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2009 SOUTHEASTERN NATURALIST 8(4):695–708
Military Training and Road Effects on
Imperata cylindrica (L.) Beauv. (Cogongrass)
Lisa Y. Yager1,*, Jeanne Jones2, and Deborah L. Miller3
Abstract - Type, level, and intensity of human activities may facilitate establishment
and spread of invasive plant species. A better understanding of how human activities
infl uence invasion can assist land managers in developing strategies for control and
monitoring of invasive plants. Spread of the invasive species Imperata cylindrica
(Cogongrass) has been attributed to human activities. During 2002−2004, on Camp
Shelby Training Site, MS, we investigated relationships between military activity
and establishment and growth of Cogongrass. In areas of soil disturbance from military
equipment, vegetative linear growth rates of 7−10 m yr-1 were recorded on firing
points. There was a positive relationship between military troop use and Cogongrass
establishment on firing points for one of the 2 years of the study (P = 0.023). Thus,
steps to minimize soil disturbance in and near Cogongrass may reduce spread. We
examined frequency of Cogongrass infestation and vegetative growth rates for roadside
areas along gravel roads subject to at least annual mowing and grading, and dirt
tracks receiving infrequent maintenance. Cogongrass spread and establishment on
roadsides did not differ for the two road types (P ≥ 0.116). These results may refl ect
activities already in place to reduce disturbance of Cogongrass patches.
Less than 3% of the 38 million ha of the highly diverse Pinus palustris
Mill. (Longleaf Pine) forests remain in the southeastern United States (Frost
2006). A large portion of these remaining well-maintained Longleaf Pine
communities occur on military bases (Frost 2006). However, managers seeking
to maintain and protect these plant communities must consider threats
from invasive species that may detrimentally impact species diversity or
ecosystem processes (Walker and Silletti 2006). Imperata cylindrica (L.)
Beauv. (Cogongrass) has negatively affected natural and managed ecosystems
in the southeastern United States since introduction from southeast
Asia in the early 1900s (MacDonald 2004). In Longleaf Pine forests, Cogongrass
displaces native vegetation, alters soil processes, reduces recruitment
of planted native woody seeds and seedlings, and produces higher fine fuel
loads that stimulate hotter fires than native grasses (Lippincott 1997, 2000;
Platt and Gottschalk 2001). Species richness of native plants in Longleaf
Pine communities is negatively impacted by Cogongrass (Brewer 2007,
1Mississippi Museum of Natural Science, Mississippi Department of Wildlife,
Fisheries, and Parks, 2148 Riverside Drive, Jackson, MS 39202. 2Mississippi State
University, Department of Wildlife and Fisheries, Mississippi State, MS 39762. 3University
of Florida, Department of Wildlife Ecology and Conservation, West Florida
Research and Education Center, Milton, FL 32483. *Corresponding author - Lisa.
696 Southeastern Naturalist Vol. 8, No. 4
Yager 2007). Forage quality of Cogongrass for domesticated animals and
wildlife is lower than native grasses or agronomic pasture grasses (Lippincott
1997, MacDonald 2004). In addition, on military installations, training
operations may be adversely impacted because firing operations may create
wildfires in Cogongrass as a result of its high fl ammability (Yager 2007). A
better understanding of factors that infl uence Cogongrass establishment and
spread would benefit military and other land managers tasked with developing
appropriate control strategies that protect important habitats and military
training operations from cogongrass infestation and establishment.
Rates of establishment and spread of invasive species are dependent on
factors which affect the number and frequency of propagules introduced into
an area, and once introduced, subsequent growth and establishment of the
plants (Radosevich et al. 2003). Human activity is often cited as an important
factor in invasive species establishment (Hobbs and Humphries 1995,
Rodgers and Parker 2003). Disturbance from human activities may enhance
establishment of invasive species by reducing competition from native species,
altering surface texture and microclimate, and increasing availability of
resources such as light, water, and nutrients (Davis et al. 2000, Greenberg et
al. 1997). Additionally, propagule pressure may be increased where humans
transport seeds and vegetative fragments on vehicles, in contaminated soil,
or on equipment (Greenberg et al. 1997, Hodkinson and Thompson 2002).
