Northern Prairie Wildlife Research Center
American elm (Ulmus americana) and box elder (Acer negundo) are present as minor components of the overstory. Undergrowth is generally dominated by choke cherry, western snowberry, western wild rose, American plum (Prunus americana), and occasionally buffaloberry (Shepherdia argentea).
In addition, many deciduous woodlands have been invaded by Kentucky bluegrass, leafy spurge (Euphorbia esula), and Canada thistle (Cirsium arvense).
Aspen (Populus tremuloides) in the NGP will be either enhanced or inhibited by fire, depending on the frequency of burns. Fire often kills the tops of aspen, but regeneration from root suckers takes place quickly after burning. Frequently, post-burn aspen abundance will exceed that of pre-burn (Anderson and Bailey 1980).
Most deciduous woodland species in the NGP exist at the edge of their ranges. Even on favorable sites, woody plants live under stressful conditions, characterized by extremes of temperature, wind, and precipitation.
Most deciduous trees and shrubs are capable of sprouting from roots, root-collars, or stems (Spurr and Barnes 1980). Many species respond favorably to increases in light intensity following burning. Seeds of most species survive fire; in some cases they are stimulated by heat to germinate (Ahlgren 1974).
In the absence of fire, shrubs and trees may become decadent, and the accumulation of downed woody material increases the fuel load and the likelihood of a hot, lethal fire.
Season of burning has been reported to differentially influence sprouting response of deciduous species (DeByle 1985). These variations in response are probably related to carbohydrate reserves stored in roots. Seasonal periodicity of carbohydrate reserves is known for many deciduous species.
Reserve carbohydrates attain their maximum at the beginning of autumn and diminish slightly through winter. In April and May, root reserves diminish rapidly and are consumed by formation of new branches and roots. Therefore, deciduous plants are most susceptible to serious damage in early to midsummer when carbohydrate levels are lowest.
However, burning in early spring before leaf-out or in autumn or winter when reserves are relatively high should result in a vigorous sprouting response.
Method of burning also influences the degree of survival and sprouting of deciduous species, because rates of spread and intensity will vary. Ferguson (1957) reported that hardwood stems killed by backing fires in loblolly pine (Pinus taeda) stands resulted in slightly but consistently fewer sprouts than those killed by headfires. However, on rough fescue (Festuca scabrella) prairie, brush up to 1 inch (2.5 cm) in diameter at flame height was top-killed by backing fires, while most brush up to 2 inches (5 cm) in diameter was top-killed by headfires (Wright and Bailey 1982).
In all cases, fuel loads and moisture, topography, and weather influence the degree of top kill of deciduous species.
Limited data are available on the response of native woodlands in the NGP to fire. Shrub densities were not reduced 1 and 2 years after a wildfire burned through a deciduous woodland in southwestern North Dakota, and the fire stimulated vigorous sprouting of many shrub species (Zimmerman 1981).
Other evidence on the response of deciduous species to controlled burning in this region is provided by Gartner and Thompson (1973) from foothills ponderosa pine (Pinus ponderosa) in western South Dakota. Burning did not appear to affect the frequencies of shrubs, and some species, such as leadplant and common choke cherry, survived the fire very well.
Bock and Bock (1984) reported that light prescription burns in early spring and late fall in ponderosa pine stands in the southern Black Hills reduced densities of currants (Ribes spp), but most other shrubs were unaffected. However, a fall crown fire resulted in an increase in most shrub species, including red raspberry (Rubus idaeus), currant, roses, and western snowberry.
Ecologists have postulated that Juniperus species are generally restricted to shallow soils on steep slopes and ridges because the species is fire intolerant (Gartner and White 1986). Unburned areas support interspersions of red cedar (J. virginiana) and American elm. In the absence of fire, trees progressively invade and will eventually dominate the tallgrass prairie (Towne and Owensby 1984).
Data from other regions suggest that fire may effectively stimulate reproduction of deciduous species. American elm seedlings established quickly after a spring burn in Kansas (McMurphy and Anderson 1965).
Because of the historical frequency of fires in the NGP and the apparent adaptability of many native plant species to fire, it is likely that fire maintained the integrity of plant communities in this region. However, the paucity of data on the impact of fires on native deciduous woodlands remains a weakness in our understanding of native woodlands ecology.