The Influence of Management Regimes and Habitat
Characteristics on the Persistence and Current Occupancy
of the Non-native Melinis repens (Natalgrass)
Kathryn E. Tisshaw and Eric S. Menges
Southeastern Naturalist, Volume 17, Issue 4 (2018): 654–670
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2018 SOUTHEASTERN NATURALIST 17(4):654–670
The Influence of Management Regimes and Habitat
Characteristics on the Persistence and Current Occupancy
of the Non-native Melinis repens (Natalgrass)
Kathryn E. Tisshaw1,* and Eric S. Menges2
Abstract - Although prescribed fires and pre-treatments (e.g., roller chopping and mowing)
are used by public and private landowners to manage natural habitats in Florida, they
can influence the invasion and spread of non-native plants in natural areas. Firelanes and
roads used to access habitat for management practices create corridors for invasive grasses.
Melinis repens (Natalgrass) is an invasive plant found throughout Florida. Fire regimes
and roads acting as corridors may affect invasion and persistence of Natalgrass, but these
topics have not been well-studied. Following up on distribution data originally collected
in 2002 at Archbold Biological Station, we explored how fire regimes, distance to road,
habitat type, and microhabitat factors influenced Natalgrass persistence through 2016, and
current Natalgrass occupancy. Persistence from 2002–2016 was not influenced by distance
to road. However, Natalgrass was currently more likely to occupy habitat closest to roads
and was more likely to persist in areas burned within 16 y. Although Natalgrass was most
likely to persist in human modified habitat, it still persisted in and occupied interior scrub
habitat. Natalgrass was more likely to occupy areas with lower litter, shrub, and palmetto
cover, which are characteristics of many habitats, including sandy roadsides and recently
burned scrub habitat. These results suggest Natalgrass is able to persist in habitats other than
roads, and distance to road did not influence its persistence; thus, land managers should treat
interior habitat where Natalgrass is persisting. At the same time, searches for new populations
of Natalgrass should be focused largely in areas close to corridors, such as in roads
and firelanes.
Introduction
Prescribed fires are used as a management practice in a wide variety of habitats
and regions to restore vegetation structure, reduce fuels, and enhance wildlife
habitat where certain endangered or threatened species require fire (D’Antonio
and Vitousek 1992, Elgersma et al. 2017, Long et al. 2004, Wade et al. 1980). Fire
severity, fire frequency, and time-since-fire can vary among vegetation types and
within the same fire or vegetation type, and can influence post-burn vegetation
patterns (D’Antonio and Vitousek 1992, Flannigan et al. 2013, Long et al. 2004).
Prescribed fire has been used extensively in Florida for many years (Boughton et al.
2016, Mitchell et al. 2006, Wade et al. 1980), but urbanization and concerns about
control and smoke have led land managers to use roller-chopping and mowing,
which allow for more effective carry and easier control during a prescribed burn
1Department of Environmental and Life Sciences, Trent University, 1600 West Bank Drive,
Peterborough, Ontario K9J 0G2, Canada. 2Plant Ecology Program, Archbold Biological
Station, 123 Main Drive, Venus, FL 33960. *Corresponding author - ktisshaw@trentu.ca.
Manuscript Editor: Matthew Heard
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(Menges and Gordon 2010), as pre-treatments or an alternative to prescribed fires.
Although these practices are employed to manage natural habitat, they can act as
disturbances that increase the risk of non-native plant invasions into natural areas
(Hobbs 1991, Hobbs and Huenneke 1992, Long et al. 2004).
In order to manage pyrogenic habitats using prescribed fires and pre-treatments,
roads or firelanes are used to access protected habitat and provide firebreaks. Roads
and firelanes act as corridors for non-native plants to colonize open areas and gaps
of natural habitats (Wace 1977). These corridors may increase the risk of non-native
invasive plants entering natural habitat (Gelbard and Belnap 2003, Hansen and
Clevenger 2005, Yates et al. 2004), and lead to a loss of native species (Tomimatsu
and Ohara 2004, Turner 1996). In general, non-native grasses are often common
along roads because of conditions such as increased nutrient availability, reduced
competitive pressure, stimulation of seed germination, and increased seed dispersal
(D’Antonio 1993, Forman and Alexander 1998). Consequently, active management
can have unintended consequences on the distribution of non-native invasive plant
species in natural areas.
