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Modeling the Distribution of the Dwarf Palmetto (Sabalminor; Arecaceae) in McCurtain County, OklahomaAuthor(s): Christopher J. Butler, Jennifer L. Curtis, Kimberly McBride, DavidArbour, and Berlin HeckSource: The Southwestern Naturalist, 56(1):66-70. 2011.Published By: Southwestern Association of NaturalistsDOI: http://dx.doi.org/10.1894/JB-15.1URL: http://www.bioone.org/doi/full/10.1894/JB-15.1

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MODELING THE DISTRIBUTION OF THE DWARF PALMETTO (SABALMINOR; ARECACEAE) IN MCCURTAIN COUNTY, OKLAHOMA

CHRISTOPHER J. BUTLER,* JENNIFER L. CURTIS, KIMBERLY MCBRIDE, DAVID ARBOUR,

AND BERLIN HECK

Department of Biology, University of Central Oklahoma, 100 North University Drive,

Edmond, OK 73034 (CJB, JC, KM)

Oklahoma Department of Wildlife Conservation, Route 4, Box 2900, Broken Bow, OK 74728 (DA)

109 Kaye Drive, Broken Bow, OK 74728 (BH)

* Correspondent: cbutler11@uco.edu

ABSTRACT—The dwarf palmetto, Sabal minor, reaches the northwestern limit of its geographic range inMcCurtain County, Oklahoma, where it is critically imperiled. We determined that S. minor was morewidespread than previously reported; 28 new localities were identified. Populations were associatedpositively with average annual minimum temperatures (i.e., plants were in the warmest parts of thecounty), negatively associated with elevation, and the species primarily occurred on silty clay soils.Although S. minor was common in a few areas near Red Slough Wildlife Management Area, it generallywas uncommon and typically represented by only a few individuals. Ecological Niche Factor Analysisprojected that suitable habitat was concentrated in southeastern McCurtain County, but that potentiallysuitable habitat was present throughout much of the county.

RESUMEN—El palmeto enano, Sabal minor, llega al lımite noroeste de su distribucion geografica en elcondado de McCurtain en Oklahoma, donde esta crıticamente en peligro. Encontramos que S. minorestuvo mas ampliamente distribuido que lo informado anteriormente, con 28 nuevas localidadesidentificadas. Poblaciones se asociaron positivamente con la media temperatura mınima anual (es decir,las plantas se encontraron en la parte mas calida del condado) y negativamente con la altura, y seencontraron principalmente en suelos de arcilla limosa. Aunque S. minor fue comun en algunas zonascercanas a Red Slough Wildlife Management Area, fue en general poco frecuente y normalmente fuerepresentado por pocos individuos. Un analisis de factores del nicho ecologico concluyo que el habitatadecuado se concentro en el sureste del condado de McCurtain, pero que mas habitat potencialmenteadecuado estuvo presente en mucho del condado.

The dwarf palmetto, Sabal minor, is a relativelysmall understory palm generally occurring inlow-lying, swampy habitats (Zona, 1990; Riffleand Craft, 2003), although it also occurs inupland sites (D. L. Immel, http://plants.usda.gov/plantguide/pdf/cs_sami8.pdf), especially incentral Texas (T. Wednt, http://129.116.69.198:427/Tex.html). Sabal minor occurs fromOklahoma and Texas eastward to Florida andNorth Carolina (Riffle and Craft, 2003), with anisolated population in northeastern Mexico(Goldman, 1999). In Oklahoma, S. minor hasbeen collected only in McCurtain County. Sabalminor is categorized as a G5/S1 plant. The globalrank G5 indicates that the species is demonstra-bly secure globally although it may be rare inparts of its range, especially at the periphery

(Oklahoma Natural Heritage Inventory, http://www.oknaturalheritage.ou.edu/index.htm). How-ever, the state ranking S1 is used for species thatare critically imperiled in Oklahoma becauseof rarity (#5 occurrences, few remaining indi-viduals, or small geographic range) or becauseof some factor that makes it especially vulnera-ble to extinction (Oklahoma Natural Herit-age Inventory, http://www.oknaturalheritage.ou.edu/index.htm). Individuals of S. minor fromMcCurtain County are of interest from aconservation standpoint as they appear to beunusually cold-hardy and it is speculated thatthere may be a genetic component to theirability to withstand harsh winters (G. L.Wade, http://www.canr.org/Wade%20CANR%20Report%2006.pdf).

