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KUCHLER TYPE FIRE ECOLOGY AND MANAGEMENT

KUCHLER TYPE: Nebraska sandhills prairie
FUELS, FLAMMABILITY, AND FIRE OCCURRENCE : Nebraska sandhills prairie occurs in a climate that is not conducive to rapid decomposition of dead plant materials. Therefore, dry fuel and mulch build up within a short period of time [22]. Old stems and leaves of little bluestem persist for several years, furnishing abundant fuel for fires [47]. A single year's accumulation of plant material is capable of supporting a grassland fire at any time of the growing season. In addition, regrowth following a spring fire produces enough fuel to support a fire in the fall of the same year [8]. Fire during the same growing season is less likely if the first fire occurred after May. Summer and fall fires are likely to decrease the potential for burning in the early spring of the following year [8,9]. The flammability of grasses varies to some extent over the growing season. The probability of a fire occuring in ungrazed, native bluestem grassland may be high even before the grasses enter dormancy [8]. Ignition is probably more important than fuel availability or condition in the initiation and spread of Nebraska sandhills prairie fires [8]. Early observers noted frequent prairie fires from spring through fall. Fire is and was a frequent event in the central Great Plains. Historic fire frequencies were greatly influenced by human-caused ignitions, both by Native Americans and European settlers [52]. Many fires were ignited by lightning during electrical storms. Peak lightning fire frequencies occurred in July and August [8,9]. Moore [36] attributed 32 percent of presettlement fires in the north-central plains to Indians, 14 percent to Europeans, and the remainder he left unascribed due to lack of information. Most of those were probably caused by lightning. Presently, range fires are not uncommon, but their extent and recurrence are limited by human efforts at fire suppression and control [10]. Currently, lightning is the main source of ignition; sparks from trains also start a number of fires each year [49]. Kirsch and Kruse [30] observed four fires started by lightning in a single afternoon in 1958. In South Dakota, eleven fires were ignited within a 20 by 40 mile (32 by 64 km) area within a 2-week period in 1971 [30]. In May 1965 a lightning fire, driven by 48 mile-per-hour (76 k/hr) winds, burned over 18,000 acres (7,200 ha) of grassland and forest (including over 42% of the 26,000 acres [10,400 ha] of planted pine) in the Nebraska sandhills prairie [49]. FIRE EFFECTS ON SITE : Fire in grasslands generally causes soils to warm earlier in spring, and achieve temperatures that average 9 degrees Fahrenheit (5 deg C) higher than on unburned sites [5,31,52]. In Missouri little bluestem-big bluestem grasslands, the maximum spring soil surface temperatures were 5.22 to 17 degrees Fahrenheit (2.9-9.8 deg C) higher on burned plots than on unburned plots [32]. Soils in burned grasslands usually have slightly higher pH than in unburned grasslands due to the release of alkaline earth metals. In neutral or alkaline soils this usually has little effect, but in acidic grassland soils, pH increases may improve germination or have other effects on vegetative responses [46]. Spring and summer fires reduce vegetative cover and increase the likelihood of blowout formation [39]. Burzlaff [13] suggested that the limiting of fire has been of major importance in stabilizing dune soils in the Nebraska sandhills prairie. Burning recycles nutrients and releases energy stored in aboveground plant biomass. Fire effects differ depending on fire frequency and the length of fire-free intervals. The species- and nutrient-specific effects of fire on nutrient uptake suggest that fire exclusion has the potential to effect long-term changes in species composition in nutrient-limited ecosystems, including Nebraska sandhills prairie [38]. FIRE EFFECTS ON VEGETATION : The effects of fire on grasses depend on climate, site and topography, soil moisture, site condition (including fire frequency), species composition, and timing of fire. Spring burning usually favors warm-season species over cool-season species. The later the fire occurs in spring, but still prior to the emergence of green shoots, the greater the production of warm-season species in tallgrass prairie [31]. However, many grasses respond negatively to burning in dry areas (approximately 275-300 mm summer rainfall) [31], or during drought [14,52]. Fire followed by drought can kill enough little bluestem and other bunchgrasses to create blowout conditions [39]. Fire combined with drought reduced little bluestem cover as much as 42 percent in Texas ranges; burning in wet years resulted in an 81 percent increase in little bluestem