Prescribed fires and pre-treatments can increase the risk of non-native plant invasions
through soil disturbance and reduction of competition (Catling et al. 2002,
Dodson and Fiedler 2006, Hobbs and Atkins 2006, Lonsdale 1999). A disadvantage
to using pre-treatments is that they create more soil disturbance than using prescribed
fires alone (Menges and Gordon 2010). Some non-native plants have been
observed to increase in abundance following fire (D’Antonio and Vitousek 1992,
Just et al. 2017, Lippincott 2000) and following soil disturbances (Catling et al.
2002, Dodson and Fiedler 2006). For instance, Imperata cylindrica (L.) P. Beauv.
(Cogongrass) is a non-native grass found throughout Florida that is more likely
to invade natural habitat when soil disturbance has occurred (Lippincott 2000,
Schmalzer and Foster 2016). Cogongrass is able to colonize areas after prescribed
fire and, once established, increases fuel load, fire severity, and Pinus (pine) mortality
(Lippincott 2000, Matlack 2002).
Melinis repens (Willd.) Zizka (Natalgrass) is a grass native to Africa that has
become a problematic invasive species in many areas around the world, including
Florida (Stokes et al. 2011). Its approximate date of arrival in Florida was in 1893
(Scott 1913). Natalgrass is a Category I invasive plant which can displace native
plants and alter community structure (FLEPPC 2015). It is a short-lived perennial
C4 grass which is primarily wind-dispersed (Possley and Maschinski 2006).
Natalgrass is one of the few grasses observed to invade scrub habitat (Gordon
et al. 2005, Hutchinson and Menges 2006). The optimal habitat of Natalgrass is
along roads and in disturbed areas (David and Menges 2011, Gordon et al. 2005);
however, it is unknown if these sites can serve as continuous source populations
to disperse seed into adjacent scrub habitat that has been recently disturbed or
burned. In some Florida habitats, Natalgrass has been observed to increase in density
after a prescribed burn, especially when accompanied with soil disturbance
(Hutchinson and Menges 2006, Williges et al. 2006). David and Menges (2011)
speculated that fire may influence Natalgrass distribution through creating open
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habitat and decreasing competitive effects of shrubs. Despite this research, no
one has quantified the influence of fire regimes on Natalgrass distribution, the
conditions under which Natalgrass can persist, and whether roads and firelanes
act as corridors for Natalgrass invasion or persistence.
In this paper, we examine how effects of land management, such as distance
to roads or firelanes, time-since-fire, fire severity, and historical fire frequency
influenced the persistence of Natalgrass from 2002 to 2016, and how persistence
differs from current Natalgrass occupancy (hereafter current occupancy). We
also examine how habitat type, vegetation cover, and gap cover influence current
occupancy. Undisturbed Florida scrub seems resistant to most plant invasions
(Greenberg et al. 1997); thus, we predicted that Natalgrass would persist only
in road habitats and that it would have the greatest persistence and current occupancy
closest to roads. Since some non-native invasive grass species are able
to colonize areas after prescribed fires, we predicted that Natalgrass would have
greater persistence and current occupancy in recently burned management units,
those that have been burned with the highest severity, and those that have been
burned most frequently. We predicted that Natalgrass would persist more frequently
in human-modified areas, pastures, and disturbed scrub habitat than in
other interior scrub habitats. Certain non-native invasive species can take advantage
of areas where competition is reduced, so we predicted that Natalgrass would
occupy open areas with characteristically low litter-cover, high bare-sand cover,
and low shrub and palmetto cover.
In order to test these predictions, in 2016, we conducted a resurvey of the distribution
of Natalgrass, previously surveyed in 2002, to determine how its persistence
was affected by management regimes. In addition, we surveyed random road transects
in different habitats in 2016 to determine how current Natalgrass occupancy
is affected by management regimes and microhabitat characteristics.
