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Plant Ecology 160: 61–78, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Community structure along elevation gradients in headwater regions of longleaf pine savannas Paul B. Drewa 1,3,*, William J. Platt 1 and E. Barry Moser 2 1
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA; 2Department of Experimental Statistics, Louisiana State University, Baton Rouge, LA 70803, USA; 3Current address: USDA-Agricultural Research Service, Jornada Experimental Range, New Mexico State University, MSC 3JER, Box 30003, Las Cruces, NM 88003, USA; *Author for correspondence (e-mail:
[email protected]) Received 10 October 2000; accepted in revised form 31 May 2001
Key words: Baygalls, Classification, Ordination, Seepage bogs, Southeastern United States, Titi/cypress swamps Abstract Savanna community structure has been described mostly at the regional level worldwide. Quantitative descriptions of vegetation patterns and relationships with substrate characteristics at more localized scales have received less attention. Our primary objective was to examine the distributions of herbs and shrubs/trees along local topographic gradients in headwater regions of longleaf pine savannas in the southeastern United States. We also examined whether herb patterns were structurally similar to those of shrubs/trees along the same topographic gradients and whether patterns were correlated with edaphic factors. Abundance data were collected within quadrats placed along transects from upslope savannas through mid-slope seepage bogs into lower-slope shrub/tree zones within Louisiana and Florida. -flexible cluster analysis and nonmetric multidimensional scaling were used to delineate herbaceous species communities. Ordination was performed separately on shrub/tree abundance data. The herb-based classifications were also used to delineate shrub/tree communities, providing an indirect means of comparing herb to shrub/tree distributions. In Louisiana, three herbaceous communities were sharply delineated along elevation gradients of several meters and were strongly correleated with soil moisture. In Florida, three similar herbaceous communities were less discrete along elevation gradients of < 1 meter. In both regions, shrub/tree distributions were much broader and appeared less sensitive than herbs to changes in environmental gradients. Coefficients of variation indicated that, in general, herbaceous species were more narrowly distributed than shrubs/trees along localized elevation gradients in both Louisiana and Florida. Alterations of fire regimes (fire suppression, dormant-season fires) may have resulted in expanded distributions of shrubs/trees, but not herbs. Introduction Across the southeastern US, the floristic composition of longleaf pine savannas has been quantitatively described almost exclusively at the regional level. Further, these vegetation patterns have been related primarily to major differences in soil texture (Harcombe et al. 1993; Peet and Allard 1993; Taggart 1994; van Kley 1999a, 1999b). Similarly, soil texture, derived from different parental materials, and climate may be the most important determinants of regional differences in savanna community structure worldwide (Solbrig 1996). Such relationships have been estab-
lished in savannas of northern Bolivia (Haase 1990), Venezuela (Sarmiento and Monasterio 1969; Sarmiento 1983), the northern Transvaal (Yeaton et al. 1986), as well as oak savannas and barrens of the American Midwest and Northeast (Heikens and Robertson 1995; Leach and Givnish 1999; Arabas 2000). Less explored, by contrast, are more localized patterns of savanna vegetation that tend to be more strongly associated with topography and geomorphology (Solbrig 1993). Despite floristic differences, structural similarities of localized plant communities may occur in longleaf pine savannas throughout the Southeast. Diverse as-
62 semblages of herb and shrub/tree species are arrayed along localized topographic gradients in many “headwater” areas, where stream drainages commence (Chapman 1932; Vogl 1973; Bridges and Orzell 1989; MacRoberts and MacRoberts 1990, 1993; Peet and Allard 1993; Platt 1999). These patterns based on floristic composition, have been qualitatively described as upslope longleaf pine savannas, mid-slope seepage bogs, and downslope evergreen shrub/tree zones (Means and Moler 1979; Clewell 1986; Brooks et al. 1993; Olson and Platt 1995). However, the structure of these plant communities has yet to be elucidated quantitatively and related to environmental factors. Southeastern longleaf pine savannas have been altered anthropogenically, affecting herbaceous and shrub/tree species in different ways. Historically, longleaf pine savannas were maintained by frequent (more than once a decade) lightning-season fires (Platt 1999). These pyrogenic systems (sensu Mutch (1970)) most often burned during rain-free periods of the early growing season (usually May-June) when high frequencies of lightning strikes were likely to ignite dry vegetation (Komarek 1974; Platt et al. 1988; Goodman and Christian 1993; Olson and Platt 1995). These natural fire regimes have been hypothesized to stimulate growth and flowering of many herbs (Platt et al. 1988, 1991; Streng et al. 1993; Brewer and Platt 1994) and reduce densities and distributions of woody species (Williamson and Black 1981; Glitzenstein et al. 1995; Olson and Platt 1995). During much of the 20th century, southeastern pine savannas have been subjected first to fire suppression and then dormant season fires (Frost 1993). These practices resulted in increased densities and expanded distributions of woody species (Heyward 1939; Platt and Schwartz 1990; Platt et al. 1991; Robbins and Myers 1992; Waldrop et al. 1992; Streng et al. 1993; Glitzenstein et al. 1995; Gilliam and Platt 1999; Drewa et al. 2002), as well as decreased abundances of herbs (Platt and Schwartz 1990; Peet and Allard 1993). Although each of the two groups can be used to describe communities quantitatively along elevation gradients, they may reveal patterns with very different structural properties. In this paper, we addressed three questions regarding the floristic composition of plant communities along localized elevation gradients in headwaters of southeastern longleaf pine savanna drainages. First, are there patterns to the distributions of herbs and shrub/tree species along localized topographic gradients? Second, are herb patterns structurally similar to
those of shrubs/trees along the same topographic gradients? Third, are herb patterns and those of shrubs/ trees correlated with ground level elevation, surfacesoil moisture, and other edaphic factors? We addressed these questions using longleaf pine savannas in east and west Gulf coastal plain regions of the Southeast where soil types and groundcover plant species composition tend to be different.
