Surface Temperatures and Durations Associated with Spring
Prescribed Fires in Eastern South Dakota Tallgrass Prairies
Author(s): Michelle K. Ohrtman, Sharon A. Clay, and Alexander J. Smart
Source: The American Midland Naturalist, 173(1):88-98. 2015.
Published By: University of Notre Dame
DOI: http://dx.doi.org/10.1674/0003-0031-173.1.88
URL: http://www.bioone.org/doi/full/10.1674/0003-0031-173.1.88
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Am. Midl. Nat. (2015) 173:88–98
Surface Temperatures and Durations Associated with Spring
Prescribed Fires in Eastern South Dakota Tallgrass Prairies
MICHELLE K. OHRTMAN,1 SHARON A. CLAY
AND
ALEXANDER J. SMART
Departments of Plant Science and Natural Resource Management, South Dakota State University, Brookings, 57007
ABSTRACT.—Fire and grazing are commonly used to manage nonnative grasses in the
Northern Great Plains, but the effects of fire frequency and management between fire events
on fire behavior in this region are poorly understood. We examined temperature and
duration of prescribed spring fires at two locations where plots were treated with two fire
frequencies (annual or biennial), simulated grazing (1 mo of weekly clipping in spring) and
no simulated grazing. In May 2011 and 2013, soil surface fire temperatures and heat duration
were monitored in treatment plots using thermocouples. Probes also were placed at 1, 2, and
3 cm depths to measure soil heat transfer. Lethal heat duration (.60 C) at the surface tended
to be longer in plots treated with biennial fires compared to plots treated with annual fires.
Fires in 2011 had higher maximum temperatures than 2013. Cooler fires in 2013 were
characterized by longer durations of lethal heat. However, simulated grazing increased
residence time of lethal temperatures in biennial plots and reduced lethal temperature
duration in annual plots. Surface heat did not influence soil temperatures even at the 1 cm
depth. Greater fuel loads, characteristic of plots treated with biennial fires, generally were
associated with higher maximum temperatures and longer heat durations. Results suggest
decreasing fire frequency to once every 2 y, perhaps combined with biennial grazing
management, may enhance fire behavior to better meet management objectives.
INTRODUCTION
More than 20,000 ha of grasslands are managed with prescribed fire each year in the
Northern Great Plains (NGP), for the primary purpose of exotic plant control and native
vegetation enhancement (Willson and Stubbendieck, 1997; NPS, 2009; GPC, 2009, 2010;
USFWS, 2010). While grassland fire temperature and duration can, in part, determine the
level of plant tissue damage from lethal heat (.60 C) (Whelan, 1995), their effect can be
highly variable depending on interactions between numerous abiotic and biotic factors
(Pyke et al., 2010). Management (e.g., grazing) between fire events can undoubtedly impact
fire behavior in NGP tallgrass prairies, yet few studies have quantified these effects.
Fuel quantity and type are among the best predictors of plant tissue heat damage during
grassland fires (De Luis et al., 2004; Savadogo et al., 2007; Pyke et al., 2010) with greater
fuel loads having consistently higher maximum temperatures and longer plant lethal
temperature (.60 C) durations (Stinson and Wright, 1969; Morgan, 1999; Savadogo et al.,
2007; Vermeire and Rinella, 2009; Fidelis et al., 2010; Vermeire and Roth, 2011). Fuel
moisture content can also impact fire performance (Leonard, 2009). For example dormant
vegetation (dry) produced a fire intensity exceeding 700 C for up to 2 min (Stronach and
McNaughton, 1989; Stocks et al., 1996) whereas mixed dormant and growing vegetation
(higher water content) only reached 100 C for 2 to 3 min durations (Savadogo et al., 2007;
Fidelis et al., 2010).