Field Site Description
We conducted this study at the Archbold Biological Station (ABS; Swain
1998) in Lake Placid, Highlands County, FL (27º11'N, 81º21'W). ABS is one of
the largest remaining tracts of the Lake Wales Ridge ecosystems (Weekley et al.
2008), and includes over 2100 ha of southern ridge sandhill, Florida scrub, flatwoods,
and seasonal-pond habitats (Abrahamson et al. 1984). For the purpose of
this study, we categorized the habitat types at ABS into xeric yellow sands, flatwoods,
scrubby flatwoods/Ceratiola ericoides Michx. (Florida Rosemary) scrub,
human-modified areas, and roads. Xeric yellow sands combines the Abrahamson
et al. (1984) categories of Quercus (oak)-Carya (hickory) scrub and sandhill.
Scrubby flatwoods/rosemary combines scrubby flatwoods, Pinus clausa (Chapman
ex Engelmann) Vasey ex Sargent (Sand Pine) scrub, and rosemary scrub
(Abrahamson et al. 1984). Human-modified areas includes old-field habitats.
Roads includes sandy roadsides adjacent to interior habitat. We also sampled
invasive species in the Archbold Reserve (a former ranchland; hereafter the
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Reserve). Reserve habitat types were categorized into either pasture or disturbed
scrub with a history of mechanical disturbance.
Methods
Data collection (persistence)
To evaluate persistence, we resurveyed 220 plots where Natalgrass was found
within ABS and the Reserve in 2002 and 2003 (Hutchinson and Menges 2006).
These plots included interior habitats, >2.5 m from the vegetation edge along roads.
This distance is beyond the range of edge effects in vegetation (E. Menges, unpubl.
data). These plots were in management units with various habitat types, fire histories
(Menges et al. 2017), and distances to roads. We navigated to these plots using
the 2002 coordinates and a Trimble GeoXT GPS unit with submeter accuracy. We
recorded whether Natalgrass was present or absent in 2.5-m–radius circular plots
centered on the GPS point. We did not record data at 20 of these plots, which were
ponded or within new development.
Data collection (current occupancy)
We used a stratified random-transect design (Holly and Hampton 1990) to evaluate
current occupancy of Natalgrass in interior habitats. We randomly generated
32 transects among 10 different management units with varying fire histories at
ABS, beginning at the vegetation edge along unpaved, sandy roads or firelanes, and
running perpendicular to the road into adjacent interior habitat. Using a randomtransect
generator tool in ArcGIS, we created 30-m transects with one 5-m plot
randomly established within each of three 10-m intervals: within 10 m from the
road, 10–20 m from the road, and 20–30 m from the road (resulting in 3 plots per
transect, and 96 plots total). Using a Trimble GeoXT GPS unit, we navigated to the
coordinates of the start of the transects and placed 30-m transects by hand using
a transect tape and the predetermined coordinates for plot intervals as reference
to ensure a perpendicular transect from the road into interior habitat. Following
establishment of the 5-m circular plots, we recorded whether Natalgrass was present
or absent within each plot, and collected microhabitat data including relative
percent cover of shrubs (Serenoa repens (W. Bartram) Small [Saw Palmetto] and
Sabal etonia Swingle ex Nash [Scrub Palmetto]) and gaps using the line-intercept
method (Canfield 1941). We defined gaps as areas without woody or herbaceous
plant-cover, but included ground lichens, bare sand, and litter, and which intercepted
more than 10 cm along the transect line.
Statistical analysis
We obtained data on time since last fire (years), fire frequency (number of times
an area burned since 1967), fire severity of the most recent fire: never burned,
patchy (low severity), scorched (intermediate severity), or consumed (high severity),
and habitat type (defined above under Field Site Description from a 5 x
5-m–grid database [Menges et al. 2017]). In ArcGIS 10.3.1, we measured distances
to the nearest road or firelane in meters using shapefiles of habitat types and road
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boundaries. We measured from the vegetation edge of the nearest road or firelane
to the edge of the plot.