cover [51]. In tallgrass prairie, big bluestem almost always increases after burning, as does indiangrass and probably switchgrass. But the amount of increase depends on the community state before burning, as well as the time of fire occurrence [16,31,53]. Fire consumes excessive mulch, which builds up rapidly in Nebraska sandhills prairie and retards plant growth. However, the exposure of bunchgrass bases contributes to drought stress [22]. On Nebraska sandhills prairie choppy sands areas burned in May, vegetative cover in August averaged 16 percent less on burned sites than on adjacent unburned sites. By 2 years following the fire, vegetative cover averaged 8 percent less than on adjacent unburned sites [9]. One growing season after a May, 1965 wildfire, forb cover and total vegetative cover were reduced. Vegetative growth was 53 to 91 percent greater on unburned sited than on burned sites. Species diversity was unchanged. Total grass cover was not substantially altered. Sand bluestem, sand dropseed, and sandhill muhly relative cover decreased; prairie sandreed and needle-and-thread grass relative cover increased [49]. A study to evaluate the effects of grazing on fire response was conducted on choppy sands sites. Both grazed and ungrazed sites burned in May averaged 26 percent lower total cover in August than unburned sites. After 2 years, vegetative cover averaged 11 percent less on burned sites than on unburned sites. The decline in total cover as a consequence of burning was greatest on north-facing slopes (where prefire grass cover was greatest). A suggested reason for this effect was that greater grass cover on a site promotes a hotter, more intense, and therefore more destructive fire [78]. The Arapaho Prairie has not been grazed since its purchase by the Nature Conservancy in 1976. Areas of this prairie were burned by wildfire (lightning ignition) in October 1981. Aboveground plant biomass was measured the following growing season. In early spring, prairie sandreed exhibited a higher rate of growth on burned areas than on unburned areas, but by mid-August, burned and unburned sites were approximately equal in prairie sandreed aboveground biomass. Needle-and-thread grass had greater biomass on burned sites than on unburned sites in August, but the reverse was true in October, indicating that growth slowed earlier on burned sites. A similar pattern was exhibited by blue grama [37]. No studies have been conducted on the long-term effects of fire on Nebraska sandhills prairie vegetation [49]. Nebraska Sandhills Prairie Species In Other Communities: In the loess hills of Nebraska, a late spring prescribed fire was used to control Kentucky bluegrass (Poa pratensis), Canada bluegrass (P. compressa), and Japanese brome (Bromus japonicus). Precipitation was lower than average the first postfire growing season and above average the second year following the fire. Herbage yield of warm-season grasses (blue grama, sideouts grama, and sand dropseed) was generally higher in June and September on burned plots in both the first and second postfire years [41]. In northern mixed prairie, the ratio of cool-season (needle-and-thread grass, big bluestem, switchgrass) to warm-season (blue grama, sideouts grama, little bluestem) species was unaffected by burning; however, herbage production of cool-season species increased during the warm-season growth period on spring-, summer-, and fall-burned plots. Warm-season herbage increased with spring fire as expected. The increase in cool-season species is contradictory to the prevailing view that spring burning is detrimental to cool-season grasses. The author suggested that there is a native cool-season guild of species that are well adapted to fire, and that the warm-season species are less well adapted to fire on dry northern sites [44]. In the mixed-grass prairie of Wind Cave National Park, South Dakota, spring prescribed fires were conducted for ponderosa pine control. The fires were lit when cool-season grasses were in the process of green-up and the fires were relatively low-severity and slow-moving. Green needlegrass (Stipa viridula), needle-and-thread grass, little bluestem, and western wheatgrass decreased in ground cover the first growing season after the fire, but showed rapid increases the 2 years following the fire. Bare ground cover increased 26 percent immediately after the fire but decreased 29 percent to below prefire estimated levels by 2 years after the fire [20]. Wright and others [54] charted information on the response of species by prairie type and location. Glenn-Lewin and others [23] determined the flowering response of prairie species to burning. In locations other than Nebraska sandhills prairie, big bluestem, little bluestem, sideoats grama, switchgrass, indiangrass, and prairie dropseed all increase