Environmental factors that influence prescribed burning in the Northern Plains

Abstract

Several environmental conditions were recorded and analyzed for 192 prescribed burns in the Northern Great Plains. The purpose of these burns was to improve wildlife habitat and manipulate native prairie vegetation. All of the fires occurred in grassland and shrubsteppe vegetation types. Fuels were predominantly grasses and forbs intermixed with patches of shrubs. Nearly all of the fuels were <1.5 m tall. Shrub stems averaged <2.5 cm in diameter.In northern grasslands, prescribed burns can be conducted successfully during all months when there is no snow or ice cover on the ground. However, the burns analyzed here were conducted between March and October during all hours of the day or night.Burns were conducted under a wide range of environmental conditions. For example, one plot of mixed-grass prairie burned successfully just 9 hours after it received 1.2 cm of rain; another plot of mixed-grass prairie burned successfully when ambient air temperature was only 1.7 deg.C. Heavy fuels will often burn for short periods of time though light rain or snow are falling.Precipitation, relative humidity, temperature, cloud cover, and wind speed and direction are important climatic factors to consider before and during prescribed burns. These factors influence ignition of fuels, fire behavior, and fire effects. The 4 basic environmental conditions that influence the burning of northern plains grasslands and shrubsteppe are those that:1) Prohibit ignition or spread of fires. Plots with complete snow or ice cover, or those ignited during rainfall rates >0.05 cm/h, do not burn. However, these are good conditions to burn stockpiles of unwanted fuels that are usually high risk elements during regular prescribed burns.2) Produce partial burns. Partial burns are defined as those where fire is discontinuous and patches of standing and lodged vegetation are left unburned. Partial burns occur most often when fine fuels feel moist when handled, where less than 2 days have passed since the last measurable precipitation, and when cloud cover is complete. Other conditions associated with partial burns are relative humidities >50 percent, temperatures <21 deg.C, and wind speeds <10 km/h. These conditions occur most often during May and June.3) Produce complete burns. Complete burns are defined as those in which fire is continuous and nearly all vegetation (standing, lodged, and ground litter) is consumed by the fire. Complete burns occur most often when fine fuels feel dry when handled, 2 or more days have passed since the last measurable precipitation, and the sky is partly cloudy to clear. Complete burns occur with relative humidities between 25 and 50 percent, temperatures between 21 and 32 deg.C, and wind speeds of 13 to 24 km/h. These conditions occur most often in July, August, and September, but can occur anytime from March through November.4) Produce high risk fires. High risk fires are defined as those fires that are conducted during undesirable climatic conditions. High risk fires can always be expected with a combination of high winds, low humidity, high temperatures, and no recent precipitation. These conditions are most probable with wind speeds >32 km/h, relative humidities <20 percent, and temperatures >35 deg.C. These conditions occur most often in July, August, and September, but can occur anytime from April through October.