Repeated fire events in grasslands are recommended to minimize nonnative plant reinvasion or promote recovery after the initial event (Whisenant, 1990; Kyser and DiTomaso,
2002). However, prescribed grassland fires can remove .99% of aboveground biomass
1
Corresponding author present address: Plant Science Department, South Dakota State University,
Brookings, South Dakota 57007. e-mail: michelle.ohrtman@sdstate.edu.
88
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(Bragg, 1982; Howe, 1994b) and annual fire frequencies may not have sufficient fuel loads
present to provide the necessary dose of lethal heat for suppression (NPS, 2009). Many
studies suggest grassland fuel loads do not accumulate in a linear manner. Vegetation
biomass can reach prefire levels in a minimum of 2 y (Morgan, 1999; Prober et al., 2007;
Fidelis et al., 2010; Kirkpatrick et al., 2011; MacDonald and McPherson, 2011), although little
or no further build up may occur if return times are greater than four or more years
(Govender et al., 2006; Oluwole et al., 2008; Cianciaruso et al., 2010).
Grazing management between fire events is another important variable that can impact
the success of a fire treatment. Grazing used in combination with fire increases grassland
species diversity (Collins et al., 1998) and effectively manages invasive plants (Davies et al.,
2009; Diamond et al., 2009; MacDonald and McPherson, 2011). Although grazing removes
less plant biomass than fire, grazed lands have lower fuel loads than ungrazed areas
(Savadogo et al., 2007; Davies et al., 2009; Diamond et al., 2009; Leonard et al., 2010;
Kirkpatrick et al., 2011; MacDonald and McPherson, 2011), which may reduce the initial
effectiveness of fire to kill undesirable vegetation. The combination of frequent fires, with
grazing between fire events, can further reduce post treatment fuel accumulation
(Archibald et al., 2005; Kirkpatrick et al., 2011); the unintended consequence can be
milder fire conditions compared to areas managed with either method alone. Similar to fire,
the effects of grazing management on fuel loads may be negligible beyond 2 to 3 y
(Archibald et al., 2005; Kirkpatrick et al., 2011; MacDonald and McPherson, 2011).
Spring prescribed fires are well known to reduce exotic cool season grass cover when set
during their elongation phase of growth by damaging plant meristems through prolonged
exposure to lethal heat (Willson and Stubbendieck, 1997). Consequently, spring fires can
promote dominance of native warm season grasses by altering competitive interactions
between plant species (Howe, 1994a, b, 1995; Smith and Knapp, 1999; Howe, 2000). In
addition spring fires remove thatch and standing dead biomass which can enhance warm
season grass growth by allowing greater levels of light at the soil surface and higher soil
temperatures early in the growing season (Knapp and Seastedt, 1986). Both annual (Abrams
and Hulbert, 1987; Abrams, 1988; Towne and Kemp, 2003) and biennial (Svedarsky et al.,
1986; Towne and Kemp, 2008) spring fires have achieved these management goals.
In the North American Great Plains, a region where the fire and grazing interaction is
considered a single ecological process (Hamilton, 2007; Fuhlendorf et al., 2008), no
research has examined the combined effects of fire frequency and grazing on fire behavior.
We investigated the influence of spring fire return frequency and simulated spring grazing
treatments on fire temperature and lethal heat duration at two locations in eastern South
Dakota. We hypothesize: (1) biennial fires will produce higher maximum temperatures and
longer durations of plant lethal temperatures than annual fires and (2) post fire simulated
grazing will decrease maximum fire temperature and lethal heat duration. Research
findings can be used to better understand how grazing and prescribed fires in northern
tallgrass prairies influence fuel dynamics and fire behavior to optimize management effects.
