The Effect of Shrubs on the Establishment of an
Endangered Perennial (Asclepias curtissii) Endemic to
Patrick Mondo, Kristen D. Marshall Mattson,
and Cynthia C. Bennington
Southeastern Naturalist, Volume 9, Issue 2 (2010): 259–274
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2010 SOUTHEASTERN NATURALIST 9(2):259–274
The Effect of Shrubs on the Establishment of an
Endangered Perennial (Asclepias curtissii) Endemic to
Patrick Mondo1, Kristen D. Marshall Mattson2,
and Cynthia C. Bennington1,*
Abstract - Asclepias curtissii (Curtiss’ Milkweed) is an endangered perennial herbaceous
plant endemic to Florida scrub habitat. Although many scrub perennials are
gap specialists, Curtiss’ Milkweed is often found growing in close association with
woody vegetation. We asked whether seed germination and seedling establishment
are enhanced by the microsite conditions created beneath woody shrubs. In addition,
we asked whether adult plants occur in association with shrubs more frequently than
would be expected by chance and whether this distribution could be explained by
seed dispersal patterns. Seeds were germinated, ex situ, in a factorial experiment
with leaf litter and shade as main effects. In a separate experiment, to determine the
effect of shrub cover on seedling establishment, 144 Curtiss’ Milkweed seedlings
were planted into a total of twelve fenced plots within Lyonia Preserve, Deltona, fl.
Within each plot, six seedlings were planted in sandy gaps, and six were planted in
close association with existing woody shrubs. We assessed survival over a five-month
period. We also measured the distance to the nearest woody stem from extant adult
plant locations and compared these to the distance of random points around those
extant plants. Similarly, we released 70 milkweed seeds from extant plant locations
and measured the distance from the spot where they landed to the nearest woody
stem and compared this to the distance between random points and woody stems. We
found that seed germination was significantly enhanced by shade (P < 0.0001) but
not by leaf litter, and that seedlings growing in the shade of close neighboring shrubs
had significantly higher rates of survival (P < 0.001) than those seedlings planted
in gaps. Extant plants tended to grow close to shrubs, and seeds tended to land near
shrubs, but neither of these distances were less than would be expected by random
chance (P > 0.10 in both cases). The facilitation of seedling establishment by woody
plants has been documented in other arid environments, but not in Florida scrub.
Facilitative or positive interactions among individuals of different plant
species can influence community composition (Bertness and Callaway
1994) through positive effects on individual plant growth and survival (see
reviews in Brooker et al. 2008, Callaway and Walker 1997). For an interaction
between two plants to be considered facilitative, the competitive effects
of the heterospecific neighbor must be outweighed by the positive effects of
the microenvironment created by that neighbor (e.g., Holmgren et al. 1997).
1Department of Biology, Campus Box 8264, Stetson University, DeLand, fl32723.
2The LowCountry Institute, 40 Mobley Oaks Lane, Okatie, SC 29909. *Corresponding
author - firstname.lastname@example.org.
260 Southeastern Naturalist Vol. 9, No. 2
The best-known examples of the positive effects of “nurse plants” come
from studies of seedling establishment in desert succulents (e.g., Cody 1993,
Franco and Nobel 1988, Jordan and Nobel 1979, McAuliffe 1984, Shreve
1931, Turner et al. 1966). Subsequent studies have centered on arid and other
stressful environments, with the expectation that the positive effect of plant
neighbors will increase, relative to competitive or negative interactions, with
increasing abiotic stress (e.g., Bertness and Callaway 1994, Callaway and
Walker 1997, Callaway et al. 2002, Tewksbury and Lloyd 2001). Although
some experiments have failed to find evidence for increasing facilitation
with increased stress (e.g., Riginos et al. 2005, Smit et al. 2007, Tielbörger
and Kadmon 2000), facilitative interactions among plants have been documented
in stressful environments such as tundra (Callaway et al. 2002) and
dry grasslands (Greenlee and Callaway 1996) in addition to deserts. Established
adult plants can buffer new recruits against environmental extremes
in a variety of ways (Bertness and Callaway 1994). In xeric habitats, nurse
plants can provide microhabitats in which stress experienced by a seedling
is reduced through increased shade (Franco and Nobel 1989) and reduced
transpiration losses (Prider and Facelli 2004), increased nutrient availability
through increased litter deposition (Callaway et al. 1991, Franco and Nobel
1988, Shumway 2000), or protection from herbivory (McAuliffe 1986,
Rousset and Lepart 2000, Smit et al. 2007).
