Preliminary Evidence that Patch Burn-Grazing Creates
Spatially Heterogeneous Habitat Structure in Old-field
Grassland
Devan Allen McGranahan, Gina M. Raicovich, W. Nathan Wilson,
and C. Kenneth Smith
Southeastern Naturalist, Volume 12, Issue 3 (2013): 655–660
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D.A. McGranahan, G.M. Raicovich, W.N. Wilson, and C.K. Smith
22001133 SOUSToHuEthAeSaTstEeRrnN N NaAtuTrUaRlisAtLIST 12V(o3l). :1625,5 N–6o6. 03
Preliminary Evidence that Patch Burn-Grazing Creates
Spatially Heterogeneous Habitat Structure in Old-field
Grassland
Devan Allen McGranahan1,*, Gina M. Raicovich2, W. Nathan Wilson3,
and C. Kenneth Smith4
Abstract - Heterogeneity created through patchy disturbance is an important component
of grassland conservation, but little research has focused on patch burn-grazing in
grassland of the eastern United States. To test the viability of patch burn-grazing in a
Tall Fescue-invaded old-field grassland on the Cumberland Plateau, we conducted a prescribed
patch burn, stocked cattle, and measured vegetation structure and plant functional
group composition. We found that patch burn-grazing creates spatial heterogeneity (patch
contrast) for grass height, litter cover, bare ground, and canopy cover of native and exotic
grasses. These results suggest that patch burn-grazing is a viable tool for heterogeneitybased
grassland management in the region.
Introduction
Heterogeneity is an important component of conservation, and in grassland
primarily results from spatially patchy disturbance (Fuhlendorf and Engle 2001).
In many rangeland ecosystems, fire and grazing interact to create a unique
disturbance—pyric-herbivory (Fuhlendorf et al. 2009)—which in turn creates
heterogeneity in habitat structure along with wildlife and plant diversity (Derner
et al. 2009).
Relatively little research has contributed to an understanding of the fire-grazing
interaction in eastern North America. In the southeastern United States, specifically,
many grasslands are characterized by small tract sizes and are frequently
invaded by Schedonorus phoenix (Scop.) Holub (Tall Fescue), an Eurasian coolseason
grass that occurs across 14 million ha in the region (Fribourg et al. 1991).
As an invader in native warm-season grassland and an abundant species in oldfields,
Tall Fescue has been implicated in reduced fire spread and associ ated with
decreased native plant species richness (McGranahan et al. 2012a, 2013a).
We established a pilot project to determine whether the fire-grazing interaction
will create spatial heterogeneity in a pasture stand common in the southeastern
United States. We implemented a patch burn-grazing management scheme in
old-field grassland dominated by Tall Fescue and Andropogon virginicus L.
(Broomsedge Bluestem) and compared habitat variables related to vegetation
structure (grass height, bare ground, and litter cover) in addition to plant
1Mellon Environmental Fellow, Department of Environmental Studies, The University of
the South, Sewanee, TN 37375.2Farm Manager, The University of the South, Sewanee,
TN 37375. 3Domain Manager, The University of the South, Sewanee, TN 37375. 4University
Forester and Professor, Department of Forestry and Geology, The University of
the South, Sewanee, TN 37375. *Corresponding author - mcgranah@alumni.grinnell.edu.
D.A. McGranahan, G.M. Raicovich, W.N. Wilson, and C.K. Smith
2013 Southeastern Naturalist Vol. 12, No. 3
656
functional group composition. We predicted that bare ground, native grass cover,
and forb cover would be greater in the burned patch than the unburned area, while
grass height, litter, and Tall Fescue cover would be lower in the burned patch. We
discuss our results with respect to fuel load and fire-behavior characteristics.
Field-site Description and Methods
Our inquiry was conducted at the University of the South in Sewanee, TN, located
on the Cumberland Plateau. We selected a 16-ha old-field not treated with
grazing, fire, mowing, soil amendments or herbicide for >10 yrs aside from periodic
tree removal to maintain the grassland state. The majority of the grassland
vegetation was dominated by Tall Fescue and Broomsedge Bluestem, with less than 10%
canopy cover of upland hardwood species (Acer rubrum L. [Red Maple], Liquidambar
styraciflua L. [Sweetgum], and Quercus spp. [oak]) and Juniperus virginiana L.
(Eastern Red Cedar) in addition to small patches of blackberry (Rubus spp.).
We conducted a prescribed patch burn on 3 ha in mid-March 2012. Prior to
the burn, we measured fuel moisture and fuel load for both live and dead fuel
components using the constituent differential method (Gillen and Tate 1993).