METHODS
STUDY SITES
This research was performed at two sites in Brookings County, located in eastern South
Dakota. The first site, nearest to the city of Brookings (44u2096.330N, 96u48928.620W), was a
cool season pasture that had been seeded with big bluestem (Andropogon gerardii Vitman) in
2005. Soil at the Brookings site was well drained Barnes clay loam with 0 to 2% slopes
(NRCS, 2010a, b). Kentucky bluegrass (Poa pratensis L.), big bluestem, and smooth
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bromegrass (Bromus inermis Leyss. subsp. inermis) were the dominant grass species and yellow
sweetclover [Melilotus officinalis (L.) Lam.] was the predominant forb species. Average
vegetation biomass in untreated areas at this site in 2011 was 3400 kg ha21. The second site,
near the city of Volga (44u2391.530N, 96u57929.390W) was a native prairie with well drained
soils in the Buse Poinsett complex with 9 to 15% slopes (NRCS, 2010a, b). Common plant
species at the Volga site included little bluestem [Schizachyrium scoparium (Michx.)], big
bluestem, sideoats grama [Bouteloua curtipendula (Michx.) Torr.], smooth bromegrass, and
yellow sweetclover. Kentucky bluegrass was also present but less prevalent than at the
Brookings site. Average vegetation biomass collected in untreated plots at this site in 2011
was 2300 kg ha21. Annual precipitation recorded at Brookings, South Dakota was 41.5 and
46.8 cm in 2011 and 2013, respectively (South Dakota Climate and Weather, 2013).
Field plots were established in 2009 in a split block – split plot design containing four
replicates of each treatment at each site (Brookings and Volga). Fire treatments were
applied to 6 3 6 m blocks and consisted of no fire (control), annual spring fire, and
biennial spring fire. Each block was split into two 3 3 6 m plots and one plot of each block
was treated with simulated grazing. Simulated grazing consisted of clipping vegetation to
2 cm height once each week for 4 wk following the fire treatments; the first clipping
treatment occurred 7 d after fire. Clipped biomass was removed from the study plots. All
prescribed fires were conducted in May using a ring fire method when there was a mix of
dormant and actively growing vegetation that is common during spring prescribed fires in
the NGP. Weather conditions during fire treatments consisted of wind speeds between 8
and 29 kph, air temperatures between 10 and 24 C, and relative humidities between 30 and
50%. Biennial prescribed fires and the subsequent simulated grazing treatments were
applied in 2009, 2011, and 2013. Annual prescribed fires and postfire simulated grazing
treatments were applied in 2009, 2010, 2011, 2012, and 2013.
FUEL CONDITIONS
In 2011 fuel load was quantified in all treatment plots (biennial and annual fire with and
without simulated grazing) at the Brookings site and in annual fire treatments at Volga. Live
and dead herbaceous vegetation was clipped to ground level within 0.25 m2 quadrats (n 5 4
treatment21) and dried for 3 d at 70 C. Fuel weights were recorded immediate following
collection and after drying to determine percent moisture. In 2013 fuel load was quantified
only in plots without simulated grazing from both the Brookings and Volga sites; however
fuel moisture was not measured. Surface soil (0 to 7.5 cm depth) moisture values were
obtained within treatment plots but found to be negligible, and therefore these data are not
discussed.
MAXIMUM TEMPERATURE AND LETHAL HEAT DURATION
Surface and subsurface temperature was measured during prescribed fires on May 6, 2011
and May 16, 2013 at both study sites. To measure surface temperatures during prescribed
fires, mineral insulated thermocouples (Type K) attached to data loggers (TC Direct,
Hillside, IL) were placed within the surface litter layer taking care not to embed probes in
the soil. However, litter depth varied by plot and site and therefore probes recorded within a
range of 5 cm above the soil surface. Four probes were placed in each plot within three of
the four blocks (12 probes total for three replicates) for each site. However, probes were
often disturbed during fire ignition and containment, leaving two or three probes per
replicate (seven to nine probes total) with accurate temperature recordings treatment21
site21 year21. Thermocouples were placed at least 1 m inside the plot boundary and spaced
at least 1 m apart. Temperatures were recorded at 1 s intervals from just prior to fire ignition
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until surface temperatures were below 60 C. Subsurface temperatures were monitored in
one block at each site by installing thermocouples at 1, 2, and 3 cm depth (n 5 3
thermocouples depth21) in the vertical face of a shallow pit. However, no significant change
in subsurface soil temperature was observed during annual and biennial fires so results are
not shown.