Cogongrass establishment into new areas has been linked to rhizomes that
are moved by land-management activities, such as disking, mowing, and
transport of contaminated soil (Shilling et al. 1997). These disturbances also
may facilitate seedling establishment. For example, Shilling et al. (1997)
reported enhanced establishment of Cogongrass seedlings when Paspalum
notatum Fluegge (Bahiagrass) cover was reduced. Areas of more frequent
rights-of-way maintenance operations (e.g., intersections and interstate
interchanges), were more likely to be infested with Cogongrass along roadsides
than areas subject to less maintenance (Patterson et al. 2004).
Eussen (1980) reported that one rhizome of Cogongrass can produce 350
shoots in 6 weeks and cover up to 4 m2 in 11 weeks. At these growth rates,
rhizomatous growth plays an important role regarding localized spread of
Cogongrass. Human activities may increase growth rates and vegetative
spread of cogongrass. For example, growth of Cogongrass seedlings was enhanced
with soil disturbance (tilling) in a coastal wet pine savanna (King and
Grace 2000). However, type and frequency of disturbance may be important.
Disking and mowing Cogongrass have been reported to reduce Cogongrass
biomass, if only temporarily (Gaffney 1996, Johnson 1999, Willard et al.
1996). Repeated deep tillage has even been suggested as an effective method
to control Cogongrass, and Cogongrass is generally not a problem in intensively
tilled crops (Willard et al. 1996).
Cogongrass is widespread on and near the military installation, Camp
Shelby Training Site (CSTS), MS. Control of Cogongrass is considered
essential to protect military training operations, along with Longleaf Pine
2009 L.Y. Yager, J. Jones, and D.L. Miller 697
and other habitats important to federally listed species, such as Gopherus
polyphemus Daudin (Gopher Tortoise). On CSTS, Cogongrass patches (see
Methods for definition) appear to be concentrated in human-created ruderal
areas, such as roadsides and firing points (a firing point is here defined as an
upland area cleared for military training operations), which receive regular
disturbance from human activity. Therefore, it is important to understand
how human activities associated with these areas infl uence spread and
growth of Cogongrass. The first objective of this study was to determine if
frequency and area of Cogongrass patches and rates of patch formation and
growth differed on firing points with different training regimes. If propagule
pressure and other environmental factors were equal across CSTS, then
larger firing points should be more likely to be infested with Cogongrass.
As Cogongrass has become more prevalent near Camp Shelby, firing points
constructed more recently may have had a greater probability of propagule
introduction from equipment or fill dirt. Military tanks which could disturb
the soil and transport rhizomes from one area to another may be another
potential source of infestation. From these considerations, we developed a
working hypothesis that presence, numbers, and area of Cogongrass patches
on firing points were related to date of construction and size of firing points
as well as use of firing points for tank training. We also hypothesized that
changes in numbers and area of Cogongrass patches on firing points was
related to days of use for military training activities involving tracked vehicles
and other equipment that could potentially result in soil disturbance.
The second objective of this study was to determine if frequency of Cogongrass
patches and rate of patch formation and growth differed on maintained
roadsides (those with annual mowing and frequent grading) of gravel roads
compared to roadsides on dirt tracks that were infrequently maintained. We
hypothesized that the more highly maintained graveled roads would be more
likely to be infested with Cogongrass and that rates of vegetative growth of
Cogongrass would be greater along their adjacent roadsides.
Camp Shelby Training Site is located in Perry, Forrest, and George counties,
MS, and at 54,315 ha, is one of the largest National Guard Training installations
in the United States (Yager 2007). Most of the land is owned and managed by
the United States Forest Service, but about 6900 ha are owned and managed
by the Department of Defense, State of Mississippi, or Department of the Army
(Yager 2007). Land is managed for timber production, wildlife, recreation, and
water protection, in addition to military training (Yager 2007).