Occurring on well-drained, sandy, nutrient-poor soils (Myers 1990),
Florida scrub is a xerophytic ecosystem in which many of the same environmental
stresses that account for facilitation in desert ecosystems may be
found. Because scrub is a pyrogenic ecosystem, however, there are two important
differences that could affect facilitative interactions. First, frequent
disturbance through fire results in significant temporal variability in the size
and spatial distribution of plants (e.g., Menges and Hawkes 1998). Thus, the
aboveground canopy provided by a nurse plant may be highly variable over
a relatively brief time period. Secondly, the high-intensity fires that are common
in scrub can create open sandy patches in what can otherwise be a dense
competitive community. For this reason, the literature on perennial plant
distribution and seedling establishment in this ecosystem has emphasized the
importance of gaps (e.g., Hawkes and Menges 1996, Johnson and Abrahamson
1990, Menges et al. 2008, Petrü and Menges 2003, Weekley and Menges
2003). Hawkes and Menges (1996) found that 55% of variation in herbaceous
plant density was explained by the presence of open space in scrub habitat. In
addition, a number of scrub perennials appear to be gap specialists (Menges
and Hawkes 1998, Menges and Kimmich 1996, Menges et al. 1999, Petrü
and Menges 2003, Quintana-Ascencio and Morales-Hernandez 1997). The
association of herbaceous species with gaps has been observed in other dry
shrublands as well (e.g., Shmida and Whittaker 1981). These gaps may provide
space free of competition for light, nutrients, or water (Forseth et al.
2001, Hawkes and Menges 1996, Weekley and Menges 2003).
2010 P. Mondo, K.D. Marshall Mattson, and C.C. Bennington 261
Here, we assess the facilitation by shrubs, if any, in the establishment of
an endangered plant, Asclepias curtissii Gray (Curtiss’ Milkweed), endemic
to Florida scrub (Coile and Garland 2003). Putz and Minno (1995) reported
that Curtiss’ Milkweed plants tend to be found along roadsides, but they
were unable to explain adult plant distribution based on the amount of shrub
cover or bare soil. Our casual observations at one field site, however, suggested
that Curtiss’ Milkweed tends to be found growing amongst shrubs
and, unlike some other scrub perennials, is unlikely to be found in open
gaps. We tested the hypothesis that facilitation by shrubs is occurring in this
species by asking: 1) are seed germination and early seedling establishment
positively affected by the microsite provided by a shrub canopy?, 2) are adult
plants, in fact, found in association with shrubs more often than expected by
chance?, and 3) could such an association be explained by the retention of
wind-dispersed seeds by shrubs?
Curtiss’ Milkweed is a long-lived perennial that dies back to the tap root
each fall and resprouts in spring (Putz and Minno 1995; C.C. Bennington,
pers. observ.). In our population, we have observed aboveground shoots
typically persisting into October and new shoots appearing in early April. In
addition, plants frequently resprout several times during the growing season
following herbivory by an animal (based on fecal evidence, we suspect rabbits)
that clips the stem back to the ground. Anecdotal evidence presented
by Putz and Minno (1995) suggests that individual plants can live 25 years
or more. While they have opposite leaves, the foliage of adult plants generally
resembles that of the scrub oaks, making plants very cryptic when not
in flower. Flower production typically begins in June and reaches a peak
in late July (Putz and Minno 1995; C.C. Bennington, pers. observ.). The
flowers of Curtiss’ Milkweed are visited by a variety of insects, including a
large number of skipper butterflies (Putz and Minno 1995; C.C. Bennington,
pers. observ.). Most members of the genus Asclepias are self-incompatible
or show inbreeding depression (Ivey et al. 1999, Lipow et al. 1999), but
the breeding system of Curtiss’ Milkweed has not been investigated. In our
population, individual plants frequently produce a large number of flowers
(mean = 152.2, s.d. = 110.1, n = 22, in 2006), but the proportion that set fruit
can be quite low (mean = 0.018, s.d. = 0.029, n = 22). We have not attempted
to quantify spontaneous seedling recruitment at our field site, and the cryptic
nature of the plants makes them difficult to spot through casual observation,
but we have only recorded three seedling recruits in five years.