Methods Study sites The initiation of streams that drain upland regions of the southeastern coastal plain occurs along an often gentle topographic gradient (Bridges and Orzell 1989; Clewell 1986; Harcombe et al. 1993; Peet and Allard 1993). Similar changes in vegetation along these gradients have been described in many southeastern longleaf pine savannas (Figure 1). Upslope regions are characterized by porous, weathered soils that drain readily and generally do not flood. Seepage bogs form downslope in areas where water seeps onto hillsides from small underground, often intermittent aquifers. These aquifers are recharged by rainwater that percolates through sandy, upslope savanna soils and moves down a sloping gradient created by a subterranean layer of impermeable clay (Martin and Smith 1991). Eventually, water seeps laterally out of the hillsides where the clay layer is near the surface (Olson and Platt 1995). Downslope from seepages, soils of evergreen shrub/tree zones are often saturated. Here, intermittent streams coalesce into larger, more permanent drainages (Means and Moler 1979). Along these more permanent drainages, upslope savannas grade directly into bottomland hardwood forests (Clewell 1986; Bridges and Orzell 1989). Here, seepage bogs are absent. We selected sites that included mid-slope areas where streams are intermittent and first initiated (i.e., that include seepage bogs and evergreen shrub/tree zones) and thus constitute true headwater regions, which contain unique assemblages of often rare and endemic species (Bridges and Orzell 1989; Clewell 1986; Peet and Allard 1993) and have yet to be quantitatively characterized at localized scales. In the western Gulf coastal plain region, we selected sites in the Vernon District of the Kisatchie National Forest, Vernon Parish, Louisiana (latitude 31° N, longitude 93° W; 75–90 m above sea level).
63
Figure 1. Diagrammatic representation of plant communities arrayed along topographic gradients in the headwater drainages of a) Louisiana and b) Florida longleaf pine savannas: 1) upslope longleaf pine savannas, 2) seepage bogs, and 3) evergreen shrub/ tree zones. Hydric conditions in bogs and evergreen shrub/tree zones are created when rain water percolates down through sandy soils, moves laterally along an impermeable layer of clay, and surfaces. In our study areas of Louisiana and Florida, evergreen shrub/ tree zones are called “baygalls” and “titi/cypress swamps” respectively. Diagrams are based on past, empirical studies (e.g., Wolfe et al. (1988)) and our field observations, but are not intended to illustrate a 50% tree cover of Pinus palustris, characteristic of longleaf pine savannas.
This region contains Tertiary and Quaternary sediments, superimposed on Paleozoic and Precambrian rock (Murray 1961; Hart and Lester 1993). Upslopehillside longleaf pine savannas are characterized by rolling topography and well drained, highly weathered, nutrient poor, sandy loams (Martin and Smith 1991; Hart and Lester 1993). Soils are classified as Arenic Paleudults, Typic and Vertic Hapludalfs, or Typic Albaqualfs (Bridges and Orzell 1989; Hart and Lester 1993). Osier series (Entisol) soils are patchily distributed and associated with mid-slope seepage bogs and evergreen shrub/tree zones called “baygalls”
(Figure 1a). In addition, soils of the Osier series are fine sandy loams that are poorly drained, nutrient poor, and strongly acidic (Martin and Smith 1991; Hart and Lester 1993). In the eastern Gulf coastal plain region, our study sites were located in the Panacea Unit of the St. Marks National Wildlife Refuge, Wakulla County, Florida (latitude 30° N, longitude 84.5° W; 1–2 m above sea level). Here, sediments are old Pleistocene stream channels of the Ochlockonee River and are characterized by low, flat topography with waterlogged, well-leached, acidic, sandy soils classified as Humaqueptic Psamments or Aquic Quartzipsamments, usually of the Scranton and Ridgewood series (Clewell 1986; Glitzenstein et al. 1995). Soils in midslope seepage bogs are similar to upslope flatwoods but have a longer hydroperiod. In lower-slope evergreen shrub/tree zones or “titi/cypress swamps,” soils, classified as Humaquepts, Haplaquods, and Psammaquents of the Osier and Rutledge series, are poorly drained and sandy (Figure 1b) (Coultas et al. 1979; Clewell 1986). Different suites of species are associated with these three habitats in Louisiana as well as Florida (Clewell 1986; Wolfe et al. 1988; Bridges and Orzell 1989). Longleaf pine (Pinus palustris Mill.) is commonly the dominant overstory tree in the uplands and seepages. Grasses, especially Aristida beyrichiana Trin. & Rupr. or Schizachyrium scoparium (Michx.) Nash, tend to be most abundant in upslope longleaf pine savannas, but there also are large numbers of forb species as well as species of trees and shrubs in the ground cover. Seepage bogs contain a rich herbaceous flora that includes species of Sarracenia, Pinguicula, and Eriocaulon (Folkerts 1982; Wolfe et al. 1988). Evergreen shrub/tree zones are composed mainly of woody species, such as Taxodium distichum (L.) L.C. Rich, Cyrilla racemiflora L., Magnolia virginiana L., and llex coriacea L. (Clewell 1986; Brooks et al. 1993). Our study areas share a history of anthropogenic effects. Though large pines in both study areas were removed about 60 years ago, soils were not appreciably disrupted (Platt et al. 1988; Olson and Platt 1995). Species composition of the ground cover resembles that in extant patches of old growth longleaf pine savannas (Wells and Shunk 1931; Platt et al. 1988; Hart and Lester 1993; Streng et al. 1993). Both areas were subjected to fire suppression in the 1930s – 1940s, and both have been intermittently burned in the winter during the latter half of the 20th century.