Camp Shelby is located within the Gulf Coastal Plain Physiographic
Region and within the Longleaf Pine-Bluestem portion of the Longleaf
Pine ecosystem. As a result of past fire regimes and silvicultural practices,
a variety of upland Longleaf, Pinus taeda L. (Loblolly), and Pinus elliottii
Engelm. (Slash) Pine forest associations occur on CSTS (Yager 2007).
Embedded within these forests are isolated areas of disturbance, including
698 Southeastern Naturalist Vol. 8, No. 4
roadsides, military training areas (firing points and ranges), and powerlines,
which have a predominantly herbaceous groundcover and are maintained by
mowing and/or burning. Military training areas have been cleared of trees
and are used for a variety of training purposes. Training areas are mostly
upland sites with well-drained sandy loam soils. Non-native species, such
as Paspalum notatum Fluegge (Bahiagrass), Cynodon dactylon (L.) Pers.
(Bermudagrass), and Lolium perenne L. (Ryegrass), have been planted for
erosion control during construction and to rehabilitate areas of soil disturbance.
Native species, such as Schizachyrium scoparium (Michx.) Nash.
(Little Bluestem), Schizachyrium tenerum Nees (Slender Bluestem), Andropogon
virginicus L. (Broomsedge), Pityopsis graminifolia (Michx.) Nutt.
(Silkgrass), Desmodium spp. (beggar’s tick), and Rhus copallina L. (Winged
Sumac), also occur within these areas (Yager 2007).
Roads of CSTS, which include paved, graveled, and dirt tracks, are variously
maintained and vegetated. Roadsides of paved and graveled roads are
maintained with a predominantly herbaceous groundcover by mowing at least
annually including periodic prescribed fires. These herbaceous roadsides are
about 3 m wide and easily differentiated from adjacent forest or other vegetation
associations. Graveled roads receive grading multiple times per year, as
needed. Dirt tracks are unimproved roads without gravel that receive mowing
and grading less than once a year, if ever. Roadside areas of dirt tracks transition
rapidly into the adjacent vegetation associations and may have substantial
woody vegetation mixed with early successional herbaceous species.
Climate is warm-temperate to subtropical. Mean monthly normal temperatures
are least in January (9 °C) and greatest in July and August (27 °C)
(National Oceanic and Atmospheric Administration 2006). The 30-year mean
annual rainfall at the nearest weather station (Wiggins, MS), is 163 cm/year
(National Oceanic and Atmospheric Administration 2006). Annual rainfall
recorded on Camp Shelby Training Site from 2002−2004 ranged from 179
to 206 cm (Yager 2007). Soils are predominantly silty loams, sandy loams,
and loams, and most are classified as Ultisols or Alfisols (USDA Natural
Resource Conservation Service 1999, USDA Soil Conservation Service and
Forest Service 1979).
Cogongrass is a clonal plant that generally produces closely aggregated
tillers with an extensive rhizome system. For this research, Harrington’s
(1977) definition of a tiller as an erect, lateral grass shoot was used. A
Cogongrass patch was defined as the area of ground occupied by an aggregation
of tillers. Tillers within 2 m of each other were considered part of
one patch. Patches included more than one plant (i.e., one or more genets).
Although rhizomes may extend beyond the tillers of the patch underground,
a patch edge was defined as the outermost surface-occurring tillers along the
perimeter of the patch.
2009 L.Y. Yager, J. Jones, and D.L. Miller 699
Military training activity effects on Cogongrass growth
The following data were recorded for military firing points: 1) area (m2),
2) number and area of Cogongrass patches, 3) soil disturbance from military
use as evidenced by rehabilitation (presence or absence of rows of recently
planted Ryegrass and Bahiagrass), 4) designation for tank use—restricted
or permitted, 5) age class—construction pre- or post-1968, and 6) military
use—number of days a firing point was requested for training involving
tracked vehicles and other equipment that could potentially result in soil
disturbance. Cogongrass patches on extant artillery firing points (n = 99) on
CSTS were mapped with a Trimble Pro−XR GPS unit (Trimble Navigation
Inc., Sunnyvale, CA) during winters of 2002, 2003, and 2004. Evidence of
recent rehabilitation (revegetation of plants after soil disturbance) on firing
points also was recorded during mapping in 2002. A few firing points were
not mapped in each year because of entry restrictions or problems with equipment.