Field site description
Lyonia Preserve, located in Volusia County, fl(28°55'N, 81°13'W), is a
146-ha area of preserved scrub habitat surrounded by suburban development.
262 Southeastern Naturalist Vol. 9, No. 2
The dominant woody species include three species of oak (Quercus geminata
Small [Sand Live Oak], Q. myrtifolia Willd. [Myrtle Oak], and Q. chapmanii
Sarg. [Chapman Oak]), Lyonia ferruginea (Walt.) Nutt. (Rusty Lyonia),
Ceratiola ericoides Michx. (Florida Rosemary or Sand Heath) and, in some
sections, Pinus clausa (Chapman ex Engelm.) Vasey ex Sarg. (Sand Pine).
Before the first restoration activity in 1994, the site was dominated by Sand
Pine and scrubby oaks. At that time, a variety of methods (i.e., controlled
burning, root raking, roller chopping, and harvesting) were used to remove
the large trees and to encourage the regeneration of scrub shrubs. The area
continues to be managed to maintain a scrub community that likely existed
on the site prior to human habitation of the area.
In the fall of 2004, we recovered between 40 and 99 seeds per follicle
from eight different parent plants, for a total of 512 seeds. At the same time,
we collected field soil and filled thirty-two 6-cm2 plastic cells in each of
sixteen 26-cm x 53-cm plastic trays. One seed was added to each plastic cell
with no attempt made to ascertain viability prior to planting. Within each
flat, half of the cells were randomly assigned to a “litter” treatment. In these
cells, litter (mostly leaves, twigs, and organic matter) collected from Lyonia
Preserve was scattered to a depth of approximately 1 cm. On 29 October
2004, plastic flats were moved from the greenhouse to a sunny, grassy field
on the Stetson University campus. Once in place, 8 of the 16 flats were randomly
assigned to a shade treatment. Shade cloth that reduced light by 50%
was placed 10 cm above each of those eight trays in the shade treatment. For
the first five weeks, trays were supplementally watered to keep the soil moist
and to prevent water availability from limiting germination. Trays remained
in their original positions throughout the six months of the experiment.
Between 29 October and 3 December 2004, cells were censused every
three or four days for germination. Between 3 December and 15 May 2005,
trays were censused once every two weeks. Because it is possible that a seed
germinated during this time but that the seedling died before being detected
in a census, we recognize that any effects we observed could have been due
to very early seedling mortality, rather than a lack of germination.
Seeds of Curtiss’ Milkweed were collected from mature follicles of
three plants in the fall of 2006 and 2007. Seeds from both collection years
were germinated in pots containing soil brought back to the greenhouse
from Lyonia Preserve. In December 2007, seedlings were transplanted into
Cone-tainers (4 cm diameter by 20 cm depth; Stuewe and Sons, Corvallis,
OR) in field soil. In March 2008, seedlings were transplanted into plots in
Lyonia Preserve. We transplanted 144 plants into twelve 3-m x 2-m plots,
surrounded by 1-m tall plastic fencing to reduce mammalian herbivory.
All of the plots were located along the edge of a sandy gap, and plots were
2010 P. Mondo, K.D. Marshall Mattson, and C.C. Bennington 263
oriented so that one half (3 m x 1 m) had woody vegetation providing shrub
cover while the other half lacked woody vegetation (Fig. 1). Within each half
of the plot, seedlings were spaced 0.5 m apart. The number of aboveground
Curtiss’ Milkweed seedlings remaining was evaluated five times between
March 2008 and August 2008. We performed a final census in May 2009
to determine which plants had successfully overwintered and resprouted.