THE SOUTHWESTERN NATURALIST 56(1):66–70 MARCH 2011

McCurtain County is in southeastern Okla-homa, bordering Texas to the south andArkansas to the east. Elevations are 86–582 mabove sea level (National Elevation Dataset,http://ned.usgs.gov/). Average annual precipi-tation is 132 cm, average annual temperature is17uC, and there is an average of 15 days/yearwhen temperature is below 26.7uC. The growingseason averages 215 days with the average firstfreeze occurring on 3 November and the last on30 March (Oklahoma Climatological Survey,http://climate.ok.gov/). Vegetation associationsin McCurtain County (B. W. Hoagland, http://geo.ou.edu/botanical) include bottomland for-est, cypress-bottoms (Cupressaceae) forest, lob-lolly pine (Pinus taeda), and oak-pine (Quercus-Pinus) forest (Reinking, 2004). All S. minorcollected were in the southeastern portion ofMcCurtain County (B. W. Hoagland et al.,http://geo.ou.edu/botanical).

Despite S. minor being listed as criticallyimperiled in Oklahoma, little has been pub-lished about its geographic range. Other studiesthat have examined the distribution of palmsrelied upon aerial photographs (Everitt et al.,1996) or satellite imagery (McMorrow, 2001).However, because S. minor is an understoryspecies, detection by aerial photographs orsatellite imagery is problematic due to vegetationthat obscures these plants from above.

One approach to mapping the range of S.minor in Oklahoma is to use modeling of itsecological niche. Traditional models use pres-ence-absence data in conjunction with variousecogeographical variables (e.g., temperature,precipitation) to identify factors that affectdistribution (Anderson et al., 2003). This infor-mation can be analyzed using Genetic Algorithmfor Rule-set Prediction, which uses a heuristicapproach to identify factors correlated withpresence or absence of a species (Anderson etal., 2003). However, data indicating absence at asite can be misleading because the species mayhave been either truly absent or not detected(Hirzel et al., 2002). As a result, false absencesmay potentially skew predictions of the model.An alternative approach is to use only dataindicating presence to identify the geographicrange of an organism (Hirzel et al., 2002). AnEcological Niche Factor Analysis identifies eco-geographical variables associated with specimensand generates a habitat-suitability map (A. H.Hirzel et al., http://gi.leica-geosystems.com/

default.aspx) using the program BioMapper(A. H. Hirzel et al., http://www.unil.ch/biomap-per). This analysis summarizes multiple ecogeo-graphical variables into a few independentfactors similar to a principal-components analy-sis. The first factor is the marginality factor,which maximizes the difference in environmen-tal conditions between the niche and the studyarea. Subsequent factors are specialization fac-tors and are created by computing the directionthat maximizes the remaining variance betweenthe study area and locations where the speciesoccurs. We used Ecological Niche Factor Analysisto identify ecogeographical variables that affectthe distribution of S. minor and to model itsdistribution in McCurtain County, Oklahoma.

MATERIALS AND METHODS—To determine where S.minor was documented to occur in McCurtain County,Oklahoma, we obtained data from the OklahomaBiological Survey Oklahoma Vascular Plants Database(B. W. Hoagland et al., http://geo.ou.edu/botanical)and from the United States Department of AgricultureForest Service. We also traveled to McCurtain Countyin February 2008 to confirm sites where S. minor hadbeen reported and to record locations where S. minorwas encountered incidentally. We then created apreliminary model of the ecological niche usingArcGIS 9.2 (Crosier et al., 2004) that was based onelevation, use of land, and vegetational cover toidentify areas that potentially were suitable for S. minor.In March 2008, we collected data at 46 additional sitesin the county.