Patches were considered separate if there was more than 2 m distance
between patch edges. Firing-point size and potential to be used for tank training
were determined using the Mississippi Army National Guard (MSARNG)
GIS database. Aerial photography from the MSARNG GIS database was used
to classify firing point construction as pre- or post-1968. Records for military
use were obtained from MSARNG Range Control (2005).
The relationships of Cogongrass presence, numbers of patches, and patch
area on firing-points in 2002 to firing-point age, size, and tank use were examined
using regression analyses appropriate to the type of data (binomial
or continuous data) (Quinn and Keough 2002). For Cogongrass presence
or absence data, logistic multiple regression was used because the data followed
a binomial distribution (Quinn and Keough 2002). A general linear
model was used to perform multiple regression for the continuous data of
total area of Cogongrass (Quinn and Keough 2002).
Changes in numbers of Cogongrass patches on firing points from 2002 to
2003 and 2003 to 2004 were analyzed using a SAS PROC GENMOD poisson
regression for each year (SAS Institute, Inc. 2004). The model used was:
Increase in number of Cogongrass patches = military use + presence
of Cogongrass patches in previous year + area of Cogongrass patches
in previous year.
Data were underdispersed, and therefore quasi-likelihood analysis was determined
to be appropriate (Quinn and Keough 2002). The PSCALE option
was added to the analysis to calculate parameter estimates (SAS Institute,
Inc. 2004). Changes in area of Cogongrass for each of the 2 years were analyzed
using PROC GLM to perform multiple regression (SAS Institute, Inc.
2004). The model used was:
Increase in area of Cogongrass patches = military use + presence of
Cogongrass patches in previous year + area of Cogongrass patches in
Infestation data from previous years were included in all models as a covariate
because existing infestations were a potential source of propagules and growth.
700 Southeastern Naturalist Vol. 8, No. 4
Growth and disturbance from tracked vehicles
In another study on CSTS which examined the effect of patch size on
rates of linear growth of Cogongrass (Yager 2007), baseline size data on 30
Cogongrass patches located on firing points were collected using a S.P. Constructor
50 Total Station GPS unit (Trimble Navigation, Inc., Sunnyvale,
CA) in April 2002, 2003, and 2004. Universal Trans Mercator coordinates
were collected every 30−50 cm along patch edges, and ArcView 3.2 (ESRI,
Inc., Redlands, CA) was used to calculate patch area, distances of 2003
points from the 2002 perimeters of each patch, and distances of 2004 points
from the 2003 perimeters of each patch. Mean linear growth was determined
for each patch by averaging distances measured. During the study period, 3
of the 30 patches showed evidence of soil disturbance from tracked vehicles.
Linear growth rates for the 3 patches with evidence of soil disturbance are
reported because they are examples of effects of soil disturbance on linear
spread of Cogongrass growth. Linear growth rates for Cogongrass patches
without soil disturbance are reported for comparison purposes.
Infestation rates and growth on roads and tracks
To determine if graveled roads or tracks were more likely to exhibit
infestation, thirty 100-m sections of different graveled roads were randomly
selected. For each selected gravel road, an additional 100-m section of track
was selected randomly from the track nearest the selected road section. Occurrence
of Cogongrass infestations from road or track edge to 15 m into
adjacent vegetation association was recorded using a Trimble ProXT GPS
device during January 2003 and March 2004. Chi-square analysis was performed
to evaluate differences in frequency of infestation for graveled roads
compared to tracks (Ott 1988).