STATISTICAL ANALYSIS
Mixed effects ANOVA models and student’s t-tests were used to test the effects fire return
frequency (annual or biennial), simulated grazing treatment (postfire clipping or no
clipping), location (Brookings and Volga), and year (2011 and 2013) on maximum fire
temperature and duration of plant lethal heat (.60 C). Site, fire frequency, and simulated
grazing were considered fixed effects and year and block were considered random effects.
Differences among treatments were also examined for fuel load and fuel moisture by
location and year. Means (6SE) are reported with significant differences separated using
Fisher’s Protected LSD Test when P ,0.05. Analyses were performed in JMP 10.0.2 (SAS
Institute Inc.).
RESULTS
FUEL CONDITIONS
Brookings.—Fuel loads in 2011 were 3700 6 670 kg ha21 in plots treated with biennial
prescribed fires and 2500 6 310 kg ha21 in plots treated with annual prescribed fires (P 5
0.05). Fuel loads did not differ between clipped and unclipped plots at the time of burning
during this year (P 5 0.25), regardless of fire return frequency. Vegetation moisture was
about 16% irrespective of fire frequency (P 5 0.36) and simulated grazing treatment (P 5
0.68). In 2013 fuel load was only measured in plots that were not subjected to the simulated
grazing treatment. During that year fuel loads were 8000 6 430 kg ha21 in plots treated with
biennial prescribed fires and 5200 6 780 kg ha21 in the plots treated with annual prescribed
fires. Fuel loads in plots treated with biennial prescribed fires were 35% more than that fuel
loads in plots treated with annual prescribed fires, irrespective of year (P 5 0.001).
Volga.—There was a trend of lower fuel load in plots treated with simulated grazing than
unclipped plots in annual fire treatments at this site in 2011 (2800 6 150 kg ha21 vs. 3500 6
360 kg ha21; P 5 0.07). In 2013 fuel loads in biennial prescribed fire plots at the Volga site
were 48% greater than annual plots (4800 6 820 kg ha21 vs. 2500 6 380 kg ha21; P 5
0.004); these values are 35 and 52% less than respective fuel loads at Brookings (P 5 0.002).
MAXIMUM TEMPERATURE AND LETHAL HEAT DURATION
Maximum fire temperature.—Maximum fire temperature differed by year (P 5 0.001) and
there was an interaction between fire return frequency and simulated grazing (P 5 0.05)
(Table 1). Averaged across all treatment combinations and sites, maximum fire temperatures in 2011 were approximately 230 C hotter compared to fire temperatures in 2013.
Maximum fire temperatures in plots treated with biennial fires but not treated with
simulated grazing were higher than in plots treated annual fires and simulated grazing. The
highest recorded temperature during our study, 836 C, was recorded during 2011 in a plot
treated with biennial fire but not treated with simulated grazing.