64 Data collection
Statistical analyses
Vegetation abundance data were collected along elevation gradients in 16 plots of 30 × 80 m, eight in the Louisiana study site and eight in the Florida study site. Within each plot, we ran two transects, parallel to a plot’s long axis, from upslope longleaf pine savannas down into seepage bogs and evergreen shrub/ tree zones, and a quadrat was located every 10 meters. In total, we positioned eight quadrats along each transect. Starting points of the two transects were randomly selected along the top edge or short axis of each plot located in an upslope longleaf pine savanna. Quadrats were 1 m × 1 m in uplands and seepage bogs and 1.5 m × 1.5 m in evergreen shrub/tree zones. Within each quadrat, all herbaceous species and vines were identified and assigned values of 1, 2, 3, 4, or 5 designating their occurrence in one of five aerial coverage classes (< 5%, 5–25%, 26–50%, 51–75%, and > 75% respectively). We also identified woody species, and their abundance was determined by counting stems all of which were < 2 cm diameter. In evergreen shrub/tree zones, we employed a larger quadrat size to accommodate the wider spatial distribution of almost exclusively woody stems. Data from these quadrats were standardized to 1 m 2 prior to any statistical analyses to minimize our influence of identifying evergreen shrub/tree zones a priori. We identified 195 species in Louisiana and 148 species in Florida. Nomenclature followed Clewell (1985); Godfrey (1988); Godfrey and Wooten (1981); Peet (1993); Radford et al. (1981). Environmental data were collected from a subset of the quadrats in our study sites. For each study area of Louisiana and Florida, one of the two transects within each of four plots was randomly selected. Relative elevations (to the lowest quadrat location in meters and accurate to ± 0.5 cm) were measured using a laser leveller (Topcon Rotating Laser RLH1SBT). Also, a 15 cm × 15 cm × 15 cm soil sample was randomly collected no more than 1 m away from each quadrat during the fall season. Soil samples were weighed, dried at 80 °C for five days in an oven, and reweighed to determine moisture content. Each soil sample was passed through a 2 mm sieve to remove vegetation debris. Soil samples were individually analyzed for soil texture (% sand and clay) using the hydrometer method (Bouyoucos 1951), soil pH using a 2:1, distilled water:soil ratio, and organic matter using a muffle furnace (Rhoades 1982).
Classification and ordination were used to explore patterns in the vegetation. Our raw data were organized into four matrices comprised of the following: 1) 128 quadrat samples × 166 Louisiana herbaceous species, 2) 108 quadrat samples × 29 Louisiana shrub/ tree species, 3) 110 quadrat samples × 114 Florida herbaceous species, and 4) 92 quadrat samples × 34 Florida shrub/tree species. The three latter matrices were comprised of fewer than the maximum number of quadrats (128) because we did not encounter these species groups in every quadrat. For each data matrix, a dissimilarity matrix, using a Bray-Curtis distance metric (Bray and Curtis 1957) was generated using PROC IML in SAS (Version 6.11; SAS Institute 1989). -flexible cluster analysis ( = −0.25) (Lance and Williams 1967) was then performed seperately on the two dissimilarity matrices based on Louisiana and Florida herbaceous species abundance data using PROC CLUSTER. -flexible cluster analysis contains an agglomerative hierarchical algorithm (for algorithm explanation see Scheibler and Schneider (1985); Milligan (1989); Everitt (1993)). The -flexible method ( = −0.25), recommended by Lance and Williams (1967), was demonstrated to recover the underlying clustering structure of artificial data sets containing only a few outliers (Milligan 1989). Beginning with a random configuration of quadrat samples that served as a starting point to find a configuration of minimum stress, 10 runs of nonmetric multidimensional scaling (NMMDS) were performed on each of the four dissimilarity matrices using PROC MDS. For each dissimilarity matrix, we constructed an ordination diagram using dimension scores from that configuration with the lowest stress. A configuration of minimum stress is defined as the most accurate diagrammatic representation of distances between quadrats, based on a Bray-Curtis dissimilarity matrix (Kruskal and Wish 1978; Kenkel and Orlóci 1986). All configurations were of two dimensions because stress values were not appreciably different at three or more dimensions. To assist us in interpreting vegetation patterns, Pearson correlation coefficients were calculated between dimension scores for each ordination diagram separately and the abundance data for each species used to generate them (PROC CORR). We used cluster analysis results to delineate communities on ordination diagrams. For each geographic
65 region, a dendrogram cut-level of three clusters, based on herbaceous species data, was used as a quantitative guideline for grouping quadrats on an ordination diagram based on herb abundance data, as well as on an ordination diagram based on shrub/tree abundance data. This procedure was similar to those by van der Maarel (1979); Bradfield and Porter (1982); Lewis (1991); Zhang and Skarpe (1996). We selected cut-levels of three because internodes of both dendrograms tended to be much longer compared to other levels, suggesting an underlying clustering structure. Also, imposing a cut-level of three groups on ordination diagrams permitted not only a comparison between our data and qualitative designations in the literature (e.g., Wolfe et al. (1988)), but also an indirect comparison between herb and woody species distributions along environmental gradients. Relationships between vegetation patterns and environmental data were examined using Pearson correlation. For each of the four NMMDS ordinations, dimension scores were correlated with the corresponding raw environmental data. In these analyses, we used scores for only those quadrat samples where soil characteristic and elevation data were collected. We selected Bray-Curtis distance and NMMDS from among other alternatives because they are robust to deviations from underlying ordination model assumptions of species distributions in ecological space. This combination has successfully recovered intrinsic community properties using artificial and natural data sets (Faith et al. 1987; Ludwig and Reynolds 1988). Further, -flexible cluster analysis and NMMDS were used instead of other methods such as TWINSPAN because we were uncomfortable with the assumption that all encountered species would be normally distributed along environmental gradients using the Chisquare distance metric. Also, we considered the definition of cut levels using pseudospecies as too arbitrary a method for delineating communities. Canonical correspondence analysis was not used to examine vegetation-environment relationships because it also uses chi-square distance where species are assumed to be distributed normally along environmental gradients. We more directly compared the degree to which herbs were distributed along elevation gradients as opposed to shrub/tree species. For each study area of Louisiana and Florida, quadrats were divided into three groups based on the -flexible cluster analysis that was performed on herbaceous species abundance data. However, only those quadrats where we col-
lected elevation data were included. For each of the three clusters of quadrats, the coefficient of variation was calculated using relative elevation data. Similarly, we performed cluster analysis on our shrub/tree data after which these same procedures were repeated. Here, high and low coefficient of variation values indicate that species are respectively distributed more broadly and narrowly along elevation gradients. Statistical analyses were performed on vegetation data using quadrats as the observation unit. Quadrats in our study were of suitable size to detect subtle changes in vegetation composition along localized topographic gradients. Given the multi-scaled sampling design, we could have used transects and plots as units of observation also. However, they would have been too large to detect these subtle changes in vegetation composition within sites. Our sampling design was part of a larger experiment used to examine fire effects on shrubs in longleaf pine savannas in Louisiana and Florida (Drewa 1999; Drewa et al. 2002).