To determine differences in vegetative linear growth between these two
road types, 11 patches along roads and 4 patches along tracks were mapped
using a GPS device (S.P. Constructor 50 Total Station) to record points in
30- to 50-cm intervals along the patch edge during April 2002, 2003, and
2004. Cogongrass patches on tracks ranged in size from 18 m2 to 286 m2,
with a mean of 104 m2 and a median of 56 m2. Patches along graveled roads
ranged in size from 24 m2 to 302 m2, with a mean of 113 m2 and a median
of 76 m2. Points were classified as being within adjacent forest or roadside
(mowed area adjacent to road). Roadside patches were selected randomly
from graveled roads which had been surveyed east of Mississippi Highway
29 during 2001. Because tracks had not been as extensively surveyed, track
patches were selected based on knowledge of where they occurred. Areas
adjacent to tracks were not mowed and thus did not have an area that was
clearly defined as roadside. Therefore, for comparison purposes between
the two road types, points collected for patches along tracks were classified
as roadside if they fell within 2.85 m of the track edge. This width equaled
the mean width of roadsides of graveled road patches. ArcView 3.2 was
used to calculate patch area, distances of 2003 points from the 2002 perimeters
of each patch, and distances of 2004 points from the 2003 perimeters
of each patch. Mean linear roadside growth was determined for each patch
2009 L.Y. Yager, J. Jones, and D.L. Miller 701
by averaging distances measured (>8 measurements per patch) within the
roadsize zones. Patch growth data was analyzed using a repeated measures
analysis of variance with year and road type as explanatory variables (Ott
1988; SAS Institute, Inc. 2004). Markers were lost for two of the graveled
road patches during 2002, hence, 2002−03 linear growth analysis only included
9 data from 9 graveled road patches.
Military activity effects on Cogongrass
Infestation rates based on firing-point age and tank use ranged from
36 to 55% (Table 1). Logistic regression did not indicate a relationship
between firing-point age, size, or use by tanks and probability of Cogongrass
infestation (likelihood ratio: χ2
3= 1.91, P = 0.592). The general
linear model also did not indicate a relationship between size of Cogongrass
infestation on firing points and firing-point age, size, or use by
tanks (P = 0.173). Twenty-two new infestations on 14 firing points were
mapped in 2003. In 2004, 21 new infestations were mapped on 16 firing
points. Regression analyses indicated positive relationships between
new infestations with prior infestations in 2003 and with military use in
2004 on firing points (Table 2). Increase in Cogongrass area for 2003 and
2004 on firing points was related positively to area of infestation in the previous
year, but not to number of days of military use (Table 3).
Table 2. Summary of regression analysis for numbers of new Cogongrass infestations on military
firing points on Camp Shelby Training Site, MS in 2003 and 2004 related to independent
variables: Cogongrass surface area in previous year, presence of infestation in previous year,
and days of military use in previous year.
Variable NDFA DDFB Parameter S.E. χ2 P < χ2
Change in number of infestations 2003
Cogongrass area 2002 1 92 -0.73 1.05 0.48 0.489
Infested 2002 1 92 0.31 0.11 7.80 0.005
Days of use 2001 1 92 -0.30 0.03 1.28 0.259
Change in number of infestations 2004
Cogongrass area 2003 1 92 0.05 1.08 0.00 0.960
Infested 2003 1 92 0.15 0.11 1.94 0.164
Days of use in 2002 1 92 0.03 0.01 5.12 0.023
ANominator degrees of freedom.
BDenominator degrees of freedom.
Table 1. Percentage of firing points (n) infested with Cogongrass and mean area infested categorized
by date of construction and tank use on Camp Shelby Training Site, MS in 2002.
Constructed pre-1968 Constructed post-1968
% Mean area % Mean area
Firing points n infested infested (m2) n infested infested (m2)
Designated for tank use 34 53 332 20 45 507
Restricted for tank use 31 55 202 11 36 524
702 Southeastern Naturalist Vol. 8, No. 4
Growth and disturbance from tracked vehicles
Mean linear growth rates for undisturbed patches were 0.54 m (n = 19)
for 2002−03 and 0.90 m (n = 25) for 2003−04, whereas mean linear growth
rates for the two patches that exhibited soil disturbance in 2003−04 were 1.46
and 3.22 m, respectively (Fig. 1). Maximum growth rates recorded for the
undisturbed patches were 0.92 for 2002−03 and 1.51 for 2003−04. Maximum
growth rates for the two disturbed patches were 6.35 and 9.85 m, respectively.