In June 2009, we measured soil temperature at one point under shrub cover
and one point in open sand in each of the twelve plots using a Li-Cor 1000
data logger (Li-Cor, Lincoln, NE). At those same points, we collected soil
samples to a depth of 8 cm. All data were collected in the morning of a
single day following a week of typical afternoon showers. Soil samples were
weighed to the nearest 10 mg and then dried at 60 °C for one week before being
reweighed. The difference between weights was used to calculate percent
Adult plant locations
We have monitored the population of adult Curtiss’ Milkweed plants
since 2004 and continue to do so. Although not every plant can be found
as an adult in each year, we have documented over 50 plant locations over
We measured the distance to the nearest woody stem (herein referred
to as DNWS) from the stem of the Curtiss’ Milkweed plant for each plant
for which an aboveground shoot was identified in the summer of 2008 and for
which the flag and tag remained intact through the fall of 2008 (n = 29). To
determine whether plants are closer to woody shrubs than would be expected
Figure 1. Diagram of the 3-m x 2-m fenced plots used in the survivorship experiment.
The shaded area represents the portion of the plot that had natural shrub cover. The
other half of the plot lacked woody vegetation.
264 Southeastern Naturalist Vol. 9, No. 2
by chance, we also recorded DNWS for four random points within a 3-m
radius of each extant plant. Although many of our plants are located along
trails, we did not include random points that fell within a trail or sand road.
Seeds from three different Curtiss’ Milkweed follicles were used to determine
whether seeds are more likely to land and settle near a shrub than in
open sand. To perform this portion of the study, we chose seven extant plant
locations that included a mixture of open sand and closed shrub canopy. At
each of these sites, we placed one to three seeds at a time on the open palm of
a hand held at approximately 1 m above the soil surface until the wind took
them. Wind speeds varied from 3–9 km/hr for these trials. We followed each
seed for 10 minutes, being careful not to interfere with its movement. At the
end of 10 minutes, we marked the final resting place of each seed with a wire
stake flag. If the plume was still attached, we collected the seed and reused
it. Once 10 seeds had been released from a particular location, the DNWS
for each was recorded.
To determine whether DNWS for dispersed seeds was less than would
be expected by chance, we determined DNWS for random points that were
within the observed range of seed resting places given the wind speed and
direction at the time. Thus, rather than considering the entire area within a
circle with 3-m radius around a plant as possible sites, we only considered
points that were within the seed shadow that contained all 10 of the seed
landing sites. Approximately every 30° in the arc, we extended a transect to
the distance travelled by the farthest-flying seed at that site. At five random
points along each of these transects, we measured DNWS.
To determine the effect of shade and leaf litter on seed germination, we
calculated percent germination for each of the litter treatments within each
tray. Percentages were transformed using the arcsine square root to improve
normality. A two-way analysis of variance (JMP, ver 6.0.2, SAS Institute
Inc., Cary, NC) was conducted to determine the effect of light, litter, and
their interaction on germination.
For each of the dates for which we surveyed seedling presence in the 12
fenced plots, we conducted a loglikelihood test (JMP, ver 6.0.2, SAS Institute
Inc., Cary, NC) with main effects of plot and cover (shrub cover versus
open sand). We used paired t-tests (JMP, ver 6.0.2, SAS Institute Inc., Cary,
NC) to compare soil temperature and soil moisture between shrub cover and
open sand microsites within the twelve plots.
We also used paired t-tests to determine whether: a) adult plants tend
to occur closer, and b) hand-dispersed seeds tend to land nearer to woody
shrubs than expected by chance. To compare DNWS between extant adult
plant locations and random points within a 3-m radius, we paired the DNWS
for each plant and the average DNWS for the four random points sampled at
2010 P. Mondo, K.D. Marshall Mattson, and C.C. Bennington 265
the same plants. For the seed-dispersal experiment, the average DNWS for
the 10 seeds dispersed at each of the seven plant sites was paired with the
average for the random points sampled within the dispersal shadow at each
of those sites. In both cases, we used one-tailed tests since we predicted both
adult plants and seeds to be closer to shrubs than random points.
Of the 512 seeds that were planted in October 2004, only 36 had germinated
at the end of five weeks. By 15 May 2005, however, a total of 111
of the 512 seeds had germinated. Eighty-five of these 111 seeds were in the
shade treatment, representing a significant positive effect of shade on germination
(F1,28 = 54.8, P < 0.0001; Fig. 2). Litter did not affect germination rate
(F1,28 = 0.68, P = 0.41), and there was no differential effect of litter between
light treatments (shade x litter interaction: F1,28 = 0.73, P = 0.40).