We used ArcGIS 9.2 (Crosier et al., 2004) to create agrid of 100 by 100 1-m2 cells (10,000 m2 or 1 ha) forMcCurtain County. As McCurtain County consists of494,743 ha, the grid consisted of 494,743 cells. Eachcell then had variables assigned to it, including land-use, and vegetational cover from both the United StatesGeological Survey and aerial imagery (downloadedfrom United States Department of Agriculture Geo-spatial Data Gateway, http://datagateway.nrcs.usda.gov/; classified using ERDAS IMAGINE 9.1; ERDAS,Atlanta, Georgia), distance to surface water, amount ofwater flowing into each cell (calculated using ArcGIS9.2; Crosier et al., 2004), elevation, temperature,precipitation, slope, aspect, type of soil, and whetherS. minor was discovered at each cell on the grid. Wethen used BioMapper 4 to perform an Ecological NicheFactor Analysis (A. H. Hirzel, http://www2.unil.ch/biomapper/). Based on observations in the field,populations of S. minor appeared to be spatiallyclumped, so we added a spatial-autoregressive compo-nent (i.e., eastings and northings in the UniversalTransverse Mercator coordinate system) to the model.

We evaluated four methods of constructing maps ofhabitat suitability (median, geometric mean, harmonicmean, and minimal distance) using a continuous Boyceindex (Hirzel et al., 2006). The median algorithm forcalculating habitat suitability assumes that the mediandistribution of the species for each ecogeographical

March 2011 Butler et al.—Dwarf palmetto in Oklahoma 67

variable is the preferred habitat and it assumes thatdistributions are symmetrical. The geometric-meanalgorithm is similar to the median algorithm but doesnot assume symmetry. The harmonic mean is similar tothe geometric mean but it gives a high weight to allobservations (rather than mean values for distributionof the species in each ecogeographical variable).Minimal distance gives each observation the sameweight, rather than accounting for density. A furtherdescription of each method is in A. Hirzel et al.(http://www.unil.ch/biomapper). We used a continu-ous Boyce Curve (Hirzel et al., 2006) to classify habitatas either suitable (suitability . 50%) or unsuitable(suitability , 50%). Robustness of the model wasassessed using a continuous Boyce index (Hirzel et al.,2006), which can range from 0 to 1, with models closerto 1 being more robust.

RESULTS—Sabal minor was at 41 locationsrepresenting 29 1-ha cells on grids (Fig. 1a). Ofthe locations, 28 were new and 13 were fromherbarium collections. Although all records inthe Oklahoma Vascular Plants Database werefrom southeastern McCurtain County, S. minor

occurred as far north as Beavers Bend State Park(34u08.349N, 94u41.169W) ca. 40 km north of theRed River. There were $20 individuals in thispopulation, primarily clustered along the Moun-tain Fork River (a specimen from here wasdeposited in the Herbarium Division of theMuseum of Natural History at the University ofCentral Oklahoma). Although we sampledpoints north of Beavers Bend State Park priorto performing the Ecological Niche FactorAnalysis, we did not find S. minor further north.The greatest density of S. minor was in southeast-ern McCurtain County, particularly near RedSlough Wildlife Management Area (33u44.319N,94u40.159W) where in some areas it was thedominant understory plant with #74 matureplants/100 m2. Most S. minor were in low-lyingdeciduous forests (primarily dominated by Celtislaevigata, Nyssa sylvatica, Quercus lyrata, and Q.phellos), but some were in open fields and, at twolocations, in upland stands of loblolly pine.

FIG. 1—a) The 29 1-ha cells where the dwarf palmetto (Sabal minor) was observed in McCurtain County,Oklahoma, during February and March 2008. The dashed line indicates the approximate distribution of S. minorbased on records in the Oklahoma Vascular Plants Database. b) The projected distribution of S. minor is shown inblack; the greatest density of suitable habitat is in southeastern McCurtain County, but scattered areas of suitablehabitat occur throughout the county.