We employed generalized linear-model logistic regressions for model formation
with multiple predictor variables to ascertain the effects of distance to road,
fire-regime variables, and habitat type on the persistence and current occupancy of
Natalgrass, and microhabitat data on the current occupancy of Natalgrass. We used
2 models to analyze (1) persistence, and (2) occupancy as binary response-variables
against the predictors: distance to road, time-since-fire, fire severity, fire frequency,
and habitat type (Table 1: m1 and m2, respectively). Using backwards stepwise
procedures, we determined which predictors were the most import ant in determining
Natalgrass persistence and occupancy. We used a binomial-regression model to
test the effects of microhabitat variables on the current occupancy of Natalgrass in
interior native habitat (Table 1: m3).
We removed from the models observations that did not have fire-history data.
We used Bartlett tests to assure homogeneity of variances among predictors within
models. In m2, distance to road did not meet the assumption of homogeneity of
variance; therefore we used a non-parametric function for this predictor within
a generalized additive model using the gam package in R v 3.4.4 (Hastie 2018).
There were no predictors with correlations exceeding 0.5 (Pearson correlation coefficient),
therefore all predictors stayed in the model before stepwise procedures.
We employed the Wald statistic to test the significance of the regression variables,
wherein levels of factors were compared to 1 level assigned as the reference
category. We ran the chi-square goodness-of-fit test (χ2) to assess the model goodness-
of-fit. Details on the model diagnostics are outlined in Table 1. We conducted
all analyses in R v.3.4.4 within R studio v.1.1.383 (R Core Team 2017; RStudio
Team 2016).
Results
Distance to road and fire-regime effects on Natalgrass
The effects of distance to roads on persistence and current occupancy of
Natalgrass were variable. Although persistence was not affected by distance
to roads (P = 0.811; Table 2; Fig. 1A), current occupancy of Natalgrass decreased
significantly with distance to roads (P = 0.002; Table 3; Fig. 1B).
Natalgrass persistence and current occupancy were affected by the fire regime.
Persistence of Natalgrass was significantly influenced by time-since-fire (P = 0.036;
Table 1. Model diagnostics including: the chi-square goodness-of-fit test statistic (χ2), degrees of freedom
(df), and the significance of the fitted models (P > 0.5 indicates the fitted model does not differ
significantly from our observed values).
Model
name Model (y ~ x1 + x2 + x3 + x4) χ2 df P
m1 Persistence ~ time-since-fire + fire severity + habitat type 68.418 13 0.968
m2 Current occupancy ~ distance to road + fire frequency 9.405 8 1.000
m3 Current occupancy ~ bare sand + litter + shrub + palmetto 84.448 4 0.617
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Figure 1. The (A) persistence or (B) current occupancy of Melinis repens (Natalgrass) in
relation to distance to road in meters.
Table 2. Binary logistic regression predicting the likelihood of persistence of Natalgrass from 2002
to 2016 based on distance to roads, time-since-fire, fire severity, fire frequency, and habitat type (n =
105). Stepwise procedures resulted in the best model (m1) including the predictor’s time-since-fire,
fire severity, and habitat type. Details on the influence of distance to road and fire frequency before
being removed from the full model with all explanatory variables are also included. Factor levels in
parentheses were reference conditions in regressions. * denotes significance at P < 0.05; ** denotes
significance at P < 0.01.
95% CI for
odds ratio
Variables (m1) n z-value SE Wald df P Odds ratio Lower Upper
Time-since-fire
(3–4 y) 18 2.680 4 0.0360*
0–2 y 58 -1.724 0.519 1 0.0847 0.166 0.017 1.174
5–8 y 10 -0.570 0.411 1 0.5690 0.452 0.024 6.580
9–16 y 7 -0.003 0.372 1 0.0144* 0.026 0.001 0.467
17–34 y 12 -1.300 0.415 1 0.1940 0.185 0.012 2.211
Fire severity
High (consumed) 51 3.815 2 0.0256*
Low (patchy) 44 1.712 0.354 1 0.0870 3.394 0.869 14.967
Intermediate (scorched) 10 2.589 0.384 1 0.0096** 29.126 3.046 726.940
Habitat type
Flatwoods 27 2.748 5 0.0232*
Human modified 31 3.094 0.362 1 0.0020** 11.475 2.615 60.046
Pasture 7 0.005 995.235 1 0.9960 1.79e+9 0.000 .