Results Louisiana Herbs Using Louisiana herbaceous and vine species abundance data, cluster analysis and NMMDS revealed three distinct groups of quadrats corresponding to upslope-hillside longleaf pine savanna, mid-slope seepage bog, and lower-slope baygall communities (Figure 2a). In Table 1, abundances of one group of species were strongly negatively correlated with the first ordination dimension. Moreover, Dicanthelium aciculare Desvaux ex Poiret, Diodia teres Walt., Helianthus angustifolius L., Panicum ovale Ell., Rhynchospora globularis (Chapm.) Small, Schizachyrium scoparium, Schizachyrium tenerum Nees, Solidago odora Aiton, and Stylosanthes biflora (L.) BSP. were most abundant in upslope pine savannas. A second group of species occurred almost exclusively and thus were most abundant in seepage bogs. This group was weakly correlated with dimension 1 but strongly negatively correlated with dimension 2 and included Centella asiatica (L.) Urban, Coreopsis linifolia Nutt., Ctenium aromaticum (Walt.) Wood., Drosera brevifolia Pursh, Hedyotis nigricans (Lam.) Fosberg, Lobelia puberula Michx., Marshallia tenuifolia Raf., Panicum dichotomum (L.) Gould, Rhexia lutea Walt.,
66 Rhynchospora oligantha Gray, Sarracenia alata Wood., Scleria reticularis Michx., and Xyris ambigua Bey. ex Kunth. Species that were abundant both in upslope pine savannas and seepage bogs included Aristida purpurascens Poiret, Aster dumosus L., Eupatorium rotundifolium L., and Muhlenbergia capillaris (Lam.) Trinius. They were not strongly associated with either dimension. Only Smilax laurifolia L., abundant in baygalls, was strongly positively correlated with dimension 1. Abundance of Sphagnum spp. was weakly, positively correlated with the first and strongly, negatively correlated with the second dimension, suggesting that it occured in baygalls, but was more abundant in seepage bogs. Shrub/trees Many shrub/tree species were more broadly distributed than herbs (Figure 2b). After imposing the same herb-defined dendrogram cut-level of three groups to delineate communities on an ordination of shrub/tree abundance data, quadrats were not tightly clustered in shrub/tree ordination space (cf. Figure 2a). Further, shrubs/trees were weakly correlated with sample scores of NMMDS dimensions 1 and 2, many of which were abundant in more than one community (Table 2). Hypericum hypericoides (L.) Crantz, Liquidambar styraciflua L., Quercus incana Bartram, Rhus copallina L., Vaccinium arboreum Marsh., and Vaccinium stamineum L. were negatively associated with dimension 1. Almost all occurred exclusively in upslope longleaf pine savannas. Myrica cerifera L., Rubus spp., and Vaccinium elliotti Chapm. were negatively correlated with the first dimension also but were present in all three communities. Further, Myrica cerifera was negatively correlated with the second dimension and equally abundant in upslope pine savannas and mid-slope seepage bogs. Rubus spp., poorly correlated with the second dimension, was > 2.5 times more abundant in uplands than in bogs while Vaccinium elliottii was mostly present in upslope pine savannas. All tree and shrub species in seepage bogs, except Hypericum brachyphyllum (Spach) Steud., were also encountered either in upslope pine savannas or downslope baygalls. Acer rubrum L. and Hypericum crux-andreae (L.) Crantz were weakly positively and negatively correlated with dimensions 1 and 2 respectively; though present in upslope savannas and baygalls, they were more abundant in bogs. Additionally, Alnus serrulata (Aiton) Willd. and llex coriacea, poorly correlated with dimensions 1 and 2, were bet-
Figure 2. NMMDS ordination diagrams based on abundance data of a) Louisiana herbaceous species and b) Louisiana shrub/tree species. -flexible cluster analysis was performed on herbaceous species data to generate a dendrogram from which a cut-level of three clusters was used to delineate communities on both ordinations: (1) upslope-hillside longleaf pine savannas, (2) mid-slope seepage bogs, and (3) lower-slope baygalls. NMMDS, performed on herb and shrub/trees data, yielded stress values of 10.80% and 11.47% respectively. Numerals represent individual quadrats from which herb cover and shrub/tree density data were collected.
