FIRE CASE STUDIES
SPECIES: Andropogon gerardii var. gerardii | Big Bluestem 



CASE NAME : 
Late summer burn fuel load study


REFERENCE : 
Ewing, A. L.; Engle, D. M. 1988 [36]


SEASON/SEVERITY CLASSIFICATION : 
Late summer - Severe


STUDY LOCATION : 
This study took place at the Oklahoma Agricultural Experiment Station's
Agronomy Research Range, approximately 9 miles (15 km) southwest of
Stillwater, Oklahoma. 


PREFIRE VEGETATIVE COMMUNITY : 
The vegetation was tallgrass prairie dominated by the big bluestem
(Andropogon gerardii var. garardii), indiangrass (Sorgastrum nutans),
switchgrass (Panicum virgatum), little bluestem (Schizachyrium
scoparium), and side-oats grama (Boutoula curtipendula).


TARGET SPECIES PHENOLOGICAL STATE : 
Big bluestem was in an active growth stage at the time of this September
5 burn.  Plants were probably in flower.


SITE DESCRIPTION : 
Two similar upland study sites with different grazing histories,
approximately 2.1 miles (3.5 km) apart, were burned.  One site was
moderately grazed in recent years including the year of burning
(considered the low fuel site), while the other had not been grazed for
at least 3 years (considered the high fuel site).  Mean annual
precipitation is 32.7 inches (83.1 cm) with 75 percent falling between
April through October.  Precipitation was 17 percent above average
during the 1985 growing season.  Weather conditions were typical of
wildfire conditions - hot and dry.  The relative humidity was 36
percent, while the ambient termerature was 98.6 degrees F (37 deg C).
Wind speed at the low fuel plots was 13.8-24 mph (23-40 kph), and 7.8-18
mph (13-30 kph) at the high fuel plots.  The low fuel plots had 443 plus
or minus 74 grams/m2 of accumulated fuel, while the high fuel plots had
1032 plus or minus 60 grams/m2 of accumulated fuel.


FIRE DESCRIPTION : 
The fire was a line head fire ignited by driptorch on September 5, 1985.
Fire intensity and fire temperatures demstrate that fire on the high
fuel plot was roughly four times as intense at the soil surface as that
on the low fuel plot.  Fire intensity and duration was measured in
degree seconds.  Degree seconds is the amount of time the sampled area
differs from the ambient post-burn temperature by more than 2 degrees C
(sampled at two second intervals).  Data on fire intensity and duration
are presented below:

   Area Sampled                              degree seconds
                                      low fuel            high fuel
soil surface                      10,400 +or- 1,900    43,000 +or- 3,200
6 in (15 cm) above soil surface    6,300 +or- 40       29,000 +or- 2,100
12 in (30 cm) above soil surface   3,900 +or- 180      20,300 +or- 1,400


FIRE EFFECTS ON TARGET SPECIES : 
This late summer fire resulted in nearly complete combustion of biomass,
with the plots generally having blackened and bare soil with a dusting
of ash.  Tiller counts two months after the fire showed that big
bluestem recovered well on the low fuel plot.  Most of the regrowth came
from existing tillers which had been completely defoliated but had not
suffered apical meristem damage during the fire.  On the high fuel plot,
extensive damage to tillers occurred.  Reductions in tiller densities
were apparent 2 months after burning.  Regrowth on high fuel plots
consisted largely of newly initiated tillers.  Big bluestem tiller
density before and after burning is summarized below:

                              low fuel plot          high fuel plot
                       before burn  after burn   before burn  after burn
                        (8/15/85)   (10/22/85)    (8/15/85)   (10/22/85)

tiller density (#/m2)      23          45            89          17

By the end of the following growing season, tiller densities were
roughly equal on burned and unburned plots.  Even on the high fuel plot,
where fire induced reductions in tiller density was apparent in the
early growing season, tiller densities returned to normal by September.


FIRE MANAGEMENT IMPLICATIONS : 
The rhizomatous character of big bluestem makes it well adapted to
survive summer fires.  Initiating new growth from rhizomes allows this
grass to quickly revegetate the postburn community.  Wildfire in the
tall grass prairie region, may initially reduce big bluestem
productivity, but total aboveground biomass may return to normal by the
end of the following growing season.

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