Lethal heat duration.—Three-way interactions were noted when examining the length of
time the surface remained .60 C (Table 1). Lethal heat durations of temperatures .60 C at
the Volga site tended to be shorter than at the Brookings site (Fig. 1, Table 2). Higher
maximum temperatures were associated with shorter periods of lethal heat duration in plots
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TABLE 1.—ANOVA for maximum temperature (R2 5 0.46, F ratio 5 12.79, P , 0.0001) and
temperature duration above 60 C (R2 5 0.54, F ratio 5 12.43, P , 0.0001). Degrees of freedom 5 1 for
each parameter
Maximum temperature
.60 C duration
Parameter
Sum of squares
F ratio
Sum of squares
F ratio
year
site
fire frequency
clip
year*site
year*fire frequency
year*clip
site*fire frequency
site*clip
fire frequency *clip
year*fire frequency *clip
site*fire frequency *clip
1788196.4
19303.5
228701.6
74538.0
105586.0
5739.0
48376.9
27094.2
32294.1
114563.5
-
98.2**
1.1
12.6
4.1
5.8
0.3
2.7
1.5
1.8
6.3*
-
348126.1
298528.5
339981.8
2819.5
11178.3
1376.6
5135.1
28108.2
22344.5
399.7
35392.0
29314.6
49.2**
42.2**
48.0**
0.4
1.6
0.2
0.7
4.0
3.2
0.1
5.0*
4.1*
Significance: ** ,0.0001, * ,0.05
treated with annual fires (R2 5 0.10, F ratio 5 8.2, P 5 0.006). However, plots treated with
both biennial fires and simulated grazing at the Brookings site were characterized by longer
periods of lethal heat; this result was not found at the Volga site. At both sites and during
both years of measurement, plots treated with annual fires but not treated with simulated
grazing had shorter periods of lethal heat compared to plots treated with biennial fires and
not treated with simulated grazing. In 2013 at both sites, plots treated with annual fires and
simulated grazing had shorter duration of lethal heat compared to plots treated with annual
fires but not treated with simulated grazing. In 2013 simulated grazing had inconsistent
effects on lethal heat duration when applied to biennially burned plots. Periods of lethal
heat duration were longer in 2013 compared to 2011.
DISCUSSION
Our results support previous reports that document the importance of fuel load for
predicting prescribed fire temperature and duration (Stinson and Wright, 1969; Morgan,
1999; Fidelis et al., 2010). Biennial frequency fire plots at Brookings had 30% greater fuels
and greater durations above 60 C, that ranged between 60 to 180 s longer than annual plots,
irrespective of year. In addition the Brookings site had nearly twice the fuel loads compared
with the Volga site and Brookings fires had greater durations of plant lethal temperature
(up to 120 s longer) than Volga in both years. The difference in fuel loads between sites can
be explained, in part, by an increase in native warm season grass cover at Brookings over
time. Big bluestem cover increased from 34 to 80% in annually treated plots and from 19 to
56% in biennial plots since 2009 (Smart et al., 2013). Volga also experienced an increase in
native warm season grass cover over time with fire and simulated grazing management but
little bluestem and sideoats grama were dominant. Another South Dakota study observed
vegetation biomass to be two times greater following prescribed fires where big bluestem was
the dominant plant species (.60%) than where this species was absent (Engle and Bultsma,
1984). Frequent prescribed fires have been reported to produce high amounts of native
warm season grasses but this management technique can also reduce species diversity
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FIG. 1.—Temperature profiles (above 60 C) for spring prescribed fires at Brookings and Volga, South
Dakota in (A) 2011 and (B) 2013 for plots treated with a combination of annual or biennial (every two
years) fire and annual simulated grazing (Clip) or no grazing treatment (No Clip). Lines represent an
average of 7 to 9 temperature readings per treatment. Note: these are not true averages of high
temperature or duration but represent averages from the initial second each probe exceeded 60 C until
each probe fell below this temperature
(Collins et al., 1998; Kindscher and Tieszen, 1998; Polley et al., 2005) which may not be a
desired management response.