ter represented in seepage bogs than in baygalls. Myrica heterophylla Raf. and Pyrus arbutifolia (L.) Aiton were abundant at mean densities of > 3 stems/m 2 and > 2 stems/m 2 in seepages and baygalls respectively. Both species were positively correlated with the first but not the second dimension. Several species, including llex opaca Aiton, Lyonia lucida
67 Table 1. Mean aerial class coverage of prominent (overall frequency ⭓ 20%) herbaceous species in three Louisiana community types. Pearson correlation was used to quantify relationships between herbaceous species abundance data and NMMDS dimensions 1 (Dim1) and 2 (Dim2) sample scores. Species
Aristida purpurascens Poiret Aster dumosus L. Centella asiatica (L.) Urban Coreopsis linifolia Nutt. Ctenium aromaticum (Walt.) Wood. Dichanthelium aciculare Desvaux ex Poiret Diodia teres Walt. Drosera brevifolia Pursh Eupatorium rotundifolium L. Hedyotis nigricans (Lam.) Fosberg Helianthus angustifolius L. Lobelia puberula Michx. Marshallia tenuifolia Raf. Muhlenbergia capillaris (Lam.) Trinius Panicum dichotomum (L.) Gould Panicum ovale Ell. Rhexia lutea Walt. Rhynchospora globularis (Chapm.) Small Rhynchospora oligantha Gray Sarracenia alata Wood. Schizachyrium scoparium (Michx.) Nash Shizachyrium tenerum Nees Scleria pauciflora Muhl. ex Willd. Scleria reticularis Michx. Smilax laurifolia L. Solidago odora Aiton Sphagnum spp. Stylosanthes biflora (L.) BSP. Xyris ambigua Bey. ex Kunth
Community Type
Pearson correlation coefficients
Pine savanna
Seepage bog
Baygall
Mean n=60 SE
Mean n=41 SE
Mean n=27 SE
Dim1
Dim2
0.51 0.62 0.20 0 0.07 0.40 0.80 0.18 0.73 0.28 1.33 0.08 0.08 0.52 0.12 0.60 0.13 0.45 0.05 0 2.12 1.40 0.18 0.08 0.05 0.62 0.03 0.68 0.08
0.63 0.44 1.07 1.00 1.39 0.02 0 0.76 0.22 0.76 0.71 0.71 0.73 0.78 1.37 0.17 0.68 0.02 1.71 1.78 0.12 0.29 0.46 1.41 0.44 0 1.27 0 0.59
0 0 0.04 0 0 0 0 0 0 0 0 0.04 0 0 0.11 0 0 0 0.15 0.11 0 0 0 0 1.11 0 0.37 0 0
−0.34 −0.33 0.04 0.11 0.02 −0.56 −0.51 −0.05 −0.41 −0.06 −0.49 0.11 0.02 −0.24 0.14 −0.46 −0.01 −0.41 0.14 0.16 −0.70 −0.44 −0.04 0.08 0.74 −0.50 0.27 −0.62 0.03
−0.31 −0.04 −0.61 −0.46 −0.61 0.45 0.28 −0.66 0.24 −0.38 0.15 −0.55 −0.50 −0.38 −0.58 0.08 −0.56 0.13 −0.66 −0.64 0.41 0.23 −0.29 −0.64 0.11 0.42 −0.49 0.44 −0.50
0.08 0.08 0.06 0 0.05 0.08 0.10 0.05 0.09 0.06 0.11 0.04 0.04 0.10 0.05 0.09 0.04 0.08 0.03 0 0.13 0.16 0.06 0.04 0.03 0.08 0.02 0.06 0.04
(Lam.) K. Koch, Nyssa sylvatica Marsh., and Vaccinium arkansanum Ashe., were found almost exclusively in baygalls. Vegetation-environment relationships In general, first dimensions of Louisiana herb and shrub/tree ordinations were most strongly correlated with elevation and moisture (Table 3). In addition, surface soil moisture was negatively associated with the second dimension for herbs but positively correlated for shrubs/trees. Over a mean 2.5 m elevation gradient, soil moisture was > 35% greater in baygalls than upslope savannas (Table 4). Other surface soil characteristics were associated with the first dimen-
0.10 0.08 0.08 0.14 0.21 0.02 0 0.07 0.07 0.09 0.13 0.07 0.12 0.16 0.13 0.08 0.08 0.02 0.14 0.21 0.05 0.09 0.10 0.16 0.10 0 0.18 0 0.09
0 0 0.04 0 0 0 0 0 0 0 0 0.04 0 0 0.06 0 0 0 0.09 0.06 0 0 0 0 0.06 0 0.12 0 0
sions of the NMMDS ordinations. Organic matter was positively and pH negatively associated with dimension 1, both for herbs as well as shrubs/trees (Table 3). Over the elevation gradient, organic matter increased by about 10%, and pH decreased from 4.5 to 4.1 (Table 4). Differences in surface soil texture, and to a lesser extent organic matter and pH, may have also influenced herbaceous species distributions. Compared to shrubs/trees, dimension 1 for herbs was more negatively associated with percent sand (Table 3) where surface soil sand content was lower in baygalls than upslope savannas (Table 4). Also, the second dimension for herbs was more negatively associated with
68 Table 2. Mean density (stems/m 2) of tree and shrub species in each of three Louisiana community types. Pearson correlation was used to quantify relationships between shrub/tree species abundance data and NMMDS dimensions 1 (Dim1) and 2 (Dim2) sample scores. Species
Community Type Pine savanna
Acer rubrum L. Alnus serrulata (Aiton) Willd. Callicarpa americana L. Hypericum brachyphyllum (Spach) Steud. Hypericum crux-andreae (L.) Crantz Hypericum hypericoides (L.) Crantz Ilex coriacea L. Ilex opaca Aiton Liquidambar styraciflua L. Lyonia lucida (Lam.) K. Koch Magnolia virginiana L. Myrica cerifera L. Myrica heterophylla Raf. Nyssa sylvatica Marsh. Persea borbonea (L.) Sprengel Pinus taeda L. Pyrus arbutifolia (L.) Aiton Quercus incana Bartram Quercus laurifolia Michx. Rhododendron spp. Rhus copallina L. Rubus spp. Toxicodendron vernix (L.) Kuntze Unknown Vaccinium arboreum Marsh. Vaccinium arkansanum Ashe. Vaccinium elliottii Chapm. Vaccinium stamineum L. Viburnum nudum L.