During the 2002−03 period, one patch received significant disturbance from
military equipment, but the site of disturbance was subsequently rehabilitated
by disking and planting with ryegrass. Similar activities followed by rehabilitation
occurred during the 2003−04 period. For this rehabilitated site, the area
occupied by the Cogongrass patch decreased in the first year; and by April
2004, no Cogongrass shoots were observed at the site (Fig. 1). However, a
small amount of regrowth was observed in August 2004.
Infestation rates and growth on roads and tracks
Of the 30 sections surveyed on graveled roads and tracks, frequency of
Cogongrass infestation was 33.3% on graveled roads compared to 13.3% on
tracks. These frequencies did not change from 2003 to 2004. Infestation rates
for the two road types did not differ significantly ( χ2
1,59 =3.17, P < 0.075).
Linear vegetative growth along roadsides also did not differ between the
two road types (F1,24= 2.66, P < 0.116), or years (F1,24 = 2.41, P < 0.133); nor
did road type interact with year (F1,24 = 0.83, P < 0.371) (Table 4).
Firing-point size, construction period (pre- or post-1968), and tank use
were not related to the probability of infestation by Cogongrass. Probability
of infestation of an area is determined by both propagule pressure and biotic
and abiotic conditions occurring within the community (Radosevich et al.
2003, Rejmanek et al. 2005). Propagule pressure is a function of extent of
Table 3. Summary of regression analysis for increase in area of Cogongrass infestations in 2003
and 2004 on military firing points on Camp Shelby Training Site, MS related to independent
variables: Cogongrass area in previous year, presence of infestation in previous year, and days
of military use in previous year.
Variable NDFA DDFB Parameter S.E. t−value P < t
Change in Cogongrass area 2003 (R2 = 0.48)
Cogongrass area 2002 1 89 0.05 0.02 2.08 0.041
Infested 2002 1 89 0.01 <0.01 4.69 <0.001
Days of use 2001 1 89 <0.01 <0.01 1.40 0.165
Change in Cogongrass area 2004 (R2 = 0.41)
Cogongrass area 2003 1 84 0.25 0.07 3.62 <0.001
Infested 2003 1 84 0.01 0.01 1.77 0.081
Days of use 2002 1 84 <0.01 <0.01 1.06 0.291
ANominator degrees of freedom.
BDenominator degrees of freedom.
2009 L.Y. Yager, J. Jones, and D.L. Miller 703
nearby populations and numbers of propagules dispersed into or near the
plant community (Rejmanek et al. 2005). If abiotic and biotic conditions and
propagule pressure do not vary within a site (for example, CSTS), then the
probability of infestation per unit area should be the same across the site.
Under these homogenous conditions, larger areas within the site will have a
greater probability of infestation than smaller areas within the site (e.g.,, if
probability of infestation = 0.1 per m2 then the probability of a 10-m2 area being
infested will be greater than the probability of a 2-m2 area being infested:
Figure 1. Patterns
patches on military
on Camp Shelby
MS from April
which: A) received
during study period,
from equipment in
area of maximum
2003 and 2004,
C) received soil
disturbance from equipment in area of maximum growth during 2003 and 2004 (top
patch) or showed no evidence of soil disturbance (bottom patch), and D) received
soil disturbance followed by disking and planting ryegrass in winter 2002/2003 and
2003/2004. Patch area did decrease for patch shown in D.
