Dwarf Palmetto ( Sabal minor ) Population Increase in Southeastern Oklahoma

Abstract

A small population of dwarf palmetto (Sabal minor; Arecaceae) was described in 2011 from Beavers Bend State Park, Oklahoma, approximately 40 km farther north than the species had previously been known to occur along the Red River. The goal of this study was to determine whether this population's size was changing. A 2.75 ha study site along the Lower Mountain Fork River in Beavers Bend State Park was visited in 2011, 2013, and 2015, and all S. minor encountered were marked with a GPS with submeter accuracy. The stage of each S. minor encountered was recorded as either seedling, split leaves, or palmate leaves. Over the course of the study, the number of individuals with palmate leaves increased from 22 in 2011 to 37 in 2015. This increase was mirrored by an increase in the number of seedlings (N = 61 in 2011 and N = 173 in 2015) as well as an increase in the number of individuals with split leaves (N = 9 in 2011 and N = 59 in 2015). The increase of this species at the extreme northwestern edge of its range is consistent with changes in the distribution of other palm species that have been linked with warmer winters. However, the eventual range of this species in Oklahoma may be limited by the availability of water, as the rest of Oklahoma is drier and precipitation across Oklahoma is projected to remain relatively stable.

Full text

CASTANEA 82(2): 163–168. SEPTEMBER
Copyright 2017 Southern Appalachian Botanical Society
Scientific Note
Dwarf Palmetto (Sabal minor) Population
Increase in Southeastern Oklahoma
Christopher J. Butler
1
*andHuyenB.Tran
1
1
Department of Biology, University of Central Oklahoma, Edmond, Oklahoma 73034
ABSTRACT A small population of dwarf palmetto (Sabal minor; Arecaceae) was described in
2011 from Beavers Bend State Park, Oklahoma, approximately 40 km farther north than the species
had previously been known to occur along the Red River. The goal of this study was to determine
whether this population’s size was changing. A 2.75 ha study site along the Lower Mountain Fork
River in Beavers Bend State Park was visited in 2011, 2013, and 2015, and all S. minor encountered
were marked with a GPS with submeter accuracy. The stage of each S. minor encountered was
recorded as either seedling, split leaves, or palmate leaves. Over the course of the study, the number
of individuals with palmate leaves increased from 22 in 2011 to 37 in 2015. This increase was
mirrored by an increase in the number of seedlings (N ¼61 in 2011 and N ¼173 in 2015) as well as an
increase in the number of individuals with split leaves (N ¼9 in 2011 and N ¼59 in 2015). The
increase of this species at the extreme northwestern edge of its range is consistent with changes in
the distribution of other palm species that have been linked with warmer winters. However, the
eventual range of this species in Oklahoma may be limited by the availability of water, as the rest of
Oklahoma is drier and precipitation across Oklahoma is projected to remain relatively stable.
Key words: Dwarf palmetto, Oklahoma, McCurtain County, palm, Sabal minor.
INTRODUCTION The genus Sabal in-
cludes 16 species that range from the southeast-
ern USA to northern South America, with the
greatest diversity of species occurring in the
Caribbean (Zona 2000). Sabal minor is one of
five species of Sabal that are native to the
southeastern USA and has been documented
from Oklahoma, Texas, Arkansas, Louisiana,
Mississippi, Alabama, Georgia, Florida, South
Carolina, and North Carolina, with an isolated
population in northeastern Mexico (Goldman
1999).
Sabal minor is an understory palm that is
typically found in low-lying swamps, floodplains,
mesic hammocks, and on riverbanks (Zona 2000,
Butler et al. 2011). It is a cold-hardy species with
some individuals reported to withstand temper-
atures to 20.68C (Tripp and Dexter 2006).
Individuals from McCurtain County, Oklahoma,
at the extreme northwestern edge of their range,
are unusually cold-hardy and some authors
speculate that there may be a genetic component
to their ability to withstand relatively harsh
winters (Tripp and Dexter 2006, Butler et al.
2011).
Although S. minor is the most widespread
palm species in the United States, the Oklahoma
Natural Heritage Program Network categorized
it as a S2 (imperiled) plant in Oklahoma
(Oklahoma Natural Heritage Inventory [ONHI]
2017) where it is restricted to McCurtain County
(Butler et al. 2011). Recently, Butler et al. (2011)
documented a population of this species in
Beavers Bend State Park, approximately 40 km
north of the Red River. The objective of this
study was to quantify how the number of S.
minor changed at this location from 2011
through 2015.
*email address: cbutler11@uco.edu
Received June 6, 2016; Accepted June 1, 2017.
Published: October 9, 2017.
DOI: 10.2179/16-114
163