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The best model for calculating habitat suit-ability was the harmonic-mean model as it hadthe highest continuous Boyce-index value of0.519. The distribution of S. minor was associatedpositively with average annual minimum temper-atures and was associated negatively with eleva-tion. Populations of S. minor also were stronglyautocorrelated spatially and were most commonon silty clay loam. The habitat-suitability map(Fig. 1b) shows that the greatest concentrationof S. minor should occur in southeastern McCur-tain County, but that suitable habitat for thisspecies occurs in a wider area of the county thanrecords in herbarium collections suggest.

DISCUSSION—Although S. minor is most commonin extreme southeastern McCurtain County, itsdistribution is wider than previously reported,extending ca. 40 km north of the Red River andca. 25 km north of any record from herbariumspecimens. It is unclear whether S. minor waspresent historically this far north or whether thisis a recent expansion of its geographic range.However, the relatively large population (ca. 20plants) coupled with the observation of twoindividuals with fruit suggests that this populationhas been present for several years.

The harmonic-mean model provided the bestmodel of habitat suitability as it had the highestcontinuous Boyce-index value. In general, har-monic-mean models are more appropriate forrelatively small samples (A. H. Hirzel et al.,http://www2.unil.ch/biomapper/). This modelindicated that presence of S. minor was associatedpositively with average annual minimum temper-atures and negatively associated with elevation.McCurtain County is at the northwestern edge ofthe range of S. minor, so it was not surprising tofind that S. minor was restricted to the lowestelevations having the warmest temperatures.Sabal minor in McCurtain County usually wasclustered in areas with silty clay; previously, it wasreported to tolerate a wide range of soils fromsandy to loamy limestone-derived soils (Hender-son, 1986). Given that S. minor was primarily inswampy conditions, it is possible that thedistribution in McCurtain County may be influ-enced by the superior water-retaining capacity ofsilty clay soils as opposed to sandy and loamysoils, which have greater porosity (Rowell, 1994)and better drainage.

The greatest density of potentially suitablehabitat was in southeastern McCurtain County

(Fig. 1b). Immediately to the north of this area,the elevation rises slightly, decreases, then risesagain resulting in a narrow belt of suitablehabitat in lower elevations ca. 5–10 km north ofthe largest area of suitable habitat. North of thisbelt the elevation increases and potentiallysuitable habitat becomes sparser.

Sabal minor currently is listed by the OklahomaNatural Heritage Inventory as S1, criticallyimperiled, within the state. Given that 28 newlocations were discovered during this study, itmay be worthwhile to re-evaluate the status ofthis species in Oklahoma. However, although S.minor was locally common near Red SloughWildlife Management Area, it was scarce andlocal (i.e., only one plant or a small group ofplants were present) at most locations elsewherein McCurtain County. At least two areas withlarge numbers of S. minor have been cleared inMcCurtain County in the past 10 years. Contin-ued monitoring of the population of this plant isrecommended, both for conservation purposesand to determine if an expansion of thegeographic range may be occurring. Fruits ofthis species are dispersed by birds and water(Zona, 1997), which may facilitate expansion ofthe range. In North Carolina, a population of S.minor was reported recently from Martin County,ca. 45 km north of previously known popula-tions, suggesting that S. minor may be expandingits range northward in North Carolina (Trippand Dexter, 2006). We suggest further monitor-ing of S. minor in Oklahoma to investigatewhether a similar expansion of the range maybe occurring.

We thank R. Bastarache and personnel of the UnitedStates Forest Service for assistance in locating popula-tions of S. minor in McCurtain County, the OklahomaVascular Plants Database for herbarium data, and theOklahoma State University Forest Resources Center forproviding accommodations in McCurtain County.Thanks to L. Stabler for comments on an early draft.This research was supported by the Office of Researchand Grants at the University of Central Oklahoma.

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