Xeric yellow sands 16 -0.659 0.364 1 0.5090 0.514 0.053 3.243
Scrubby flatwoods/ 15 -0.008 890.556 1 0.9940 0.000 . 2.45e+65
rosemary
Disturbed scrub 9 1.626 0.271 1 0.1040 4.798 0.730 34.635
Variables excluded from m1
Distance to road 105 0.002 0.007 0.057 1 0.8110 137.938 3.624 5250.464
Fire frequency 105 1.308 1.358 0.927 1 0.3360 3.698 0.258 52.940
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Table 2). Compared to the 3–4 y-since-fire category, Natalgrass persistence was
significantly less likely in areas burned 9–16 y ago (P = 0.014), and marginally significantly
less likely in areas burned 0–2 y ago (P = 0.085). Natalgrass persistence
in areas that were burned 5–8 y ago and 17–34 y ago were not significantly different
than areas that were burned 3–4 y ago (P = 0.569 and P = 0.194, respectively). Persistence
of Natalgrass was also affected by fire severity (P = 0.026; Table 2; Fig. 2a).
Compared to high-severity patches where Natalgrass persisted only 41% of the time,
Natalgrass persistence in intermediate-intensity patches was significantly higher
(80%; P = 0.0096). Natalgrass persistence in low-severity patches was marginally
higher (~48%) than in high-severity patches (P = 0.087). On the other hand, current
occupancy of Natalgrass was not influenced by time-since-fire (P = 0.373; Table 3) or
fire severity (P = 0.938; Table 3; Fig. 2B). Fire frequency since 1967 was a significant
predictor of Natalgrass occupancy (P = 0.038; Table 3) but not a significant predictor
of Natalgrass persistence (P = 0.336; Table 2). Natalgrass occupied plots burned
twice 42% of the time, while it only occupied plots burned 3 times or more 11% of the
time (P = 0.035).
Habitat type effects on Natalgrass
Habitat type affected persistence (P = 0.0232; Table 2; Fig. 3A) but not current
occupancy (P = 0.979, Table 3; Fig. 3B) of Natalgrass. Compared to flatwoods
habitats where Natalgrass persisted only 26% of the time, Natalgrass persisted in
human-modified habitats 84% of the time. According to the model, Natalgrass persistence
in pasture, xeric yellow sands, scrubby flatwoods/rosemary, and disturbed
scrub was not significantly different from flatwoods habitats (P = 0.996, P = 0.509,
P = 0.994, and P = 0.104, respectively). However, in pastures and scrubby flatwoods/
rosemary plots, Natalgrass persisted 100% and 0%, respectively (Fig. 3A).
Therefore, there was no variation in which the model could prov ide an estimate.
Table 3. Binary logistic regression predicting the likelihood of current occupancy of Natalgrass based
on distance to roads, time-since-fire, fire severity, fire frequency, and habitat type (n = 79). Stepwise
procedures resulted in the best model (m2) including the predictors distance to road and fire frequency.
Details on the influence of time-since-fire, fire severity, and habitat type before being removed from
the full model with all explanatory variables are also included. Factor levels in parentheses were reference
conditions in regressions. * denotes significance at P < 0.05; ** denotes significance at P < 0.01.