Pearson correlation coefficients Seepage bog
Baygall
Mean n=44
SE
Mean n=37
SE
Mean n=27
SE
Dim1
Dim2
0.24 0 0 0 0.33 0.50 0 0 0.28 0 1.06 3.78 0.07 0 0 0.04 0.13 0.09 0 0 0.57 2.74 0 0 1.07 0.13 1.48 0.35 0
0.15 0 0 0 0.13 0.19 0 0 0.21 0 0.92 1.18 0.05 0 0 0.04 0.13 0.09 0 0 0.24 0.89 0 0 0.42 0.13 1.04 0.35 0
0.60 0.24 0 0.08 5.35 0.03 11.83 0 0 0 3.55 3.38 6.61 0 0.55 0.03 3.77 0 0 0 0 1.05 0.05 0 0 0 0.04 0 0
0.18 0.18 0 0.06 3.64 0.03 9.52 0 0 0 1.19 1.55 1.24 0 0.46 0.03 1.44 0 0 0 0 0.75 0.05 0 0 0 0.04 0 0
0.23 0.15 0.05 0 0 0 4.71 0.05 0 1.69 0.31 0.02 4.28 0.77 0.61 0.03 2.85 0 0.02 2.78 0 0.33 0.02 0.13 0 0.90 0.07 0 0.20
0.06 0.08 0.05 0 0 0 1.41 0.03 0 0.71 0.18 0.02 0.93 0.28 0.13 0.02 0.84 0 0.02 1.67 0 0.16 0.02 0.07 0 0.25 0.07 0 0.16
0.22 0.01 0.08 0.11 0.08 −0.52 0.08 0.10 −0.12 0.20 0.18 −0.17 0.41 0.14 0.09 0.04 0.30 −0.21 0.03 0.08 −0.20 −0.12 0.12 0.15 −0.37 0.12 −0.14 −0.08 0.05
−0.16 −0.05 0.04 −0.03 −0.16 0.01 −0.06 0.11 −0.12 0.20 −0.27 −0.33 −0.03 0.24 0.08 0.09 0.05 0.18 0.03 0.16 −0.06 0.05 0.02 0.27 0.24 0.33 −0.05 0.03 −0.01
Table 3. Relationships using Pearson correlation between environmental variables, dimension 1 (Hdim1) and 2 (Hdim2) sample scores from NMMDS performed on Louisiana herbaceous species abundance data, and dimension 1 (SDim1) and 2 (Sdim2) sample scores from NMMDS performed on Louisiana shrub/tree species abundance data. SOM, Clay, Sand = surface soil organic matter, clay and sand content respectively; EL = relative ground level elevation; Moist = surface soil moisture.
SOM Clay pH Sand Moist EL
Hdim1
Hdim2
Sdim1
Sdim2
SOM
Clay
pH
Sand
Moist
EL
0.52 0.08 −0.58 −0.42 0.77 −0.84
−0.03 0.36 0.42 −0.35 −0.31 0.27
0.29 −0.16 −0.73 −0.01 0.61 −0.65
0.45 0.12 −0.10 −0.26 0.44 −0.37
1 −0.16 −0.47 −0.29 0.76 −0.46
1 −0.01 −0.42 −0.04 −0.01
1 0.22 −0.58 0.54
1 −0.28 0.44
1 −0.75
1
sand and more positively associated with clay than shrubs/trees. Bog soils possessed slightly less clay
and more sand than surface soils in either upslope savannas or baygalls (Table 4).
69 Table 4. Summarized surface soil characteristic and relative ground level elevation data for each of three Louisiana community types. Community Type Pine savanna
Clay (%) Elevation(m) Moisture (%) Soil Organic Matter (%) pH Sand (%)
Seepage bog
Baygall
Mean n=14
SE
Mean n=10
SE
Mean n=8
SE
8.82 2.64 3.16 3.05 4.49 69.19
0.62 0.21 1.06 0.24 0.08 1.41
7.50 1.19 25.78 4.33 4.15 71.79
0.68 0.17 4.11 1.08 0.05 2.63
9.38 0.17 40.27 12.95 4.06 57.22
1.13 0.10 7.37 4.50 0.06 6.39
Florida Herbs Based on Florida herb abundance data, three groups of quadrats were delineated using cluster analysis and NMMDS that corresponded to upslope flatwoods longleaf pine savanna, mid-slope seepage bog, and lower-slope titi/cypress swamp communities (Figure 3a). These groups were less clearly defined along both dimensions than were Louisiana herbaceous communities (cf. Figure 2a). Abundances of common Florida herbaceous species were not strongly correlated with NMMDS dimensions 1 and 2 (Table 5). The only species with a more negative correlation with the first dimension was Sporobolus floridanus Chapm., which was considerably more abundant in pine savannas than seepage bogs or titi/cypress swamps. Aristida beyrichiana and Rhexia alifanus Walt. also were abundant in pine savannas, but were poorly correlated with NMMDS ordination dimensions because they were more abundant in seepage bogs. Moderate positive correlations with the first dimension occurred in several species: Centella asiatica Eriocaulon compressum Lam., Rhexia nuttallii/petiolata Walt., and Smilax auriculata Walt. Of these, Centella asiatica and Rhexia nuttallii/petiolata were more abundant in seepage bogs than titi/cypress swamps. A number of species were present in more than one habitat but more abundant in seepage bogs: Andropogon virginicus L., Aristida beyrichiana, Ctenium aromaticum, Drosera capillaris Poiret, Lachnocaulon anceps (Walt.) Morong., Lophiola americana (Pursh.) Wood., Polygala lutea L., and Rhynchospora chapmannii M. A. Curtis. Herbaceous ground cover was greater in Florida titi/cypress swamps than Louisiana baygalls. Eriocaulon compressum and Smilax auriculata were most abundant in swamps, as was Sphagnum spp.