flower production after a fire [23]. FIRE EFFECTS ON RESOURCE MANAGEMENT : Annual spring fires in the Allwine Prairie Preserve (Douglas County, Nebraska) appeared to cause minimal mortality to ground-nesting and burrowing species. The western harvest mouse, however, is quite vulnerable to prairie fires since it constructs aboveground nests out of prairie grasses and grass litter. Considerable mortality occurred to both adults and young [18]. Historically, prairie fires caused much mortality and injury to American bison, deer, and wolves [36]. Animal use of burned prairie was also affected; American bison, which used the same migration routes repeatedly, would alter their migration pathway where burned areas were depleted of forage. As much as a month was required in some cases for American bison to return to an area following a fire [36]. FIRE USE CONSIDERATIONS : Grassland fires can be intense and fast moving. Most grassland fires burn as headfires without completely consuming fuels [17,46]. Headfires in tall grasses can generate big flames but do not usually produce firebrands or spot fires due to the rapid consumption of fuels. Despite large and extensive fire fronts and a rapid spread rate, extremely high temperatures at ground level are uncommon [46]. Damage to the basal portions and rootstocks of plants is therefore also uncommon [17,46]. In dry conditions, backing fires generate more heat than headfires since they are slower moving [22,46], and thus consume fuel more completely. In dry conditions backing fires may cause more damage to roots and plant bases than headfires [17,25,26]. The amount and rate of heat release in grassland fires depends on weather conditions, topography (fire spreads faster with steeper slopes), and fuel characteristics [17]. Grassland fires often develop broad fronts because of extensive and unbroken terrain and continuous fuels. Fire fronts become irregular when topography and discontinuous fuel loads retard fire movement, and when winds and developing fire storms speed up fire movement [46]. Fire spread patterns in large fires may be very irregular, leaving patches of unburned vegetation [37]. Burned areas cool off rapidly behind the front allowing animals (and humans) to move into burned areas immediately [46]. Specific guidelines on conducting prescribed fire in grasslands are available [25,34] . Significant differences in flammability during the growing season arise due to vegetative moisture content and stage of development. In one study, the burn residue (inversely related to flammability, i.e. the drier the vegetation the more complete combustion and less ash formed) varied from a low of less than 1 percent of dry biomass in April to a maximum of 16 percent in June. Seasonal differences in flammability appear to depend on both moisture content and stage of vegetative development [8]. Summer prescribed fires can be successfully conducted in native bluestem prairies, probably including Nebraska sandhills prairie, even without forced ignition using petroleum-based fuels or artificial accumulation of plant matter for fuel [8]. FIRE MANAGEMENT CONSIDERATIONS : The ecological role of fire in the Nebraska sandhills prairie is unclear. Current range management practices consist of total fire exclusion. Range managers believe that fire exclusion is the best policy for maintaining domestic cattle forage [24]. The increase in Nebraska sandhills rangeland productivity in the first half of this century was at least partly attributed to fire control efforts because of reduced vegetation loss and blowout occurrence [13]. However, this may not be the most appropriate regime for the health of the ecosystem. The stabilization of dune soils, due in part to fire suppression, has contributed to the endangered status of blowout penstemon (Penstemon haydenii). This species occurs exclusively in blowouts, and is now on the Federal List of Endangered Species [24]. There is ample evidence that most types of grasslands soon begin to deteriorate when closed to both grazing and fire; however, not all grasslands are fire dependent [1,2]. In Nebraska sandhills prairie fire may not be completely beneficial. One year after an October wildfire on the Arapaho Prairie total phytomass was 23 percent less on burned sites than on unburned sites. Morrison and others [37] concluded that these grasslands are not positively affected by fire. Observations made 2 years after the fire, however, indicated that there were no long-lasting deleterious effects [37]. Few studies on fire frequency have been done for Nebraska sandhills prairie [8,9]. Ponderosa pine is increasing in some areas of Nebraska sandhills prairie due to cattle grazing and decreased fire frequency [43]. Historically, fire played a beneficial role in the preservation of tallgrass prairie, and fire