95% CI for
odds ratio
Variables (m2) n SE Wald df P Odds ratio Lower Upper
Distance to road 79 0.085 10.109 1 0.002** 0.763 0.617 0.873
Fire frequency (burned twice) 43 4.436 1 0.038*
Burned ≥3 37 1.449 1 0.035* 0.047 0.001 0.525
Variables excluded from m2
Time-since-fire 79 5.365 5 0.373
Fire severity 79 0.128 2 0.938
Habitat type 79 0.194 3 0.979
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Figure 2. Bar graphs depicting the (A) persistence or (B) current occupancy of Melinis repens
(Natalgrass) in relation to categorical fire severity (never burned, low, intermediate or
high severity). Proportions are based on averages of Natalgrass either being present (value
of 1) or absent (value of 0) among plots with varying fire severity categories. An average
value of 1 would signify that Natalgrass is always present, and an average value of 0 would
signify that Natalgrass is always absent. Reference categories are dark grey, and significant
differences from reference categories are denoted by an asterisk ( *) for P < 0.05.
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Figure 3. Bar graphs depicting the (a) persistence or (b) current occupancy of Melinis
repens (Natalgrass) in relation to habitat type categories (either pasture, human modified,
roads, disturbed scrub, xeric yellow sands, flatwoods, and scrubby flatwoods/rosemary).
Proportions are based on averages of Natalgrass either being present (value of 1) or absent
(value of 0) among plots with varying habitat types). An average value of 1 would signify
that Natalgrass is always present, and an average value of 0 would signify that Natalgrass
is always absent. Natalgrass was always absent from occurring in flatwoods. Reference
categories are dark grey, and significant differences from reference categories are denoted
by an asterisk (*) for P < 0.05.
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Table 4. Binary logistic regression predicting the likelihood of current occupancy of Natalgrass based
on relative percent cover of bare sand, litter, shrub, and palmetto. We used absent as the reference
category for the regressions (n = 94). There were no variables taken out of m3.
Average % cover
where Natalgrass 95% CI
Variables was absent or present for odds ratio
(m3) Absent Present SE Wald df P Odds ratio Lower Upper
Bare sand 5.078 10.356 0.017 1.663 1 0.197 0.978 0.945 1.012
Litter 18.245 11.669 0.018 8.846 1 0.003** 0.947 0.913 0.982
Shrub 31.205 19.921 0.017 10.213 1 0.001** 0.948 0.918 0.980
Palmetto 18.997 12.626 0.024 8.028 1 0.005** 0.933 0.889 0.979
Microhabitat effects on Natalgrass
Microhabitat affected Natalgrass current occupancy and cover. Natalgrass presence
was associated with significantly lower litter-cover than plots where it was
absent (12% vs. 18%; P = 0.003; Table 4). Natalgrass presence was 5% higher in
areas with bare sand cover, although this difference was not significant (P = 0.197;
Table 4). Plots where Natalgrass was present had 11% less shrub cover and 6% less
palmetto cover than plots where it was absent (P = 0.001, P = 0.005, respectively;
Table 4).
Discussion
In this study, we examined how the persistence and current occupancy of Natalgrass
were affected by management regimes, habitat types, and microhabitat
factors. Distance to roads did not influence the persistence of Natalgrass; however,
current occupancy of Natalgrass was lower farther from roads. We found that Natalgrass
persisted more often in areas that were recently burned with intermediate
severity and in human-modified habitats. Natalgrass was able to persist with lower
probabilities in xeric yellow sands, flatwoods, and disturbed scrub habitats. We
also found new populations of Natalgrass occupying xeric yellow sands, scrubby
flatwoods, and rosemary scrub habitats. Natalgrass preferred microhabitats with
low litter-, shrub-, and palmetto-cover.
Influence of distance to roads on persistence and current occupancy
Distance to roads had dissimilar effects on the persistence and current occupancy
of Natalgrass. Persistence of Natalgrass was not influenced by distance to
roads, implying that Natalgrass is able to persist once it has established no matter
what distance it is to the roadside source-population. In contrast, current occupancy
of Natalgrass was greatest at shorter distances to road, a result consistent with
the findings of David and Menges (2011). Although Natalgrass persists no matter
the distance to roads, it still exhibits patterns similar to those of other non-native
invasive grasses that are more likely to invade and occupy habitat closest to roads
(Barbosa et al. 2010, Christen and Matlack 2009, Spellerberg 1998). Natalgrass is
able to persist in interior habitats; thus, land managers should treat Natalgrass in all
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interior habitats (>2.5 m from vegetation edge). Managers should prioritize greater
search effort for new interior populations of Natalgrass near roads because the species
had greater current occupancy closer to roads.