Fuel moisture (dry-weight basis) and fuel-load data were combined with weather
data from the day of the fire to estimate flame length and rate of spread with BehavePlus
fire-behavior modeling software (Andrews et al. 2008).
Twenty 227–318-kg steers were stocked in mid-April 2012. Cattle had free access
to water in stock ponds without interior fencing. Beyond basic mineral lick,
no feed supplements were provided during the period of this study. Total rainfall
for the study period—mid-March–late May—was 8.7 cm.
To determine the effect of patch burn-grazing on spatial heterogeneity, we
measured habitat structure variables (grass height, bare ground, and litter cover)
and plant functional group composition (canopy cover of forbs, native grasses,
and exotic grasses) in late May 2012. We randomly located 25-m transects in the
burned patch (n = 3) and across the unburned areas (n = 5) along which we measured
the height of two randomly selected grass tillers at 1-m intervals. For bare
ground, litter, and plant functional composition, we randomly located five 0.25-
m2 quadrats along the transect within 5 m of either side and visually estimated
percent cover using the Daubenmire (1959) cover-class index. While wildlife response
to patch burn-grazing was not assessed in this study, vegetation variables
reported here are associated with habitat for grassland fauna (McGranahan et al.
2013b), and sampling was timed to fall within the grassland bird nesting season
in the Southern US (Conover et al. 2011).
To inform whether observed responses were due to the combined effect of fire
and grazing or due to fire alone, we compared aboveground biomass from three
grazer exclosures erected in the burned patch to aboveground biomass from
three points in the burned area accessible to grazers. All aboveground plant biomass
was collected from 0.25-m2 quadrats and dried for 48 hrs at 45 °C.
The lack of replicate pastures in our pilot study precluded the use of analysis
of variance in data analysis. Instead, we calculated effect sizes using Cohen’s
d (Cohen 1977) and associated 95% confidence intervals for each measured
657
D.A. McGranahan, G.M. Raicovich, W.N. Wilson, and C.K. Smith
2013 Southeastern Naturalist Vol. 12, No. 3
variable. Cohen’s d compares the mean and standard deviation of two groups to
determine a treatment effect, which is indicated when effect sizes are non-zero
and 95% confidence intervals do not overlap zero. Although we expected patch
burn-grazing to decrease some values in the burned patch relative to unburned
areas (e.g., grass height, litter cover) but increase others (bare ground), we report
the absolute value of all effect sizes to focus on the hypothesis that patch burngrazing
creates spatial heterogeneity (contrast among patches) across burned and
unburned areas (sensu McGranahan et al. 2013b).
Results and Discussion
At the time of our prescribed fire, mean total herbaceous fuel load was
4448 kg/ha (1.8 t/ac). Live fuel load, comprised primarily of Tall Fescue, averaged
346 kg/ha (0.14 t/ac) and dead fuel load, comprised of both Broomsedge
Bluestem and Tall Fescue, averaged 4102 kg/ha (1.66 t/ac). Field-measured
fuel moistures averaged ≈300% and 17% for live and dead fuels, respectively.
Eye-level wind speed ranged 3–8 km/hr, and relative humidity was approximately
48%. BehavePlus predicted flame lengths of 0.5–1.1 m and rates of spread
1.4–7.9 m/min across the grass-dominated portions of our relatively flat fuelbed,
which corresponded to typical fire behavior observed during the burn.
The spatial pattern of fire drove the spatial pattern of grazing in this oldfield
pasture, concordant with patch burn-grazing theory (Fuhlendorf and Engle
2004, Fuhlendorf et al. 2009, McGranahan et al. 2012b). Nine weeks after the
prescribed burn, mean aboveground plant biomass within burned-but-ungrazed
exclosures was an order of magnitude greater than that of the surrounding
grazed burned patch (756 kg/ha ± 307 SE vs. 56 kg/ha ± 2 SE). Furthermore,
grazing frequency and cattle occupancy time, measured by proportion of
grass tillers grazed and fecal pat density, respectively, were both greater in the
burned patch than unburned areas (J.S. Hill et al., The University of the South,
Sewanee, TN, unpubl. data). Taken together, these data indicate that cattle removed
substantial herbage through grazing that was spatially determined by the
patch burn.
Mean values varied between the burned patch and unburned areas for most
response variables with the exception of forb canopy cover (Table 1). Patch
burn-grazing created spatial heterogeneity between the burned patch and unburned
areas for five of the six measured variables: grass height, litter cover,
Table 1. Mean (± standard error) values for six habitat variables across the burned and unburned
patches in a single patch burn-grazing demonstration pasture in an old-field grassland on the
Cumberland Plateau, Sewanee, TN. Values for grass height given in cm, all other values in percent
canopy cover following Daubenmire (1959).