METHODS
Study Site
Beavers Bend State Park (3480802100N,
94841010 00W) is located in McCurtain County,
Oklahoma. It covers an area of 5.3 km
2
and the
elevation ranges from 121 to 251 m above sea
level. A forest dominated by Acer saccharum
(Marshall), Quercus rubra (L.), and Carya
cordiformis (Wangenh.) K. Koch covers the
mesic slopes of this park (Hoagland 2000). Sabal
minor is restricted to the riparian corridor along
the Lower Mountain Fork River (Butler et al.
2011).
We visited this location for one day in
February during 2011, 2013, and 2015 as part of
a recurring class field trip for GIS & Ecology
(BIO 4914). During the class activity, we used a
Trimble GeoXT (Trimble Inc., Sunnyvale, CA),
which has real-time decimeter positioning to
record the locations of all S. minor encountered.
We surveyed within 50 m of the Mountain Lower
Fork River for a distance of 430 m on the north
side of the river and 120 m on the south side of
the river for a total area surveyed of 27,500 m
2
or
2.75 ha. The coordinates of all S. minor
encountered were recorded as well as growth
stage. Growth stage was categorized as: seed-
lings with strap leaves (undivided or lanceolate
leaf blades), split leaves (bifid or costapalmate
leaves), or sexually mature individuals with
palmate leaves. These growth stages were
chosen as they are easy to distinguish in the
field and they correlate with reproductive
maturity. The length of time spent at each stage
varies, with Ramp (1989) suggesting that S.
minor may remain as seedlings with strap leaves
for approximately six years and the split leaf
juvenile stage may last for 15–20 years, while
Tripp and Dexter (2006) reported some individ-
uals may become sexually mature individuals
with palmate leaves within five years of germi-
nation. If multiple plants occurred within 10 cm,
as occurred at four locations with seedlings, we
recorded only that a clump of seedlings was
present and excluded this clump from the
analysis. We plotted sequential maps showing
demographic changes in space with ArcMap
v10.4 (Esri, Redlands, CA).
RESULTS We found 377 individuals of S.
minor plus four clumps of seedlings from 2011
to 2015. Although we attempted to survey this
area systematically, there were eight plants
(represented by five individuals with split leaves
and three seedlings) that were marked in 2011,
not found during 2013, and then rediscovered
during 2015. We located 92 S. minor in 2011, 191
in 2013, and 269 in 2015 (Table 1). Of 92 S. minor
recorded during 2011, only 50 individuals (54%)
were still alive by 2015. Seedlings experienced
45% mortality between 2011 and 2013 and 46%
mortality between 2013 and 2015 (Figure 1). We
observed no mortality of split leaf and palmate
individuals between 2011 and 2013, but observed
32% mortality of split leaf and 9% mortality of
palmate individuals between 2013 and 2015
(Figure 1).
However, the number of all three stages
increased during this time and they spread along
both sides of the Lower Mountain Fork River
(Figure 2). Seedlings increased from 61 individ-
uals in 2011 to 173 individuals in 2015. The
number of plants with split leaves increased
from nine individuals in 2011 to 59 individuals in
2015. The individuals with palmate leaves
increased from 22 individuals in 2011 to 37
individuals in 2015. Overall, the number of plants
with palmate leaves increased from 8 individuals
per hectare in 2011 to 13.5 individuals per
hectare by 2015.
DISCUSSION The population of S. minor
in Beavers Bend State Park is increasing.
However, the apparent density at this study site
(13.5 individuals per ha) is much lower than the
3,050.7 stems per ha reported in southeastern
Louisiana (Wall and Darwin 1999). Although the
density of this species alongside the Mountain
Fork River is still low, the number of seedlings,
split leaves, and palmate leaves increased from
2011 to 2015. Silva Matos et al. (1999) suggest
that recruitment by tropical palm trees is
density-dependent with the transition rates of
the smallest plants reduced by proximity to adult
plants. Given the relative paucity of adult plants
at our study site, this suggests that recruitment
should be relatively rapid at this location.
Although the seedling mortality is relatively high
Table 1. Individuals of each growth stage surveyed
in 2011, 2013, and 2015.
Growth stage 2011 2013 2015
Seedling 61 140 173
Split leaves 9 28 59
Palmate 22 23 37
Total 92 191 269
164 VOL.82CASTANEA

at 45–46% over two years, Ramp (1989) found a
seedling to juvenile transition probability of 0.6%
in a stable population of S. minor in Louisiana.
This suggests that the observed rate in Oklaho-
ma may be sufficient for the population to
increase through time, depending upon how long
it takes for seedlings to transition to juveniles. In
addition, Ramp (1989) also found that fruit and
seed production increased with age in S. minor,
which could also account for the increasing
number of seedlings observed at the Oklahoma
study site.
Although the effects of climate change on the
phenology and distribution of many plants has
been well documented (e.g., Parmesan 2006),
there are relatively few studies on the effects of
climate change on the distribution of palms.
Walther et al. (2007) linked the changing
distribution of the introduced palm Trachycar-
pus fortunei in Switzerland with warming winter
temperatures and a longer growing season. This
species is also spreading into surrounding
forests after being introduced in Japan (Koike
2006). Tripp and Dexter (2006) found that recent
collections of S. minor in North Carolina have
occurred at locations that are more northerly,
and they note a correlation with shorter winters
in more recent years. Winter temperatures in
southeastern Oklahoma have likewise generally
been above average since the 1990s (Oklahoma
Climatological Survey 2016), which may facili-
tate the spread of this species at the edge of its
range.
Figure 1. Flow chart showing the changes in each life history stage from 2011 to 2013 (A) and from 2013 to 2015 (B).
Percentages may not sum to 100% due to rounding. The eight individuals (five split leaves and three seedlings) that were
overlooked during 2013 but rediscovered in 2015 are excluded from this figure due to uncertainty of their life history
stage in 2013.
2017 165BUTLER, TRAN: DWARF PALMETTO EXPANSION IN OKLAHOMA

