FIRE EFFECTS
SPECIES: Andropogon gerardii var. gerardii | Big Bluestem 



IMMEDIATE FIRE EFFECT ON PLANT : 
Under dry conditions, up to 100 percent of live and dead aboveground
stems and leaves may be consumed by fire [36,53].  Rhizomes, however,
usually survive, even when aboveground tissue is completely destroyed.


DISCUSSION AND QUALIFICATION OF FIRE EFFECT : 
Fires that occur during the the active growth stage in the summer are
most damaging to big bluestem because of the limited carbohydrate
reserves available for new growth during this time period [23].  Burning
tallgrass prairie in Oklahoma during late summer resulted in nearly
complete combustion of aboveground biomass [36].  On grazed plots with
little fuel accumulation, tillers were completely defoliated but did not
suffer apical meristem damage.  On ungrazed plots with heavy fuel
accumulation, tillers did suffer apical meristem damage.  Tiller density
was significantly lower 2 months after burning on these high fuel plots,
and new growth consisted largely of newly initiated tillers.


PLANT RESPONSE TO FIRE : 
Big bluestem initiates new growth from surviving rhizomes after
aboveground foliage has been damaged or consumed by fire.  After summer
burns, depending on the amount of damage inflicted, regrowth is either
from undamaged apical meristems or from the initiation of new tillers
from underground rhizomes [36].  Plants burned during the spring or fall
when the aboveground foliage is dead resume growth in the spring as
normal.  In general, spring burning has a stimulating effect on the
growth and competitive vigor of big bluestem [74].

Big bluestem plants in recently burned areas start growth earlier in the
spring, develop faster, and produce more herbage than plants in unburned
areas.  This earlier and increased growth is most often attributed to
increased solar radiation reaching the soil following the removal of
standing dead material [53,56,69,70,94,102] which results in higher soil
temperatures [76,100].  Following spring burning in native bluestem
prairie in Missouri, soil surface temperatures in burned areas compared
to unburned area averaged 7.1 degrees F (3.9 deg C) warmer in April,
11.4 degrees F (6.3 deg C) warmer in May, 8.3 degrees F (4.6 deg C)
warmer in June, and 7.1 degrees F (3.9 deg C) warmer in July [76].
Increased soil temperatures promote earlier root growth and activity and
thus earleir emergence of shoots [72,99,106].  Also, the emerging shoots
receive more sunlight because they are not shaded by the standing dead
shoots and leaves of the previous year.  Knapp [69] reported that
following spring burning in Kansas tall grass prairie, big bluestem had
a greater photosynthetic rate and increased stomatal conductance and
leaf thickness in response to increased solar radiation.

In the tallgrass prairie, late spring burning, when warm-season grasses
are about to resume growth, is the most beneficial to big bluestem.  Big
bluestem stands burned at this time show the highest aboveground biomass
gains compared to unburned stands or stands burned at other times of the
year [4,8,123].  In the true prairie region, spring burning of big
bluestem stands that have not been burned within the past few years
almost always results in increased yields.  Two to three fold increases
are common [46,53,76,94,99,125].  Increases in big bluestem growth
following late spring burning are associated with increases in (1) the
density of tillers [53,68,121], (2) root and rhizome biomass [46], and
(3) size and number of leaves [4,69,70,121].

Increased flower stalk production is also common following burning
[21,32,33,46,54,94].  Following burning in Wisconsin, flower stalk
production increased six-fold [21]; after a spring burn in Iowa, flower
stalk production increased seven fold but returned to normal by the
third postburn growing season [32].  Although numerous researchers
report dramatic increases in seed production, Abrams [1] observed that
seedlings were less frequent in areas burned annually or on a 4-year
cycle than in unburned areas.

Big bluestem rapidly produces large amounts of foliage following fire.
Generally, it takes 2 to 5 years after burning for litter accumulation
to equal that of unburned areas [23].  When the fire interval is greater
than about 5 years, bluestem prairie becomes unproductive because the
large accumulations of standing dead material stifle growth.  However,
fires occurring more frequently than every 2 years will probably lower
biomass production.


DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : 
Postburn herbage production indicates that the season of burning, the
amount of annual precipitation the area receives, and site conditions
greatly influence big bluestem's response to burning.  Big bluestem
increases as influenced by different annual burning dates in Kansas are
summarized below [123]:

           Kansas prairie burned annually 48 of 56 years

time of burning             percent big bluestem in the grass community
        
prestudy (unburned)                             16.7
poststudy (unburned)                            18
late spring, about May 1                        46
mid-spring, about April 10                      24
early spring, about March 20                    23
winter, about December 1                        35

Farther west, in the mixed-grass prairie which receives less
precipitation, big bluestem generally increases following fire if
precipitation is average or above average but decreases if drought
conditions occur.  Response is somewhat variable though, and dependent
upon site characteristics.  On upland sites in the mixed-grass prairie
where big bluestem occurs infrequently, it is generally not stimulated
by burning [115].  Big bluestem is more common on lowland sites, and
because lowlands receive some surface water runoff, burning these sites
in the spring results in increased big bluestem biomass [35,115].  In
the mixed-grass prairie of north-central South Dakota, big bluestem
production increased following spring burning on overflow sites, even
though 2 years of below average precipitation followed [35].  Results
from this study are summarized below:

                                                           current years
 burning     phenological       leaf          flower       growth (g/m2) 
  date          state        length (cm)     stalks/m2      (air dried)
                             8/80   8/81    8/80   8/81        8/80

May 15, 1980   dormant      21.3   45.2     132.4  69.8        128.8
June 16, 1980  5-10 cm tall 16.8   37.0      95.2  53.8         71.4
control                     42.4   43.4       2.0  15.1         23.0

Fires that occur during the summer cause the most harm to big bluestem.
Summer burning during a below average precipitation year in
north-central South Dakota resulted in the reduction of big bluestem on
overflow sites, and shifted dominance towards cool-season grasses [115].
In the true prairie of Oklahoma, late summer burning resulted in initial
reductions of big bluestem tiller density; however, by the end of the
growing season, tiller density returned to normal [36].  Measurements
taken the following August showed that productivity was within the
seasonal normal range.  In the Black Hills of South Dakota, big bluestem
basal cover remained unchanged following an October burn [13], and
increased slightly following spring burning [41].


FIRE MANAGEMENT CONSIDERATIONS : 
Late spring is the best time to burn big bluestem stands.  The later in
the spring burning occurs, just prior to the emergence of new growth,
the greater the postburn herbage production will be.  If burns are
conducted too early, production may decrease as a result of the
increased evaporation of soil moisture in the interval between the fire
and the resumption of new growth [74].  Spring burns leave the soil
exposed for the least amount of time and thus reduce soil moisture
levels over the growing season less than winter, early, or mid-spring
burns [6,85].  Regardless of season of burn, however, soil moisture
content is consistently lower in burned areas than in unburned areas.
Thus in drought years, in areas of low precipitation, or in areas where
soil moisture is limiting, big bluestem will probably not show the high
postburn biomass increases generally reported for tallgrass prairie
sites.

Late spring burning can be used to increase grass productivity and
improve cattle use.  Cattle prefer vegetation on burned sites over that
on unburned sites [80].  Compared with weight gains of cattle grazing in
nearby unburned pastures, weight gains of cattle grazing on late spring
burned pastures were 17 percent higher in Oklahoma [115], and 11 percent
higher in Kansas [80].

Late spring burning can be used to increase big bluestem and other warm
season grass composition in warm season pastures or rangelands infested
with undesirable cool season grasses such as Kentucky bluegrass.  Late
spring burning favors warm season grasses because they are dormant at
the time of ignition, and resume growth as normal from stored food
reserves held in underground organs.  Burning at this time generally
harms cool season grasses, however, since they begin spring growth
earlier, and are actively growing at the time of ignition.

Related categories for Species: Andropogon gerardii var. gerardii | Big Bluestem