Northern Prairie Wildlife Research Center
The data necessary to define the conditions and to predict when a fire will kill 25% of the wormwood (Artemisia absinthium) on an area or decrease dead plant litter depth by 5 cm (2 in) in the NGP are not yet available. Thus the prescription guides in this paper are a summation of results (Appendix D) and general observations, i.e. the state-of-the-art of burning grassland habitats for wildlife in the NGP.
Environmental conditions dictate fire effects on the vegetation within these habitats, which is the basis for developing fire prescriptions. The prescription in itself is meaningless unless the biologist or fire boss can identify the plant species, communities, and habitats on the potential burn site.
Fuels in the following prescriptions were predominately mixed grasses and forbs with patches of shrubs and in mostly prairie habitats. Nearly all of the fuels were less than 1.5 m tall, and brush and shrub stems averaged less than 2.5 cm in diameter.
Low-risk environmental conditions restrict the ignition or the spread of fires. Plots with complete snow or ice cover, or those ignited during rainfall rates higher than 0.05 cm/h, do not burn.
These are, however, good conditions for burning stockpiles of unwanted fuels such as manure piles and buildings, since such fuels are usually high-risk elements during regular prescribed burns.
Partial burns are defined as those where fire is discontinuous and patches of standing and lodge 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 rain, and when cloud cover is complete. Other conditions associated with partial burns are relative humidities greater than 50%, air temperatures less than 2 degrees C (70 F), and wind speeds less than 10 km/h (6 mph). These conditions occur most often during May and June.
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 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%, air temperatures between 21 an 32 degrees C (70 to 90 F), and wind speeds of 13 to 24 km/h (8 to 15 mph). These conditions occur most often in July, August, and September, but can occur any time from March through November.
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 greater than 32 km/h (20 mph), relative humidity less than 20%, and air temperatures higher than 35 degrees C (95 F). These conditions occur most often in July, August, and September, but can occur any time from April through October.
The climatic conditions during which recent prescription fires were conducted on USFWS lands are shown in Figs 35-38. Over 90% of USFWS prescription burns were conducted with air temperatures of 5-33 degrees C (41-90 F) (Fig 35), relative humidities of 21-80% (Fig 36), and wind speeds of 1.6-32 km/h (1-20 mph) (Fig 37). No wind direction appeared to be favored (Fig 38).
Nearly all upland habitats in the NGP can be burned during any month of the year when there is no snow or ice cover on the ground. Our purpose here is not to describe burning strategies for individual species but to present some examples of when to burn selected habitats.
Best increases in seed production, vigor, and canopy cover are obtained with late spring (May-June) burns.
Best responses are obtained with very early spring (March-April) or late summer (August-September) burns.
Best response with burns during March to June; least response by legumes in late summer-early fall burns.
Spring burns (May-June) generally induce shrubs and brush to sprout, but frequent fires may reduce the frequency of woody plant cover.
For details see Schlichtemeier (1967) and Kantrud (1986).