Response variable
Patch Grass height Litter cover Bare ground Forbs Native Grasses Exotic grasses
Burned 14 ± 2 18 ± 4 41 ± 7 16 ± 3 34 ± 6 28 ± 6
Unburned 29 ± 3 57 ± 4 1 ± 1 18 ± 4 56 ± 6 44 ± 6
D.A. McGranahan, G.M. Raicovich, W.N. Wilson, and C.K. Smith
2013 Southeastern Naturalist Vol. 12, No. 3
658
bare ground, native grass canopy cover, and exotic grass canopy cover (Fig. 1).
Although the effect size for forb canopy cover was positive, the 95% confidence
interval included zero.
These results suggest patch burn-grazing can create heterogeneity among
patches within old-field grassland on the Cumberland Plateau. Combined with our
data and observations on cattle response to the spatial pattern of fire, the response of
patch burn-grazing on the reported variables (Fig. 1) suggest that patch burn-grazing
creates a disturbance unique from fire or grazing alone (Fuhlendorf et al. 2009).
As an example of the conservation value of these effects, reduced vegetation cover
has been associated with Grasshopper Sparrow nest survival in Tall Fescue-invaded
eastern grassland managed with fire and grazing (Hovick et al. 2012).
Despite low wind speed and the high live-fuel moisture content of the Tall
Fescue, which decreases fire spread in invaded grassland (McGranahan et al.
2013a), our prescribed fire achieved sufficient spread and intensity to consume
litter, create bare ground, and reduce grass cover in the herbaceous-dominated
Figure 1. Effect sizes and associated 95% confidence intervals demonstrate the response
of six variables to patch burn-grazing in a single patch burn-grazing demonstration
pasture in an old-field grassland on the Cumberland Plateau, Sewanee, TN. Effect size,
measured as Cohen’s d (McGranahan et al. 2012b), compares the mean and standard
deviation of three transects in the burned patch to five transects in the unburned area.
Absolute values of effect sizes are reported to focus on the effect of patch-burn grazing
on creating spatial heterogeneity (patch contrast) in each variable (McGranahan et al.
2013b). For actual values and direction (increase or decrease) in each value across burned
and unburned areas of the pasture, see Table 1.
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D.A. McGranahan, G.M. Raicovich, W.N. Wilson, and C.K. Smith
2013 Southeastern Naturalist Vol. 12, No. 3
areas of the burned patch. Patch contrast in the variables studied here should be
easily achieved in similar old-field grassland fuel beds with at least 4448 kg/
ha (1.8 t/ac) total fuel load under typical prescribed-fire conditions, although
decreased total fuel load, increased live-fuel load, and/or increased live-fuel
moisture might require greater wind speed and/or decreased relative humidity for
sufficient fire spread (McGranahan et al. 2013a).
While long-term vegetation responses to patch burn-grazing in this ecosystem
remain unknown, heterogeneity in plant functional group abundance at the tract
scale is likely important to maintain forage production through temporal cycles
in plant productivity (Fynn 2012). Additionally, other research has shown lower
abundance of invasive species under patch burn-grazing compared to grazing
with homogeneous fire regimes (Cummings et al. 2007). Continued research
should monitor changes in relative abundance of exotic (i.e., Tall Fescue) versus
native (i.e., Broomsedge Bluestem) grasses.
Ecologically sound grazing systems—low-input systems that rely on perennial
vegetation and enhance environmental outcomes (Franzluebbers et al.
2012)—have the potential to contribute to biodiversity conservation and ecosystem
protection in the eastern United States. Both fire and grazing have a long
tradition in the region, with the best economic returns shown under moderate
stocking rates (Grelen 1978, Pearson and Whitaker 1973). Fire has been shown
to increase forage quality of native Andropogon spp. stands similar to the oldfield
pasture community described here (Grelen and Whitaker 1973, Lewis et
al. 1982). The results of this pilot project indicate that patch burn-grazing might
represent a viable tool to manage diverse, perennial grassland for both biodiversity
conservation and livestock production on the Cumberland Plateau. We
recommend replicated research on the vegetation and wildlife ecology of patch
burn-grazing using moderate stocking rates in grassland in the eastern US.
Acknowledgments
We appreciate the support of the Treasurer and Office of Domain Management of The
University of the South, the Sewanee Environmental Institute, and the contributions of D.
Carter, S. Gilliam, R. Petropoulos, J. Hill, C. Henderson, and the 2012 Sewanee Summer
Farm Team.
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