Shrub cover had a strong positive effect on seedling survival in the field
(Table 1). Percent survival of seedlings was approximately ten times greater
under the shrub canopy than in the open sand throughout the months following
their transplant (Fig. 3). At the final census, more than one year after
transplant, only two of the 22 seedlings that survived more than one year
were from the open sites within plots. Soil temperatures were, on average,
Figure 2. Average percent germination (± 1 s.e.) of Curtiss’ Milkweed seeds planted
into field soil in individual cells of 16 plastic trays, eight of which were covered
with 50% shade cloth. In each tray, the seed and soil in 16 of the 32 cells was covered
with leaf litter to a depth of 1 cm.
266 Southeastern Naturalist Vol. 9, No. 2
2.85 °C warmer and soil moisture was approximately 5% lower in microsites
with open sand compared to those under shrub canopy in our seedling transplant
plots (Table 2).
Although our initial observations led us to hypothesize that Curtiss’ Milkweed
plants tend to occur close to shrubs, we were unable to demonstrate
Table 1. Results of log-likelihood tests comparing aboveground presence of Curtiss’ Milkweed
seedlings planted under shrub cover or in open sand in each of 12 fenced plots at each of six
census dates. This is a conservative estimate of survival since some individuals were counted
as dead (not aboveground) at one census date prior to 31 August 2008, but then resprouted to be
found aboveground in the following census. At the final census, all plants found aboveground
were also aboveground in August of the previous year.
Effect d.f. 5/16/08 5/30/08 7/09/08
Plot 11 18.14 (0.08) 18.82 (0.06) 22.81 (0.02)
Habitat 1 56.03 (0.0001) 56.39 (0.0001) 38.60 (0.0001)
Effect 8/01/08 8/31/08 5/15/09
Plot 19.31 (0.06) 16.46 (0.12) 29.66 (0.01)
Habitat 41.58 (0.0001) 29.53 (0.0001) 23.29 (0.0001)
Figure 3. Percent aboveground presence of Curtiss’ Milkweed seedlings transplanted
into plots in either open sandy areas or under a shrub canopy. This graph presents a
conservative estimate of survival because several plants died back to the tap root and
then resprouted between censuses. Log-likelihood tests at each of the six census dates
following transplant revealed significantly higher remaining seedlings among those
planted under shrub cover (P < 0.001 in all cases).
2010 P. Mondo, K.D. Marshall Mattson, and C.C. Bennington 267
that this association is greater than would be expected by chance. While the
average distance from an extant milkweed plant to the nearest shrub was less
than the average distance between random points and the nearest shrub, this
difference was not significant (Table 3). Furthermore, the wind-dispersed
seeds of Curtiss’ Milkweed tended to settle close to shrubs, but this difference
was not greater than would be expected by chance (Table 3).
We found strong evidence for facilitation of Curtiss’ Milkweed seed
germination and seedling establishment by the shrubs of Florida scrub. Previous
studies in stressful environments have demonstrated positive effects
of nurse plants through the amelioration of extremes in soil fertility, water
availability, temperature, and light (see reviews in Brooker et al. 1998, Callaway
1995). In Florida scrub, where soils are characterized by low fertility
(e.g., Kalisz and Stone 1984), leaf litter that accumulates beneath shrubs
may enhance nutrient levels (e.g., Callaway et al. 1991). Litter tends to build
up under shrubs of Florida scrub (Schmalzer and Hinkle 1996), and nitrogen
levels have been demonstrated to be higher under shrub canopy than in gaps
in this ecosystem (Maliakal-Witt et al. 2005). We found no evidence for a
positive effect of leaf litter on seed germination, but did not measure litter
depth or nutrient levels in our seedling transplant plots. However, while
increased soil fertility under shrubs may provide positive effects on seedling
growth (e.g., Armas and Pugnaire 2005, Carlsson and Callaghan 1991,
Gomez-Aparicio et al. 2005, Moro et al. 1997, Pugnaire et al. 2004, Tirado
and Pugnaire 2003), seedling survival of plants with a well-developed tap
Table 3. Average distance to the nearest woody stem (DNWS) for a) extant adult Curtiss’ Milkweed
plants and random points within a 3-m radius of the plants and b) landing sites of handdispersed
Curtiss’ Milkweed seeds and random points within the area of the dispersal shadow.