Influence of fire regimes on Natalgrass
Natalgrass persisted more frequently in areas that were last burned 0–16 y ago,
consistent with our prediction that it would persist more frequently in recently
burned units. The 17–34 y-since-fire category, where persistence was lower, is the
only category that included areas that had not been burned since the 2002 survey
was conducted (~15 y ago). These results suggest that Natalgrass could persist in
an area for up to 16 y in the absence of fire. Other studies have shown that certain
grasses persist for many years after fire (Lippincott et al. 2001). For example, cover
of non-native invasive Urochloa maxima (Jacq.) R. (Guineagrass) was 6 times
higher in burned plots compared to unburned plots 3 years after a fire (Veldman et
al. 2009). Alien grasses in Hawaii increased in abundance following a fire compared
to pre-burn levels, and they maintained high cover-levels even 18 y after the fire
(Hughes et al. 1991). Although, in our study, current Natalgrass occupancy did not
depend on time-since-fire, and Natalgrass did not occur more frequently in recently
burned areas, Natalgrass sometimes invades areas shortly after fire (Hutchinson and
Menges 2006, Williges et al. 2006). Duration of typical Natalgrass persistence after
a recent invasion is not known, but our results suggest that once established, Natalgrass
patches are capable of persisting for many years. This finding again suggests
that interior populations of Natalgrass need to be treated aggr essively.
In other circumstances, prescribed burning has been effective in managing invasive
plants. Prescribed burning to reduce Poa pratensis L. (Kentucky Bluegrass) invasions
has been effective (Abrams 1988, Becker 1989, McMurphy and Anderson
1965) and has even stimulated populations of native grasses such as Schizachyrium
scoparium Michx. (Little Bluestem) and Andropogon gerardii Vitman (Big
Bluestem) (Sverdarsky et al. 1986, Towne and Owensby 1984). Populations of
the invasive grass Bromus inermus Leyss. (Smooth Brome) were reduced after
repeated prescribed burns (Willson and Stubbendieck 1996). These contrasting
findings on the effects of fire on non-native grass distribution emphasize the need
for land managers to understand which grasses in the ecosystem are controlled by
or benefit from the use of prescribed fires. Eradicating or reducing invasive grass
populations before prescribed burns to prevent subsequent post-fire invasion would
be an efficient management strategy.
Our results showed the highest persistence of Natalgrass in intermediate burnseverity
patches, while other studies have found mixed results varying from no
effects of burn severity on the persistence of non-native species (Kuenzi et al.
2008), to high proportions of non-native species compared to native species associated
with high-severity burns (Crawford et al. 2001, Fornwalt et al. 2010,
Griffis et al. 2001, Turner et al. 1997). Studies that found an association between
high occurrences of non-native species and high-severity burns attributed their
findings to non-native species that resprouted after fire (Turner et al. 1997),
dispersed into recently burned areas (Fornwalt et al. 2010), or were intentionally
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seeded into the area (Kuenzi et al. 2008). Collectively, the findings from these
studies are inconclusive, suggesting that post-burn vegetation patterns following
fires of varying fire severity are influenced by the interplay of past management
practices, species composition pre-burn, dispersal abilities of neighboring
individuals, and resprouting ability of individuals within burn patches. Our observations
suggest Natalgrass is often killed by fire, in contrast with grasses that can
reproduce asexually from belowground rhizomes that are not destroyed by fire
(e.g., Cogongrass; Dozier et al. 1998). Once Natalgrass has been consumed by a
high-severity fire, it would need to recolonize from neighboring Natalgrass seed
sources or surviving seeds in the soil. It is unlikely that Natalgrass would resprout
once consumed by a high-intensity fire, which may contribute to its greater persistence
in patches burned with intermediate severity.