Table 4. Annual linear growth (m) of Cogongrass patches on roadsides next to graveled roads
or tracks on Camp Shelby Training Site, MS from April 2002 to April 2003 and April 2003 to
Initial patch sizes (m2) 18−286
n 9 11
Mean (S.E.) linear growth (m/yr) 0.80 (0.09) 0.88 (0.10)
Maximum linear growth (m/yr) 2.11 2.16
Initial patch sizes (m2) 24−302
n 4 4
Mean (S.E.) linear growth (m/yr) 0.48 (0.13) 0.80 (0.17)
Maximum linear growth (m/yr) 1.53 3.48
704 Southeastern Naturalist Vol. 8, No. 4
10 m2 x 0.1 m-2 > 2 m2 x 0. 1 m-2). The lack of a positive relationship between
firing-point size and probability of infestation suggests that at least some of
the factors that affect propagule pressure and Cogongrass establishment vary
among firing points. Results of this study suggest that factors other than construction
period or designation of tank use may be important for explaining
infestation patterns. Land managers may view conditions on military firing
points as fairly homogenous based on their similar topographical locations,
physiognomies, species composition, and land use. However, proximity to
nearby Cogongrass infestations, forestry activities, recreational uses, and
other conditions vary among firing points and may result in increased likelihood
of infestation. Military training includes many activities other than
tank training that may bring in propagules or result in soil disturbance, such
as paladin training and live-fire exercises. Additionally, factors that affect
likelihood of infestation may change annually, or even weekly, as environmental
conditions vary and interact with human use (Davis and Pelsor 2001,
Davis et al. 2000). A more complete analysis of past use and infestation
patterns was not feasible because a complete history of military or other use
on firing points was not available. Future studies could be strengthened if,
in addition to accurate records of actual amounts and types of military use,
records of amounts and locations of ground disturbance from military training,
rehabilitation, or other actions were maintained.
Analysis indicated a positive relationship between new infestations of
Cogongrass in 2004 with days of military use in the previous year, but a
similar relationship was not found for new infestations in 2003. One possible
explanation of these results may be that timing of activities is important.
Activities occurring when soils are wet are more likely to result in ground
disturbance, such as rutting. Disturbance in Cogongrass patches may result
in transport of rhizomes on vehicles, whereas disturbance in uninfested areas
may provide better sites for establishment. Activities occurring during seed
production may be more likely to transport seeds. Thus, under some conditions
related to seasonal infl uences, military training would be more likely
to result in spread of Cogongrass. Another explanation for results observed
may be that certain types of training activities were more likely to result
in ground disturbance or transport of propagules. For instance, although
wheeled vehicles may result in ruts under wet conditions, they would generally
be less likely to result in soil disturbance than tracked vehicles Thus,
data on weather conditions, specific duration and types of training activities,
and amounts of ground disturbance would be helpful for future studies.
Cogongrass growth may be affected by the type of soil disturbance.
One-time tilling increased growth of Cogongrass seedlings in a Mississippi
fl atwood site (King and Grace 2000). However, other studies indicated that
disking reduced Cogongrass biomass (Gaffney 1996, Johnson 1999, Willard
et al. 1996). Two possible explanations for this result were proposed:
1) exposure of rhizomes by disking led to rhizome dessication, or 2) rhizome
fragmentations reduced available carbohydrates for growth (Gaffney
2009 L.Y. Yager, J. Jones, and D.L. Miller 705
1996, Johnson 1999, Willard et al. 1996). Observations of linear growth
from 3 Cogongrass patches, each of which received disturbance from military
equipment within and adjacent to the patch at 2 unrehabilitated and 1
rehabilitated site, also suggested that type of soil disturbance may result in
different growth patterns. For the 2 Cogongrass patches at the unrehabilitated
site where much of the patch remained intact but surface vegetation
adjacent to the patch was removed, one-time soil disturbance appeared to
enhance Cogongrass growth in the area of disturbance. This effect may
have been because of reduced competition from surrounding vegetation,
along with the remaining intact shoot and rhizome systems benefitting from
increased resources in the disturbed area. It should be noted, however, that
if the response observed was a result of the disturbance, then other types of
equipment used for logging, rehabilitation, or rights-of-way maintenance
could have a similar effect. For the patch where rehabilitation occurred following
initial disturbance, Cogongrass growth appeared to be impeded and
total area covered by the patch was reduced. Rehabilitation included disking
the entire patch and surrounding area, followed by planting ryegrass. In
this instance, rehabilitation would have fragmented the rhizome system and
killed shoots, while offering increased competition from ryegrass. Revegetation,
with species such as ryegrass, may play an important role in integrated
management programs for certain invasive species (Miller 2003).
