Fine-Fuel Moisture Considerations in Prescribed Burning
Underburning
Fine-fuel moisture is strongly influenced by rainfall, relative humidity, and temperature. The preferred range in actual (not calculated) fine-fuel moisture of the upper litter layer (the surface layer of freshly fallen needles and leaves) is from 10 to 20 percent. Burning when fine-fuel moisture is below 6 or 7 percent can result in damage to plant roots and even the soil. When fine-fuel moisture approaches 30 percent, fires tend to burn slowly and irregularly, often resulting in incomplete burns that do not meet the desired objectives. However, when areas with very heavy fuel buildups or extensive draped fuels are burned, moisture content should be 20 to 25 percent to keep fire intensity manageable, especially if aerial ignition techniques are used. Fine-fuel moisture values obtained from National Fire Danger Rating System (NFDRS) tables on fire-behavior models are considerably less than these actual values.
Some experienced practitioners can accurately estimate fuel moisture by examining a handful of litter. However, the only sure way to tell is to take a sample and ovendry it. Tables and equations in the NFDRS and BEHAVE can be used to estimate fine-fuel moisture, but the results are invariably underestimates (because they are worst-case values designed for use in predicting wildfire behavior).
One simple test that will give a very rough estimate of the upper-litter-layer moisture content is to pick up a few pine needles and individually bend each in a loop. If the needles snap when the width of the closing loop is about 1/4 to 1/2 inch, their moisture content is between 15 and 20 percent. If they do not snap in two, they are too wet to burn well. If they crumble into small pieces they are exceedingly dry and even if the lower litter is moist, the fire may cause damage and be difficult to control. Fuel moisture sticks that respond to weather changes like 10-hour fuels are available. With a good set of scales and proper placement of the sticks, acceptable fuel moisture estimates can be obtained just before ignition. These values will differ slightly from actual fine-fuel moistures, but are fairly representative of most southern fuel types. They are much closer to actual fine fuel moistures than are calculated or tabular values.
Lower litter should always be checked before burning to make sure it feels damp. This will help ensure that some remains, even though charred, to leave a protective covering over the soil. Generally, the moisture content increases from the litter surface down through the duff layer to the soil. Exceptions can occur after a light shower, or in the morning after a heavy dew. In these cases, fires often burn more intensely than would be expected from just looking at the upper-litter-layer moisture content. When burning on organic soils this phenomenon can have drastic consequences. If the fire dries the moist surface layer of peat, the organic soil will ignite. These fires can impact an area for many weeks in spite of control efforts, causing extensive smoke problems.
The speed with which fine fuels respond to changes in humidity depends on fuelbed characteristics such as whether the fuelbed consists of compacted hardwood leaves or jack-strawed pine needles. Different fuel types can reach different moisture contents under the same humidity conditions. For example, grassy openings containing cured material can be burned within hours of a drenching rain if good drying conditions exist. Because of these natural variations, recommended fine-fuel moisture values are only guidelines.
On-the-ground knowledge of fuels must be incorporated into the burning prescription. Fuel moisture also influences smoke production. When very damp woody fuels burn, large amounts of characteristic white smoke are given off. Much of the visible smoke plume is actually condensed water vapor.
For background information, see Fuel Moisture.
Debris Burning
Harvested areas should be burned when fuels are dry. They will ignite easier, burn more quickly and completely, shortening the time necessary to complete the burn. Less mop-up will be required and the impact on air quality will be reduced. The short, but severe, summer droughts common throughout much of the South provide ideal burning conditions on cleared areas, provided soil moisture does not get too low.
To avoid the possibility of unnecessary damage to the site, debris should be burned as it lies (broadcast burned) rather than piled. Because fuels on logged areas receive full solar radiation, they dry before surrounding forest fuels do. It takes at least several weeks after cutting for the severed tree tops to cure. Once the needles turn a greenish-yellow and the hardwood leaves wither, the debris is ready to burn. Cleared areas can then be safely burned soon after a rain, before adjacent forest fuels dry enough to burn well. Ten-hour fuel moisture (fuels 1/4 to 1 inch in diameter, such as branches and small stems) is a better indicator of burning conditions in slash fuels than is fine-fuel moisture. Fuel moisture sticks will give excellent results. One set of "sticks" can be placed on the area to be burned and another in the nearby undisturbed forest. Let the sticks become acclimated for at least 2 weeks before reading. Many managers consider the area ready to burn when the moisture content of the sticks on the logged area reaches about 10 percent while that of those in the forest is still above 15 percent.
If the burn objective is to consume larger fuels (over 2 to 3 inches in diameter), piling will probably be necessary. Piling in wet weather should be avoided, and piles should be small and free of dirt. Fresh logging debris needs to cure for several weeks before piling, because drying conditions are exceedingly poor in the middle of a pile, especially if it is compacted or contains much dirt. Much of the smoke problem associated with burning piled debris is caused by inefficient combustion of damp, soil-laden piles. These piles may smolder for days or weeks.
For background information, see Fuel Moisture.
Encyclopedia ID: p572