Habitat preferences of Natalgrass
Our prediction that Natalgrass would persist more often in human-modified areas
than in interior scrub habitats was supported. However, Natalgrass was still able to
persist in and occupy interior scrub habitats. Propagule pressure along roads may
help explain the presence of Natalgrass in interior scrub habitats (Colautti et al.
2006). Future studies should focus on measuring the density of road populations and
monitoring dispersal of Natalgrass into adjacent interior habitat. Researchers should
investigate how long populations of Natalgrass persist once established in natural
habitats, and whether the road populations can act as continual sources for dispersing
seeds into interior habitats. Land managers should focus on eradicating large patches
of Natalgrass along or near roads and disturbed areas.
Microhabitat preferences of Natalgrass
Our results supported most of our predictions for microhabitat preferences of
Natalgrass. As predicted, Natalgrass was less likely to occupy areas with high
litter-, shrub-, or palmetto- cover. David and Menges (2011) studied microhabitat
preferences of Natalgrass and found that Natalgrass biomass decreased with
increasing litter volume, which is analogous to the results of our study in that Natalgrass
was more frequently present in areas with lower litter-cover. They also found
that Natalgrass presence significantly increased with greater distance to shrubs
(David and Menges 2011), which is comparable to our results in that Natalgrass is
more likely found in plots with low shrub-cover. Bare sand had no significant effect
on current occupancy of Natalgrass in our study, although the pattern of more
Natalgrass occupancy with bare sand was consistent with our predictions. David
and Menges (2011) found that Natalgrass was less microhabitat-limited on roads
than interior scrubby flatwoods habitats, and attributed this effect to the openness of
sandy roads. In our study, bare sand did not have a significant effect on Natalgrass
presence, although Natalgrass occurred more frequently on roads. Our contrasting
results might be explained by Natalgrass persisting in a range of interior habitat
types with variable bare sand cover in our study, since David and Menges (2011)
only studied scrubby flatwoods interior habitats, which have little bare sand (Dee
and Menges 2014, Young and Menges 1999).
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2018 Vol. 17, No. 4
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Management and research implications
To protect native habitat from the invasion of Natalgrass, land managers should
reduce the total area of firelanes and roads along borders of management units when
possible. These areas may be constant sources of invasion for Natalgrass and other
invasive species, and can affect invasions into adjacent closed habitats.
Land managers should focus on eradicating persistent populations of Natalgrass
on both roads and interior habitats. Herbicide application (Stokes 2010) and careful
hand pulling (ideally before fruit maturation) can be used to reduce Natalgrass
abundance. Elimination of seed input for several months can be effective in reducing
Natalgrass because this species does not have a long-lived seedbank (Stokes
2010). Both road and interior patches should be treated, with a priority of eradicating
large patches close to roads that may act as sources. Road populations should
be treated before a prescribed burn so that post-fire invasion into interior habitats is
discouraged. In addition, land managers need to be vigilant in surveying for other
invasive species that could take advantage of areas cleared of Natalgrass. Finally,
the use of mechanical pre-treatments or surrogates should be approached with caution
(Menges and Gordon 2010), as Natalgrass is known to increase in areas where
soil has been disturbed.
Our work demonstrates contrasting results regarding where an in vasive species
persists and occupies a landscape. Resurveys are important to understand the persistence
of invasive species. When resurveys show substantial persistence, as was
the case for Natalgrass even in interior areas, land managers do not have the luxury
of allowing natural processes to eliminate problem species. Instead, invasive species
persistence demonstrated by resurveys make a case for aggressive treatment of
invasive species.
Acknowledgments
We thank Vivienne Sclater and Rebecca Waskovich for help with ArcGIS and SQL database
queries. Thanks to Stephanie Koontz for her statistical expertise, as well as Stacy Smith,
Brian Josey, and Kevin Main for their input and support. This study was conducted as part
of Kathryn Tisshaw’s internship at Archbold Biological Station, supported by a grant from
the Vaughn-Jordan Foundation.
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