protection/suppression has led to an increase in woody vegetation, particularly in the eastern portions of the tallgrass prairie zone. Fire in tallgrass prairies suppresses the encroachment of trees and shrubs, and reduces competition from cool-season invaders (i.e., Kentucky bluegrass and smooth brome) [42]. In the absence of fire, mixed-grass prairie in the Black Hills, South Dakota, has also been invaded by ponderosa pine in the last 100 years. It is estimated that 50 percent of presettlement prairie has been converted to ponderosa pine woodlands [22]. In the Black Hills, prescribed fire can be used in bluestem grasslands to reduce pine invasion without decreasing forage production if the fire is conducted prior to May 1 [21]. Nebraska sandhills prairie contains many tallgrass prairie species. The response of these species to fire has been more extensively studied in true tallgrass prairie types than in Nebraska sandhills prairie. The particular soil and precipitation properties of Nebraska sandhills prairie may have a unique effect on these species, particularly during drought. Bailey [3] stated that wildfire during the growing season is detrimental to arid (9.6 to 16 inches [240-400 mm] mean annual precipitation) mixed prarie range, but on wetter (16 to 20 inches [400-500 mm] mean annual precipitation) areas, the vegetation is better able to withstand the detrimental effects of fire. Several major grasses that are harmed by fire in the arid areas of North Dakota were found to increase in cover after fire in the wetter eastern part of the state [30]. In tallgrass prairies, the accumulation of dead plant material reaches growth inhibiting quantities in only 3 to 5 years. This accumulation retards the warming of soil and reduces available light at the bases of grass plants [48]. Occasional fires (every 2 to 3 years) renovate unmowed prairies and are distinctly beneficial if they occur in spring before growth is renewed [48]. Vegetation is generally more palatable and more nutritious on burned sites than on unburned sites; coarse grasses such as little bluestem that have a tendency to accumulate litter are particularly improved [42,52]. Gartner and White [22] stated that prescribed fire could be used to stimulate production of desirable herbaceous species, reduce mulch accumulation, and reduce competition from unwanted species within the prairie-forest ecotone. Fire is recognized as having benefits to tallgrass prairie during periods of average to above average precipitation [15]. In tallgrass prairie, productivity of big bluestem, measured as aboveground biomass, generally increases after a fire [45]. Most experimental fires in tallgrass prairies are conducted between late March and April. In prairies dominated by warm-season grasses, growth and production are stimulated by spring burning [6]. Spring burning enhances the growth, tillering, and flowering-stem density of big bluestem and other warm-season grasses [15]. For big bluestem, the increase is greater with late-spring fires (mid- to late April) than with early (mid-March) fires [6]. Over a period of 56 years in tallgrass prairie, big bluestem increased under an annual spring fire regime and little bluestem decreased [45]. Summer (prescribed) fire effects are likely to differ substantially from those during dormant periods [8]. In tallgrass prairies, timing of burning greatly affects individual species response to fire. Early spring burning favors little bluestem and prairie dropseed; late spring burning favors big bluestem, indiangrass, and switchgrass [5,42]. Burning too early may depress big bluestem production rather than stimulating it [45]. Repeated burning of big bluestem communities favors indiangrass [5]. Wildfires during the growing season, especially in drought years, appear to cause some plant mortality [22]. Other factors including target species, geographic location and rainfall must be considered in predicting and interpreting response to burning [45]. REHABILITATION OF SITES FOLLOWING WILDFIRE : Blowout formation is sometimes triggered by denudation due to fire. Blowouts can be stabilized with mulch and/or establishment of vegetation. A test of the efficacy of various grass species in stabilizing blowouts determined that prairie sandreed had the highest stand density and greatest freqency of occurrence on seeded blowout plots. It was also rated the msot effective at holding sand in place; the authors recommend planting prairie sandreed and/or intermediate wheatgrass (Thinopyrum intermedium) under a layer of prairie hay mulch for the most rapid and complete stabilization of blowouts [35].

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Information Courtesy: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. Fire Effects Information System

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