Water availability, however, may play a role in
limiting the spread of these species. Bjorholm et
al. (2005) found that water availability limits
palm species richness in the Americas and Kreft
et al. (2006) likewise found that annual precip-
itation was an important component of spatial
variation in palm species richness. Similarly,
Rakotoarinivo et al. (2013) note that palm
species richness in Madagascar correlates with
higher precipitation during the last glacial
maximum, approximately 21,000 years ago.
Annual precipitation in McCurtain County aver-
ages 132 cm per year (Oklahoma Climatological
Survey 2016) but declines to the north and west,
and the Intergovernmental Panel on Climate
Change (IPCC; 2014) predicts precipitation
totals to remain largely the same in the coming
decades in this area. Model outputs from CCSM4
(the Community Climate System Model; see
Gent et al. 2011 for a description of this dataset)
suggest that changes in annual precipitation
across Oklahoma under the most extreme
climate change scenario (RCP 8.5) will only be
approximately –5% by 2050.
Fire regimes are also important in maintaining
populations of some southeastern palms. Both
Serenoa repens (W. Bartram) Small and Sabal
etonia Swingle ex Nash are well known for being
dependent upon fire (Abrahamson 1984, Davison
and Bratton 1988, Abrahamson and Abrahamson
1996). For example, Abrahamson (1999) found a
pronounced flowering response in Serenoa
repens and Sabal etonia following a fire. In
contrast, Maliakal et al. (2000) found that there
was an increase in the average cover of Serenoa
repens with increasing time post-burn, although
they attributed this to increasing plant size
rather than an increase in recruitment. However,
palms that are not restricted to the fire-climax
pinelands of the southeastern US appear to be
less dependent upon fire. For example, McPher-
son and Williams (1998) found that fires neither
Figure 2. The numbers and spatial extent of seedling, split leaf, and palmate leaf individuals increased from 2011 to
2015 in Beavers Bend State Park.
166 VOL.82CASTANEA

facilitated nor hindered the population dynamics
and distribution of Sabal palmetto Lodd. ex
Schult. & Schult.f. Little has been published
about the effects of fire on Sabal minor but
Hartnett and Krofta (1989) found that this
species was not present in central Florida mixed
hardwood forest until more than 25 years had
elapsed since the last burn. This suggests that
this denizen of low-lying swamps may be a
somewhat fire-intolerant species. Some authors
have suggested that fire intensity and frequency
may increase during the 21st century (e.g.,
Flannigan et al. 2000, Tang et al. 2015), which
may potentially limit the distribution of this
species in the coming decades.
Cook (1983) suggests that a knowledge of
whether a group of apparent individuals are
actually ramets that originated vegetatively and
thus are clonal (i.e., part of a larger genet) is
important when discussing demography. Some
palm species, such as Serenoa repens, are clonal
and with multiple meristems (ramets) connect-
ing to a common stem (Carrington et al. 2003).
As Cook (1983) noted, clonal plants typically
spread gradually as new ramets emerge. It is
unclear whether any of the apparent individuals
identified in this study were actually ramets
although some individuals were growing less
than 10 cm apart. However, given that many
apparent individuals were not particularly close
to each other and some apparent individuals
were on opposite sides of the Lower Mountain
Fork River, it seems likely that this population is
unlikely to consist of a single genet.
Although this small-scale study lasted only
four years, the increasing number of seedlings,
split leaf individuals, and palmate leaf individu-
als suggests that S. minor is increasing in
abundance at the extreme northwestern edge
of its range, despite relatively high seedling
mortality. Future researchers should investigate
the genetics of this species and whether this
species tends to reproduce vegetatively, as this
will affect the rate at which it spreads. Multiple
dense clumps of seedlings observed in 2013 also
suggest that seed dispersal may limit the rate of
spread. Future researchers should also investi-
gate how biotic and abiotic factors may affect
the distribution and population ecology of S.
minor and other palm species around the world
in the context of anthropogenic climate change.
ACKNOWLEDGMENTS We thank the
all the students from the GIS & Ecology (BIO
4914) classes who assisted with gathering data.
We thank C. King, G. Caddell, the subject editor,
and an anonymous reviewer for comments on
this manuscript. We also thank B. Watkins for
the use of the Trimble GeoXT.
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