Shrubs/trees Shrub/tree species were more broadly distributed than herbs in Florida (Figure 3b). Similar to Louisiana, quadrats were not well clustered in shrub/tree ordination space after imposing the same herb-defined dendrogram cut-level of three groups. Several shrub/tree species were abundant across community types and were more broadly distributed than any Florida herbs (cf. Figure 3a). Abundances of shrubs/trees generally were not well correlated with NMMDS dimension sample scores, suggesting much compositional variation within and between communities (Table 6). Gaylussacia dumosa (Andr.) A. Gray, Gaylussacia frondosa (L.) T. & G., llex glabra (L.) A. Gray, Quercus minima (Sarg.) Small, and Quercus pumila Walt. were generally negatively associated with dimension 1 and either more weakly or not associated with dimension 2. They were generally more abundant in upslope savannas and less so in seepage bogs. A second group was more weakly associated with dimension 1. Clethra alnifolia L., Gaylussacia mosieri (Small) Small, Myrica cerifera, and Pyrus arbutifolia were almost always present in all three communities, especially upslope savannas. Only three species, Vaccinium arboreum, Vaccinium darrowii Camp, and Vaccinium myrsinites Lam., were found exclusively in upslope pine savannas. Similar to the other Vaccinium species, Vaccinium corymbosum L. was negatively associated with the first dimension but was more abundant in mid-slope seepages than upslope savannas. No shrub/tree species was encountered exclusively in mid-slope seepage bogs. Though Hypericum microsepalum (T. & G.) Gray ex Wats. was abundant in seepages, it was almost equally present in upslope savannas (8–9 stems/m 2). Several species, including Cyrilla racemiflora, Hypericum brachyphyllum, and Magnolia virginiana, were abundant in both bogs and
70 and 2, suggesting compositional variation within swamps. Vegetation-environment relationships First dimensions of Florida herb and shrub/tree ordinations were most strongly associated with differences in elevation and surface soil moisture (Table 7). Over a mean 0.5 m elevation gradient from upslope flatwoods pine savannas to titi/cypress swamps, mean surface soil moisture increased > 18% (Table 8). The only other surface soil characteristic well correlated with NMMDS dimensions for both herbs and shrubs/ trees was pH which was positively associated with dimension 1 for shrubs/trees as well as herbs (Table 7). Surface soil pH increased from 4.1 in upslope savannas to 4.6 in titi/cypress swamps (Table 8). Soil texture (% sand and clay) was weakly associated with dimension 1 for herbs but more strongly for shrubs/ trees (Table 7). Sand and clay content was only slightly lower and higher respectively in seepages than in the other two communities (Table 8). Directions of change in texture and pH along Florida elevation gradients were opposite to those in Louisiana (cf. Table 4). Louisiana and Florida herb-shrub/tree distributions
Figure 3. NMMDS ordination diagrams based on abundance data of a) Florida herbaceous species and b) Florida shrub/tree species. -flexible cluster analysis was performed on herbaceous species data to generate a dendrogram from which a cut-level of three clusters was used to delineate communities on both ordinations: (1) upslope-hillside longleaf pine savannas, (2) mid-slope seepage bogs, and (3) lower-slope titi/cypress swamps. NMMDS, performed on herb and shrub/tree data, yielded stress values of 16.50% and 14.77% respectively. Numerals represent individual quadrats from which herb cover and shrub/tree density data were collected.
titi/cypress swamps. They tended to be weakly positively associated with dimensions 1 and 2. Several species (Cliftonia monophylla (Lam.) Britt. Ex Sarg., llex myrtifolia Walt., Pieris phillyreifolius (Hook.) Small, and Taxodium distichum) were found only in titi/cypress swamps. These species were either weakly or negatively correlated with dimensions 1
Shrubs/trees were distributed differently than herbs in both Louisiana and Florida (Figure 4). In upslope longleaf pine savannas of both geographic regions, the coefficient of variation was larger for shrubs/trees than herbs suggesting that the former were distributed along a wider elevation range, extending from xeric uplands down into more hydric seepage habitats. Similarly, trees and shrubs common to Florida mid-slope seepage bogs occurred downslope in titi/cypress swamps. In contrast, the coefficient of variation was less for Florida herbs suggesting that they were distributed along a narrower elevation range in seepages. This was also the case for herbs in Louisiana seepage bogs where the coefficient of variation was low. However, Louisiana shrubs/trees were not common to seepage bogs. Instead, an anomalous group of two quadrats, comprised almost exclusively of Myrica cerifera, was located in xeric upland areas. Thus, the coefficient of variation for this unique, additional upland shrub/tree-defined community was low (0.04). No quadrats were associated with mid-slope seepage habitats based on shrub/tree abundance data. In evergreen shrub/tree zones of Louisiana and Florida, the coefficient of variation was large for shrubs/trees.
71 Table 5. Mean aerial class coverage of prominent (overall frequency ⭓ 20%) herbaceous species in three Florida community types. Pearson correlation was used to quantify relationships between herbaceous species abundance data and NMMDS dimensions 1 (Dim1) and 2 (Dim2) sample scores. Species
Community Type Pine savanna
Andropogon virginicus L. Aristida beyrichiana Trin. & Rupr. Centella asiatica (L.) Urban Ctenium aromaticum (Walt.) Wood Drosera capillaris Poiret Eriocaulon compressum Lam. Lachnocaulon anceps (Walt.) Morong. Lophiola americana (Pursh) Wood. Panicum spp. Polygala lutea L. Rhexia alifanus Walt. Rhexia nuttallii/petiolata Walt. Rhynchospora chapmannii M.A. Curtis Smilax auriculata Walt. Sphagnum spp. Sporobolus floridanus Chapm. Xyris ambigua Bey. ex Kunth
Pearson correlation coefficients Seepage bog
Titi/cypress swamp
Mean n=30
SE
Mean n=44
SE
Mean n=36
SE
Dim1
Dim2
0.13 1.33 0 0.20 0.03 0 0.20 0.10 0.17 0.27 0.67 0.17 0.13 0.13 0 0.97 0
0.06 0.20 0 0.09 0.03 0 0.07 0.06 0.07 0.08 0.10 0.07 0.06 0.06 0 0.23 0
0.80 2.77 0.34 0.89 0.75 0.43 0.89 0.84 0.32 0.75 0.89 0.89 1.61 0.09 0.32 0.34 0.34
0.10 0.16 0.08 0.13 0.07 0.14 0.11 0.15 0.08 0.11 0.05 0.09 0.11 0.04 0.09 0.13 0.09
0.24 0.68 0.27 0 0.16 1.32 0 0.24 0.14 0.14 0.24 0.35 0.08 0.41 0.97 0.03 0.32
0.07 0.17 0.09 0 0.06 0.22 0 0.09 0.06 0.07 0.08 0.10 0.05 0.09 0.22 0.03 0.09
0.15 −0.01 0.26 −0.11 0.11 0.27 −0.10 0.16 −0.02 −0.04 −0.07 0.30 −0.02 0.36 0.16 −0.17 0.18
0.04 0.03 −0.17 0.08 −0.01 0.16 −0.04 0.01 −0.10 −0.18 −0.01 −0.13 −0.05 0.01 −0.06 −0.16 0.11
However, it was even larger for herbs suggesting that their distributions were even broader.
