More info for the terms: avoidance, codominant, cover, fire regime, fire severity, forbs, frequency, fuel, mesic, natural, prescribed fire, severity, shrubs
Fire is important in creating and maintaining American bison habitat. Fire
regenerates grasslands and enhances production, availability, and
palatability of many American bison forage species [9,11,67,68]. Fire frequency
has been estimated to occur once every 3 to 5 years on some prairies
[97]. During presettlement times American bison habitats were to a large extent
created and maintained by lightning-caused fires or fires set by Native
Americans [44,47,49,50]. The results of intense grazing by large American bison
herds on recently burned areas may have reduced fuel loads, making the
grazed areas less likely to burn and even allowing them to function as
firebreaks [49,75]. In contrast, unburned areas would have been little
grazed, thereby increasing fuel loads and the probability of burning.
The slaughter of American bison in the late 1800's may have shortened fire return
intervals and increased fire severity during the early settlement
period. Steuter [75] suggested that integrating a regional fire
behavior model with estimates of presettlement American bison patterns could
provide a valuable tool for natural area management in the northern
mixed-grass prairie.
Several studies have shown that American bison prefer to forage on recently
burned areas [5,7,22,28,70,87]. In tallgrass prairie on the Konza
Prairie Research Natural Area, northeastern Kansas, 45 American bison range over
an array of watersheds with different FIRE REGIMES [87]. The watersheds
are burned in April annually or at 2-, 4-, or 20-year intervals. In the
spring of 1988 and 1989, Vinton and others [87] studied American bison grazing
and use patterns among these watersheds as influenced by fire regime.
American bison used some watersheds preferentially and the pattern of watershed
use changed seasonally. During the spring of both years (April-June
30), American bison selected only watersheds that had been recently burned
(annually or biennially), and were observed up to three times more
frequently than expected on these watersheds. In 1988, preferential
grazing of recently burned watersheds persisted through the summer
months. During autumn and winter of both years, American bison preferred the
annual and 20-year burn watersheds to watersheds that were burned every
2 or 4 years.
On the same study site as above, little bluestem was sampled to
determine how fire influences its use by American bison and its responses to
grazing. Plants were marked at the beginning of the 1992 growing
season. Little bluestem was sampled in an annually burned watershed and
a watershed burned at 4-year intervals (referred to as "unburned") that
had been grazed by American bison since 1987, and nearby annually burned and
4-year burn interval watershed that were ungrazed. The 4-year burn
interval watersheds had last burned 2 years before sampling. On
unburned prairie, American bison grazed only 5 percent of the available little
bluestem, selecting it only 30 percent as frequently as big bluestem,
the codominant species. On burned prairie, grazing frequency of little
bluestem was more than 3 times as great as on unburned sites and equal
to that of big bluestem. The increased grazing frequency on little
bluestem in recently burned prairie is most likely the result of the
removal of its persistent standing dead tillers by burning. Burning did
not affect grazing on big bluestem, a plant lacking persistent standing
dead tillers. With longer intervals between fires, American bison might display
even greater avoidance of little bluestem in favor of other grasses
[93].
A combination of fire and American bison grazing may increase the standing crop
of rhizomatous grasses at the expense of bunchgrasses. Pfeiffer and
Steuter [96] conducted a study on Nebraska sandhills during the 1991 and
1992 growing seasons to determine the response of sandhills prairie
vegetation to spring and summer prescribed burns and subsequent American bison
grazing. Approximately 1,235 acres (500 ha) were burned in early May,
and another 247 acres (100 ha) were burned in late July, 1991. During
the 1992 growing season, American bison grazing on burned areas reduced
bunchgrass standing crop by 56 percent, while reducing rhizomatous grass
standing crop by only 18 percent. Forbs generally appeared unaffected
by American bison grazing. The increased grazing pressure by American bison lasted only
one season. Rhizomatous grasses of the Great Plains are better adapted
to large herbivore grazing than are bunchgrasses. Burning and grazing
would increase the amount of forage available since, in unburned
prairie, standing dead tillers deter use of bunchgrasses.
Several studies concerning American bison response to prescribed fire have been
conducted at Wind Cave National Park [5,28,32]. Two prescribed fires in
ponderosa pine (Pinus ponderosa)-grassland habitat were conducted on
October 16, 1974 and May 9, 1975. The spring fire was conducted on a
site adjacent to the fall fire. American bison were noted in the area of the
burns during the course of burning. They utilized regrowth vegetation
on the burned areas throughout the summer of 1975 [32]. A prescribed
fire conducted on April 1, 1981, burned 110 acres (44.5 ha) of
mixed-grass prairie and 134 (54.4 ha) of forest land. American bison fed within
the burn in 1981 and 1982, moving in 1983 to an area burned by wildfire
[28].
On the Wichita Mountains Wildlife Refuge, Shaw and Carter [70] studied
seasonal range use by American bison before and after spring prescribed fires on
a mixed-grass prairie interspersed with post oak (Quercus
stellata)-blackjack oak (Q. marilandica) woodlands. American bison increased use
of the burned portion of their summer range. They showed no apparent
response to prescribed burning of an area of new winter range, but they
delayed their spring departure to traditional summer range.
Some studies have shown that cow-calf herds graze burned areas more
often than bulls [5,18]. The first postfire years following a fall
prescribed fire in grassland habitat at Wind Cave National Park, bulls
were found less than cow-calf herds on burned sites. Both cow-calf
herds and bull groups tended to use the burn more in June of the first
postfire season than at any other time. However, only cow-calf herds
consistently grazed the burn during the rest of the summer [18].
Wallows enhance species diversity in American bison habitat. In Wichita
Mountains National Wildlife Refuge, ruderal species (e.g., Japanese
brome and false pennyroyal) and mesic species (e.g., purple ammania,
pepperwort [Marsilea mucronata], and seacoast sumpweed) had higher cover
values within wallow than outside them. Wallows may be especially
abundant and heavily used on burned sites because American bison are attracted to
graze in such areas [16]. Collins and Uno [16] examined the effects of
February, 1982, prescribed fire on wallow vegetation in Wichita
Mountains Wildlife Refuge. Vegetation samples taken during June and
early July, 1982, from the edge and interior of unburned wallows were
more similar to each other than were edge and interior samples from
burned wallows. Species diversity and richness were significantly lower
in burned than in unburned wallows. Winter annuals were more abundant
in unburned wallows, perhaps because they were burned during their
growing season. The authors suggested that spring fires may reduce
cover of winter annuals in wallows, but summer and fall fires could
increase their importance [16].
Sedge-grasslands, which are important winter habitat for American bison, often
increase in area after fire removes surrounding shrubs or trees [9].
Fires in open black spruce (Picea mariana) forests and shrublands may
result in expansion of sedge-grasslands. In 1977, the Bear Creek
wildfire near Farewell, Alaska, moderately to severely burned a closed
spruce-hardwood forest and an open black spruce forest with an
understory of willow, shrubs, and sedges. The fire converted 100 square
miles (260 km sq.) of predominantly open black spruce forest to
sedge-grassland. Most of the American bison in this area winter on sites with
extensive sedge cover. By postfire year 4 the sedge-grassland habitat
had more than doubled in area. Fire-related snowpack changes also may
have stimulated American bison winter range expansion. Before the fire, the
disjunct and widely scattered sedge-grasslands were separated by
extensive open black spruce forest and shrublands. This habitat
generally has a greater snowpack than sedge-grasslands and, therefore,
is likely to discourage American bison movements. After the 1977 fire,
sedge-grasslands showed less snow cover than adjacent unburned open
black spruce forests and shrublands [9].