Flame residence times in North American grasslands have been reported to be about 15 s
long with maximum flame temperatures between 800 and 1000 C (Wotton, 2009). Average
maximum temperatures at the soil surface in our plots were cooler (Table 2) but durations
of plant lethal heat recorded in biennial plots in this study were greater than previous
reports. For example annual prescribed grassland fires in Australia were observed to reside
above 100 C for 62 s on average, whereas a single replication of fire every 2, 4, and 7 y
resided for 68, 113, and 173 s, respectively (Morgan, 1999). Annual fires recorded by this
study in 2011 produced comparable heat durations to those reported by Morgan (1999), but
temperatures .100 C in annual and biennial plots in 2013 resided between 2 and 4.5 times
longer depending on site and simulated grazing treatment. Sites with frequent fire recorded
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TABLE 2.—Maximum fire temperature and duration of fire above 60 C for annual and biennial plots
by clipping treatment burned in 2011 and 2013 at Brookings and Volga, South Dakota. Values represent
an average of three replicates (with two or three temperature readings per replicate) per site. Significant
differences are separated using Fisher’s Protected LSD Test when P , 0.05. Means followed by the same
letter within a year are not significantly different across sites or across treatments within a site
2011
Annual fire
Site
Clip
No clip
Maximum temperature (C)
Brookings
438 bc
491 abc
Volga
378 c
549 ab
Temperature duration (s) .60 C
Brookings
177 bc
138 cd
Volga
131 cd
83 d
Temperature duration (s) .100 C
Brookings
91 bc
93 bc
Volga
58 c
53 c
Temperature duration (s) .200 C
Brookings
47 bc
50 bc
Volga
29 c
29 c
2013
Biennial fire
Annual fire
Clip
No clip
Biennial fire
Clip
No clip
Clip
No clip
467 abc
528 abc
523 abc
611 a
205 cd
152 d
336 ab
209 cd
414 a
283 bc
262 bc
290 bc
292 a
151 bcd
286 a
229 ab
255 cde
181 e
305 bc
223 de
442 a
233 cde
371 ab
293 cd
200 a
90 bc
213 a
121 b
142 cde
75 e
191 bc
121 de
306 a
157 cd
242 ab
180 bcd
121 a
56 bc
131 a
76 b
38 bcd
1d
95 b
20 cd
166 a
52 bcd
91 b
68 bc
15 to 235 s of lethal heat during prescribed grassland fires, whereas a site unburned for 6 y
produced these temperatures for 140 to 330 s (Fidelis et al., 2010). Stinson and Wright
(1969) observed similar durations of temperatures .60 C for head fires in southern mixed
prairie of unreported fire frequency. Whereas most of our fires recorded temperature
durations within this range, we observed plant lethal heat durations between 250 and 440 s
on average during prescribed fires at Brookings in 2013. These data suggest plant lethal
temperature duration may be underestimated in grassland fire research if the temperature
profile is not monitored. Both biennial and annual fire return frequencies have been
successful for reducing cover of cool season exotic grasses (Smart et al., in press) but waiting
2 y between fires may enhance control efforts while reducing costs and providing greater
habitat for wildlife (Herkert, 1994; Dechant et al., 2003).
Biennial simulated spring grazing combined with biennial fire produced some of the
longest durations of plant lethal heat in both years and the highest maximum temperatures
in 2013. These results are unexpected because simulated spring grazing treatments removed
surface vegetation, leading to lower (but not statistically significant) fuel loads before the
fires in clipped than unclipped plots. However, in mesic areas or during wetter years, the
interaction between fire and post fire grazing can enhance plant diversity and production
(Fuhlendorf and Engle, 2001) and perhaps change the quality of vegetation fuels. Grazing
by livestock may have different effects on fire performance than the uniform clipping
treatments used in this study. Even at heavy stocking rates, utilization has been reported to
be only 60% (Smart et al., 2010) whereas simulated grazing removed all vegetation fuels
above 2 cm height. In addition grazing animals selectively remove palatable herbaceous
vegetation with higher moisture content and leave dry litter behind; this process can lower
the moisture content of the fuel load and increase fire severity (Leonard et al., 2010).
However, most combined fire and grazing programs graze within a few weeks to months
following the fire when there is less unpalatable dry material to be avoided (Anderson et al.,
1970; Kirkpatrick et al., 2011).