Greenberg et al. (1997) reported greater exotic species richness and cover
on roads with greater disturbance (vegetation removal and soil fill, versus
vegetation removal only) in Ocala National Forest, FL. However, exotic
species richness was similar on main roads and backcountry trails in Glacier
National Park (Tyser and Worley 1992). These differing results suggest establishment
of a given exotic species is affected by its dispersal mechanisms
and biological characteristics, and also the amount and type of the particular
disturbance (Parendes and Jones 2000, Tyser and Worley 1992). Patterson
et al. (2004) reported greater probability of Cogongrass infestation in areas
adjacent to highways which were subject to more maintenance and disturbance.
Parendes and Jones (2000) observed greater frequencies of several
exotic species on roads receiving high road traffic and road maintenance
activities compared to roads without severe disturbance, but this relationship
was more apparent for species with frequencies >30% for at least one
road type. Thus, differences between infestation frequencies on gravel roads
and dirt tracks on Camp Shelby may not have been statistically significant
because of the low infestation frequencies observed at the time. If so, differences
may become more pronounced through time and with advancing
infestation. Alternatively, the lack of difference may indicate that the level
of roadside disturbance and transport of propagules was not sufficiently
different between road types to affect Cogongrass establishment. Level of
maintenance may be important for establishment of Cogongrass on paved
highways in Alabama with presumably different habitat features and humaninitiated
disturbances (Patterson et al. 2004), but less so for the unpaved
706 Southeastern Naturalist Vol. 8, No. 4
roads on Camp Shelby. Annual mowing may not increase spread if it does
not occur during the seed-production period or disturb the soil to transport
rhizome fragments. Mowing which does not disturb the soil or occur during
seed production would not likely transport propagules, including seeds or
rhizome fragments. During the past several years, mowing had been restricted
on CSTS if fruiting infl orescences were visible. On CSTS, road grading
also may be less likely to transport propagules because grading operations
occurred primarily within the gravel roadbed proper and gravel roadbeds
did not appear to be susceptible to Cogongrass encroachment. However,
grading through Cogongrass patch edges may sometimes facilitate transport
of rhizomes to previously uninfested roadside areas. Roads on CSTS also
were presumably subject to less vehicular traffic than highways, which may
reduce wind-dispersal of seeds.
Linear growth rates also did not vary between gravel roadsides and tracks.
Although infrequent mowing has been shown to reduce shoot and root biomass
(Willard 1988, Willard et al. 1996), results of this study indicate that
mowing may have little effect on overall area occupied by the Cogongrass
plant or its rhizomatous spread. These results further suggest that a factor
other than grading may be important in explaining rhizomatous growth along
roadsides. One difference should be noted for Cogongrass growth between the
two road types. Cogongrass was able to spread across dirt tracks via rhizomatous
growth, whereas gravel roadbeds served as a barrier to rhizome growth,
possibly because of greater compaction, grading, or other activities.
Rates and type of human disturbance and activity can affect spread of
invasive species; however, maintenance or traffic levels did not appear to affect
rates of Cogongrass spread and growth on unpaved roads on CSTS. This
finding may have been due, in part, to precautionary measures that have been
implemented to slow spread of Cogongrass on CSTS. Military activity rates
and resulting soil disturbance did appear to infl uence spread of Cogongrass
on CSTS. This effect was not strong for infestation rates and only evident for
rhizomatous growth when equipment disturbed soil adjacent to and within
patches. These results suggest that measures to prevent soil disturbance in
and near patches may assist managers in minimizing the spread of Cogongrass.
Precautionary measures now implemented include, but are not limited
to: education of staff and contractors regarding identification of Cogongrass,
posting warning signs around Cogongrass patches on military training areas,
and fl agging Cogongrass patches on roadsides.
We thank the Mississippi Army National Guard, The Nature Conservancy, and the
US Forest Service for their funding and support. We also thank the Mississippi Museum
of Natural Science for allowing Y. Yager time to write this manuscript. We appreciate
the assistance of Bob Lemire, Colleen Heise, Brian Mitchell, Robin Switzer, and
C.J. Sabette for their contributions to this work. We also thank Brett Serviss and two
anonymous reviewers for their comments and suggestions which improved this paper.
2009 L.Y. Yager, J. Jones, and D.L. Miller 707
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