Average DNWS (cm) Paired t-test
(± 1 s.d.) results
Adult plant position 17.62 (± 14.28) t6 = 0.90, Prob > t = 0.19
Random points (within 3-m radius around plants) 20.52 (± 10.85)
Seed landing point 17.35 (± 12.87) t6 = 0.85 Prob > t = 0.22
Random points (within seed dispersal shadow) 19.05 (± 13.28)
Table 2. Average soil temperature and soil water availability under shrub canopy and in open
sand within 12 plots that incorporated both microsites.
Microsite Soil temperature (°C) (± 1 s.d.) Soil moisture (%) (± 1 s.d.)
Open sand 30.67 (± 2.89) 4.4 (± 0.022)
Shrub canopy 27.82 (± 1.80) 9.6 (± 0.042)
Paired t-test results t11 = 4.11, P = 0.002 t11 = 3.89, P = 0.003
268 Southeastern Naturalist Vol. 9, No. 2
root such as Curtiss’ Milkweed is unlikely to be significantly affected by
soil nutrient levels. The presence of nurse plants may also benefit seedlings
through protection against the effects of herbivory (Acuña-Rodríguez et al.
2006, Garcia et al. 2000, Graff et al. 2007, Jaksic and Fuentes 1980, McAuliffe
1986, Rebollo et al. 2005, Rousset and Lepart 2000), but we eliminated
the possibility that a reduction in herbivory could be responsible for any observed
facilitation in this study by fencing our seedling plots. The seedlings
in this experiment originated from just three maternal plants and therefore
represent a potentially small fraction of the genetic variability in the population.
It is possible that seedlings from a greater number of parents would
reveal increased variability in the response to nurse plants. However, at least
for the genotypes we investigated, there were very large positive benefits to
establishing under shrub cover.
The results of the seed germination experiment suggest that shading by
shrubs is the most likely explanation for the positive effect of shrub cover
in this study. This effect, however, could be a result of a decrease in solar
radiation and reduction in photo-inhibition (Armas and Pugnaire 2005,
Egerton et al. 2000) or due to an associated reduction in soil temperature
and evaporation (Franco and Nobel 1988, Prider and Facelli 2004, Turner
et al. 1966, Valiente-Banuet and Ezcurra 1991). Under shade cloth in our
seed germination experiment, reductions in evaporation may have increased
water available to germinating seeds and new seedlings, resulting in the
positive effect that we observed. Although increases in soil water availability
under shrub canopy have been measured in natural communities (Castro et
al. 2004a, b.; Egerton et al. 2000; Holzapfel and Mahall 1999; Pugnaire et al.
2004), previous research in Florida scrub suggests that soil moisture may
be either lower (Marshall Mattson and Putz 2008) or higher (Maliakal-Witt
et al. 2005, Weekley et al. 2007) in gaps than under shrub cover depending
upon the relative rates of transpiration and evaporation. While we only measured
soil moisture and temperature at one point during one rainy season, our
data suggest that young seedlings establishing under a shrub canopy would
experience higher water availability and lower temperatures than to those
growing in open sand. This difference may be a result of reduced evaporation
coupled with an increased accumulation of organic matter under shrubs and
consequent improvements in water-holding capacity (e.g., Armas and Pugnaire
2005, Pugnaire et al. 2004). Given that Florida has distinct wet and dry
seasons, the facilitative effects of shrubs on seedling establishment may be
temporally variable. In dry months, transpiration rates that exceed precipitation
inputs may reduce soil water availability beneath the shrub canopy such
that the facilitative effects of shrubs on Curtiss’ Milkweed seedlings are lost,
or become competitive effects.