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Simulated grazing was only found to increase fire temperature and duration at the
Brookings site. Simulated grazing likely provided further setback of cool season species
attempting to recover from fire, which would enhance native warm season grass growth
through reduced competition. The effects of this combined management may have been
more pronounced at Brookings because of the high level of pre management cool season
cover and the dominance of more productive big bluestem (Engle and Bultsma, 1984; Smart
et al., in press). Post fire grazing in northern grasslands therefore has the potential to
increase the ability of future fires to achieve management goals by stimulating vegetation
growth immediately after the disturbance and promoting greater accumulation of surface
fuels.
Fire and grazing are successful for grassland management because native species that
evolved with repeated disturbance recover quickly. Plant biomass reduction following fire is
often short lived; 2 y after fire and grazing vegetation fuels on all plots were not significantly
different (MacDonald and McPherson, 2011). On the other hand, fuel loads can increase
with longer periods between fires with greater fuel accumulations 3 y after the fire than pre
fire (Ducherer et al., 2009). Indeed, greater time between fires has been observed to
translate to greater durations of plant lethal heat (Morgan, 1999). The observed negative
correlation between maximum fire temperature and duration of lethal heat for annual fire
frequencies supports lighter fuel loads, associated with more frequent fires, burn quickly
and maintain shorter durations of plant lethal heat at the soil surface. We did not test the
effects of longer periods between fire or grazing ($3 y) on fire behavior in NGP grasslands.
However, if longer periods between fire and grazing are used there may be an increased risk
of nonnative plant re- invasion or recovery. We also used a ring fire method where backfires
burned most of the plot areas in this study; such fires promote lower fire temperatures and
durations than headfires (Laterra et al., 2006; Savadogo et al., 2007). With comparable fuel
loads, headfires produce higher temperatures and durations that can be more effective for
invasive plant control (MacDonald et al., 2007; Vermeire and Rinella, 2009).
Different fire regimes (i.e., frequency, timing) may favor different species (Whelan, 1995;
Pivello and Norton, 1996; Cianciaruso et al., 2010) and therefore it is important to
understand the fire tolerance of target vegetation as well as the capacity of the fire to
produce the necessary duration of heat. Although cover responses of Kentucky bluegrass,
smooth brome, and native warm season grasses to fire management are well documented,
the specific heat tolerance of common tallgrass prairie species at various life stages (from
seed to mature plants) is unknown. Reduced cover of Kentucky bluegrass and smooth
brome and greater dominance of big and little bluestem suggest repeated late spring
prescribed fires at Brookings and Volga provided sufficient energy to suppress exotic cool
season grass growth (Smart et al., 2013). However, this study showed prescribed fires did not
provide sufficient heat to result in plant lethal heat transfer in the soil. We did not observe
an increase in temperature at 1 cm soil depth, irrespective of fire frequency treatment.
These results suggest seeds with any soil insulation and established plants with belowground
perennating buds will receive minimal damage from prescribed spring grassland fires on
lands with recent fire and grazing management (#2 y). Other control methods, in addition
to fire, may need to be considered when developing management strategies for unwanted
perennial species.
This study is the first to describe fire behavior in managed North American tallgrass
prairie and suggests fuel load is an important variable for predicting fire behavior in this
system. We found biennial fire regimes can extend the duration of plant lethal heat by
promoting greater fuel accumulations and therefore may have the greatest potential for
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delivering the necessary lethal heat dose for invasive plant control. Combined biennial fire
and post fire grazing can also stimulate fuel production but the effects of this management
are dependent on site-specific and climatic factors. This study showed fire behavior is
variable across locations and years and therefore site specific evaluations over time are
important.
Acknowledgments.—This research was supported by a grant from USDA/CSREES Rangeland (award
00486094). Additional funding was provided by South Dakota Agricultural Experiment Station and
South Dakota Cooperative Extension Service. Thanks to Stephanie Hansen, Jiyul Chang, Shauna
Waughtel-Johnson, Megan Mortallero, Emily Helms, and Heidi Myer for field assistance.
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SUBMITTED 7 SEPTEMBER 2013
ACCEPTED 27 SEPTEMBER 2014