignited to increase the effective width of the control fire. Spots are then ignited at some distance upwind of the backing fires and the process continued until the entire blocks are ignited.
Ignition-grid spacing is selected to minimise crown scorch. The closer the spacing, the more merging points there are. A large number of small fires burning at once can produce the explosive as a single large fire. So, the balancing act between spacing and timing has to be continually regulated as fire behavior reacts to the changes in fuel and weather.
This technique is useful on cutover areas where fires are needed to reduce logging debris prior to seedling or planting. It works better when winds are light and variable.
The downwind control line is the first line to be ignited and then the entire outer edges are ignited. One or more spot fires are ignited near the center of the area and allow to develop before the perimeter of the block is ignited.
This method can be used in any season, and weather conditions are not a critical. However, when the atmosphere is unstable, this type of fire tends to develop a strong convection which can cause spotting a considerable distance downwind.
PILE AND WINDROW BURNING
Logging debris should be broadcast-burnt when possible. Piles are the better choice even though it costs slightly more than windrow.
Windrowing can reduce the site quality by removing topsoil. The area beneath the windrows is often lost to production because the debris is rarely consumed completely and the remaining causes replanting difficult.
On the other hand, circular piles makes planting easier, safer burning, and less smoke problems.
Windrow burning
REASONS
There are many potential benefits to be gained from using prescribed burning. The department uses prescribed burning for a number of purposes including reducing the build-up of fuel loads, maintaining biodiversity, rehabilitating vegetation, and tackling research on fire. A single prescribed burn can achieve multiple benefits. However, prescribed burns are not always beneficial. When conditions are wrong, prescribed burns can damage resources. Since they are very complex, they should be used only under controlled conditions.
Prescribed burn is the most practical way to reduce dangerous growth of combustible fuels under the debris. The appropriate conditions to use prescribed burns for fuel reduction depend on several factors including the rate of fuel accumulation, the history of wildfires, and risk values. The setting time can be one year, or a three-year and four-year is also acceptable.
Most people also use prescribed burn to dispose of logging debris. However, this practice should be avoided because of smoke management problems and the potential of site degradation.
On open sites, fire alone can expose adequate mineral soil and control competing vegetation until seedlings become established. Prescribed burn helps recycling nutrients and making them available for the next crop. Burning should be done several weeks before seed falls. Timing varies with species and locality.
Prescribed burning is particularly appropriate for wildlife habitat management where short-leaf, long-leaf, and slash pine are the primary species. A mosaic of burnt and unburnt areas maximize 'edge effect' which promotes a large and varied wildlife population. The proper size, frequency, and timing of burns is crucial to the successful prescribed burn for habitat.
Unwanted species might retard the growth of crop. Prescribed burn is used for managing competing vegetation and controlling insects and diseases such as fungal infection that may weaken and eventually kill the leaves. Once seedlings become infected, burning is the most practical method of disease control. If the fires are operated properly, they will eliminate the diseases without killing the forests.
EFFECTS
Prescribed burning has both direct and indirect effects on the environment. Fires may injure or kill the plant, depending on how intense the fires burn and how long the plant is exposed to high temperature. Some plant characteristics such as thickness and stem diameter influence vulnerability to fires. Smaller trees are easier to kill than large ones.
For the effects on soil, frequency, duration, intensity of fire, and soil characteristics must be considered. Low-intensity fires will return nutrients to the soil where they are again available to plants.
The main effect of prescribed burn on water is the potential for temporarily increasing runoff of rainfall. The water may carry suspended soil particles, dissolved inorganic nutrients and other materials reducing water quality.
Prescribed burn can have effects on air quality by reducing visibility. Air quality on a regional scale will be affected when large areas are burnt at the same time. Local problems are more frequent because of the large quantities of smoke that can be produced during a short period of time, especially when fuels are wet and combustion is incomplete. Smoke can have negative short and long-term health effects. People who are exposed to high smoke concentrations suffer eye and respiratory system irritation. Exposure to high concentrations of carbon monoxide can result in impaired alertness and judgement. Exposure to even low smoke concentrations can contribute to respiratory problems and cancer.
The effects of prescribed burn on wildlife are changes in food and cover. They sometimes improve conditions for some animals by improving habitat, and increasing food production and availability. The potential negative effects are the destruction of the sites and animal entrapment. The fires also result in unnecessary tree damage.
FARMER'S
PERSPECTIVE
Many policymakers believe that all fires are harmful, and best to prevent and be extinguished. However, they do not realise that forest and fires have coexisted for a long time, and local communities have the knowledge to manage fires.
Slash-and-burn agriculture is the technique involving clearing a patch of land by burning. It is a very old agricultural practice, and can be considered as a small controlled burn; however, most countries have banned this type of cultivation because it results in deforestation and forest fires.
For farmers' perspectives, they indicate that not being permitted to burn would severely and negatively affect their economic status. They also mention five advantages of slash-and-burn agriculture as land clearing methods. First, this method allows the soil to retain its natural fertility as the ash improves the fertility of the soil. Second, loads of fuels on the forest floor are reduced due to the burn. Third, burning creates space for a new plant and walking paths. Fourth, the fires reduce the unwanted pests and diseases, and the last, in some cases, burning helps improve soil structure enabling the faster establishment of seedlings.
The example of controlled burning in Indonesia was in Rantau Rasao village and Sungai Rambut village, Jambi in 1997 with the existing shrubs as fuels and traditional burning method.
Controlled burn in Indonesia
Slash-and-burn agriculture