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(sabal minor; arecaceae) in mccurtain county, oklahoma - Biology ...

(sabal minor; arecaceae) in mccurtain county, oklahoma - Biology ...

March 2011 Butler et

March 2011 Butler et al.—Dwarf palmetto in Oklahoma 67 McCurtain County is in southeastern Oklahoma, bordering Texas to the south and Arkansas to the east. Elevations are 86–582 m above sea level (National Elevation Dataset, http://ned.usgs.gov/). Average annual precipitation is 132 cm, average annual temperature is 17uC, and there is an average of 15 days/year when temperature is below 26.7uC. The growing season averages 215 days with the average first freeze occurring on 3 November and the last on 30 March (Oklahoma Climatological Survey, http://climate.ok.gov/). Vegetation associations in McCurtain County (B. W. Hoagland, http:// geo.ou.edu/botanical) include bottomland forest, cypress-bottoms (Cupressaceae) forest, loblolly pine (Pinus taeda), and oak-pine (Quercus- Pinus) forest (Reinking, 2004). All S. minor collected were in the southeastern portion of McCurtain County (B. W. Hoagland et al., http://geo.ou.edu/botanical). Despite S. minor being listed as critically imperiled in Oklahoma, little has been published about its geographic range. Other studies that have examined the distribution of palms relied upon aerial photographs (Everitt et al., 1996) or satellite imagery (McMorrow, 2001). However, because S. minor is an understory species, detection by aerial photographs or satellite imagery is problematic due to vegetation that obscures these plants from above. One approach to mapping the range of S. minor in Oklahoma is to use modeling of its ecological niche. Traditional models use presence-absence data in conjunction with various ecogeographical variables (e.g., temperature, precipitation) to identify factors that affect distribution (Anderson et al., 2003). This information can be analyzed using Genetic Algorithm for Rule-set Prediction, which uses a heuristic approach to identify factors correlated with presence or absence of a species (Anderson et al., 2003). However, data indicating absence at a site can be misleading because the species may have been either truly absent or not detected (Hirzel et al., 2002). As a result, false absences may potentially skew predictions of the model. An alternative approach is to use only data indicating presence to identify the geographic range of an organism (Hirzel et al., 2002). An Ecological Niche Factor Analysis identifies ecogeographical variables associated with specimens and 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/biomapper). This analysis summarizes multiple ecogeographical variables into a few independent factors similar to a principal-components analysis. The first factor is the marginality factor, which maximizes the difference in environmental conditions between the niche and the study area. Subsequent factors are specialization factors and are created by computing the direction that maximizes the remaining variance between the study area and locations where the species occurs. We used Ecological Niche Factor Analysis to identify ecogeographical variables that affect the distribution of S. minor and to model its distribution 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 Oklahoma Biological Survey Oklahoma Vascular Plants Database (B. W. Hoagland et al., http://geo.ou.edu/botanical) and from the United States Department of Agriculture Forest Service. We also traveled to McCurtain County in February 2008 to confirm sites where S. minor had been reported and to record locations where S. minor was encountered incidentally. We then created a preliminary model of the ecological niche using ArcGIS 9.2 (Crosier et al., 2004) that was based on elevation, use of land, and vegetational cover to identify areas that potentially were suitable for S. minor. In March 2008, we collected data at 46 additional sites in the county. We used ArcGIS 9.2 (Crosier et al., 2004) to create a grid of 100 by 100 1-m 2 cells (10,000 m 2 or 1 ha) for McCurtain County. As McCurtain County consists of 494,743 ha, the grid consisted of 494,743 cells. Each cell then had variables assigned to it, including landuse, and vegetational cover from both the United States Geological Survey and aerial imagery (downloaded from United States Department of Agriculture Geospatial Data Gateway, http://datagateway.nrcs.usda. gov/; classified using ERDAS IMAGINE 9.1; ERDAS, Atlanta, Georgia), distance to surface water, amount of water flowing into each cell (calculated using ArcGIS 9.2; Crosier et al., 2004), elevation, temperature, precipitation, slope, aspect, type of soil, and whether S. minor was discovered at each cell on the grid. We then used BioMapper 4 to perform an Ecological Niche Factor Analysis (A. H. Hirzel, http://www2.unil.ch/ biomapper/). Based on observations in the field, populations of S. minor appeared to be spatially clumped, so we added a spatial-autoregressive component (i.e., eastings and northings in the Universal Transverse Mercator coordinate system) to the model. We evaluated four methods of constructing maps of habitat suitability (median, geometric mean, harmonic mean, and minimal distance) using a continuous Boyce index (Hirzel et al., 2006). The median algorithm for calculating habitat suitability assumes that the median distribution of the species for each ecogeographical The Southwestern Naturalist swna-56-01-10.3d 6/1/11 16:55:25 67 Cust # JB-15

68 The Southwestern Naturalist vol. 56, no. 1 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. minor based on records in the Oklahoma Vascular Plants Database. b) The projected distribution of S. minor is shown in black; the greatest density of suitable habitat is in southeastern McCurtain County, but scattered areas of suitable habitat occur throughout the county. variable is the preferred habitat and it assumes that distributions are symmetrical. The geometric-mean algorithm is similar to the median algorithm but does not assume symmetry. The harmonic mean is similar to the geometric mean but it gives a high weight to all observations (rather than mean values for distribution of the species in each ecogeographical variable). Minimal distance gives each observation the same weight, rather than accounting for density. A further description of each method is in A. Hirzel et al. (http://www.unil.ch/biomapper). We used a continuous Boyce Curve (Hirzel et al., 2006) to classify habitat as either suitable (suitability . 50%) or unsuitable (suitability , 50%). Robustness of the model was assessed using a continuous Boyce index (Hirzel et al., 2006), which can range from 0 to 1, with models closer to 1 being more robust. RESULTS—Sabal minor was at 41 locations representing 29 1-ha cells on grids (Fig. 1a). Of the locations, 28 were new and 13 were from herbarium collections. Although all records in the Oklahoma Vascular Plants Database were from southeastern McCurtain County, S. minor occurred as far north as Beavers Bend State Park (34u08.349N, 94u41.169W) ca. 40 km north of the Red River. There were $20 individuals in this population, primarily clustered along the Mountain Fork River (a specimen from here was deposited in the Herbarium Division of the Museum of Natural History at the University of Central Oklahoma). Although we sampled points north of Beavers Bend State Park prior to performing the Ecological Niche Factor Analysis, we did not find S. minor further north. The greatest density of S. minor was in southeastern McCurtain County, particularly near Red Slough Wildlife Management Area (33u44.319N, 94u40.159W) where in some areas it was the dominant understory plant with #74 mature plants/100 m 2 . Most S. minor were in low-lying deciduous forests (primarily dominated by Celtis laevigata, Nyssa sylvatica, Quercus lyrata, and Q. phellos), but some were in open fields and, at two locations, in upland stands of loblolly pine. The Southwestern Naturalist swna-56-01-10.3d 6/1/11 16:55:25 68 Cust # JB-15

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