Not only can temporal variability in the physical environment shift
the effect of shrubs from facilitative to competitive (Greenlee and Callaway
1996, Ibáñez and Schupp 2001, Kitzberger et al. 2000, Tielbörger
2010 P. Mondo, K.D. Marshall Mattson, and C.C. Bennington 269
and Kadmon 2000), but, in this fire-adapted community with fire-return
intervals ranging from 15 to 100 years (Myers 1990), the shrub canopy can
be variable in both spatial pattern and size over time (Menges and Hawkes
1998, Petrü and Menges 2004). Unlike desert habitats where established
shrubs are very persistent, and nurse plant effects are likely to be observed
in the current distribution of adult plants, the shifting shrub canopy typical
of Florida scrub may be responsible for the fact that our data on the current
distribution of adult plants appears to provide little or no information on
the effects of facilitation. Because both Curtiss’ Milkweed and the majority
of dominant scrub shrub species at our study site resprout following fire
or other disturbance, and because individual milkweed plants and shrubs
may live for more than twenty years, the presence and size of the current
shrub canopy may differ substantially from that which was present at the
time of seedling establishment. Thus, our measures of the distance from
an extant adult plant to the nearest woody stem do not likely reflect the
distance that existed at the time of seedling establishment. Furthermore,
if the benefit of the shrub canopy changes over the course of the life span
of Curtiss’ Milkweed (Callaway and Walker 1997, Cody 1993, Holzapfel
and Mahall 1999, Rousset and Lepart 2000), extant plant distribution may
reflect both the environmental history of the site as well as the age of an
individual and the life history of the species.
While fire-created gaps benefit the establishment and survival of some
endemic scrub perennials (Hawkes and Menges 1996, Menges and Kimmich
1996, Petrü and Menges 2003, Weekley and Menges 2003), we found that
A. curtissi germination and seedling establishment are positively affected by
the presence of a shrub canopy. The taproot and long narrow leaves of Curtiss’
Milkweed seedlings suggest that this species is well-adapted to establishment
in the hot, sunny, dry conditions characteristic of Florida scrub. Even
so, the large, positive effect of shrubs on seedling survival distinguishes Curtiss’
Milkweed from the many scrub perennials that benefit from gaps (e.g.,
Hawkes and Menges 1995, Menges and Kimmich 1996, Menges et al. 2006,
Quintana-Ascencio and Morales-Hernandez 1997, Quintana-Ascencio et
al. 2003). Although we have not yet investigated the effect of shrubs on the
subsequent growth and reproduction of adult milkweed plants, the same environmental
factors that benefit seedlings (i.e., increased leaf litter, increased
water availability, decreased soil temperatures, and decreased herbivory) may
also benefit adult plants. For example, mammalian herbivory is common at
our study site (C.C. Bennington, pers. observ.), and we previously quantified a positive effect of mammalian herbivore exclusion (using wire mesh
cages) on Curtiss’ Milkweed reproduction (C.C. Bennington, unpubl. data).
Ongoing studies are aimed at determining the extent to which shrubs afford
protection from herbivores. Alternatively, facilitative effects of shrubs may
diminish over the life span of the plant. Benefits of shrubs to Curtiss’ Milkweed
seedlings may become negative effects on growth and reproduction of
270 Southeastern Naturalist Vol. 9, No. 2
adult plants if shade limits plant growth and/or flower and fruit production or
if the physical structure of shrubs reduces pollinator visitation or the ability
of seeds to disperse away from the parent plant. Weekley and Menges (2003)
found no evidence that fire affects adult survival of Curtiss’ Milkweed, but
long-term studies are needed to determine whether changes in the shrub layer
affect growth and reproduction in adult plants resprouting from a taproot. If,
in fact, the facilitative effect of nurse shrubs seen at the seedling stage shifts
to a competitive effect later in life, regular disturbance (i.e., prescribed fire
or management techniques designed to mimic the effects of fire) that reduces
the density of the shrub canopy may have positive effects on the growth and
reproduction of adult Curtiss’ Milkweed plants. While the results of this study
illustrate the potential importance of accounting for facilitative interactions in
habitat restoration (e.g., Padilla and Pugnaire 2006), more research is needed
on the long-term effect of shrubs on this rare plant before management recommendations
can be made.
We would like to thank the Plant Ecology (BY 450) class of 2007 at Stetson
University for their help collecting seeds. We would also like to thank Volusia
County Land Acquisition and Management for allowing us to locate our experiments
within Lyonia Preserve. Finally, we thank Terry Farrell, Peter May, and two
anonymous reviewers for insightful comments that improved earlier versions of
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