Discussion Three herbaceous groundcover communities were similarly arrayed along localized elevation gradients in Louisiana and Florida. In Louisiana upslope longleaf pine savannas, Schizachyrium scoparium and forbs such as Helianthus angustifolius and Solidago odora were abundant. In Florida uplands, Sporobolus floridanus, Aristida beyrichiana, and Rhexia alifanus were dominant. These results are similar to past descriptions of other upslope longleaf pine savannas located in western and eastern regions of the Gulf coastal plain (Grelen and Duvall 1966; Peet and Allard 1993; Platt 1999). In our study, composition of mid-slope seepages was also similar to past descriptions (e.g., Bridges and Orzell (1989); MacRoberts and MacRoberts (1993); Peet and Allard (1993)). In Louisiana, Panicum dichotomum and several forbs (e.g., Coreopsis linifolia, Marshallia tenuifolia, and Sarracenia alata) were abundant. In Florida, Aristida beyrichiana, Polygala lutea, and Rhynchospora chap-
mannii were among several species common to seepages. Ctenium aromaticum, Centella asiatica, and Xyris ambigua were the only abundant species found in bogs of both Louisiana and Florida; bog composition tended to be different between the two geographic regions. In contrast to narrower herb distributions and thus lower coefficients of variation in uplands and seepages, herbs were more broadly distributed in downslope evergreen shrub/tree zones. As with past, qualitative descriptions in Louisiana and Florida (Coultas et al. 1979; Clewell 1986; Martin and Smith 1991), herbaceous species such as Eriocaulon compressum, Smilax auriculata, and Sphagnum spp. were present in seepage bogs as well as in wetter, downslope communities. In general, herb distributions were strongly associated with elevation and soil moisture in both Louisiana and Florida. Regional descriptions have suggested that soil moisture, governed by differences in soils and elevation, influences community composition (Harper 1914; Wells 1932, 1942; Abrahamson et al. 1984; Christensen 1988; Abrahamson and Hartnett 1990; Platt and Schwartz 1990; Harcombe et al. 1993; Peet and Allard 1993; van Kley 1999a, 1999b). Additionally, steepness of elevation gradients may have
72 Table 6. Mean density (stems/m 2) of tree and shrub species in each of three Florida community types. Pearson correlation was used to quantify relationships between shrub/tree species abundance data and NMMDS dimensions 1 (Dim1) and 2 (Dim2) sample scores. Species
Community Type
Pearson correlations coefficients
Pine savanna
Seepage bog
Titi/Cypress swamp
Mean
Mean
Mean
SE
n=23
SE
n=35
SE
Dim1
Dim2
n=34
Clethra alnifolia L. Cliftonia monophylla (Lam.) Britt ex
27.78 0
17.59 0
0.53 0
0.45 0
4.16 2.94
1.24 1.51
−0.08 −0.04
−0.11 0.20
Sarg. Cyrilla racemiflora L. Gaylussacia dumosa (Andr.) A. Gray Gaylussacia frondosa (L.) T. & G. Gaylussacia mosieri (Small) Small Hypericum brachyphyllum (Spach)
0 86.00 52.89 9.56 0
0 30.57 16.64 4.70 0
8.48 12.03 10.35 7.38 7.45
3.15 4.96 6.29 3.69 3.08
6.08 0 0 0.22 1.46
2.54 0 0 0.15 0.78
0.17 −0.27 −0.36 −0.10 0.04
0.08 −0.17 −0.06 0.02 0.18
0 0 8.67
0 0 4.53
0 0 9.80
0 0 2.64
0.03 0.09 0
0.03 0.09 0
−0.02 −0.13 −0.08
0.20 0.02 −0.02
5.56 33.56 0 0 0 0 2.22 0 2.00 3.78 0 0 0 0.22 9.11 29.22 12.22 0 0 4.22 0.44 2.44 54.67
5.56 6.32 0 0 0 0 2.00 0 1.38 2.32 0 0 0 0.22 4.26 10.29 5.76 0 0 2.90 0.44 2.44 17.19
6.13 33.68 0 0 0 0 0.58 0.35 0.23 1.55 1.03 0.03 0 0.03 1.48 4.30 1.60 0 0 0 1.08 0 0
5.05 6.57 0 0 0 0 0.58 0.35 0.20 0.59 0.83 0.03 0 0.03 1.31 2.73 1.40 0 0 0 1.08 0 0
0.68 0.14 0.64 0.09 0.04 0.23 3.25 0.25 0.01 0.58 0.40 0.14 1.38 0.13 0 0 0 0.06 0.16 0 0 0 0
0.30 0.10 0.52 0.06 0.04 0.14 0.92 0.18 0.01 0.26 0.29 0.14 0.92 0.07 0 0 0 0.06 0.14 0 0 0 0
−0.08 −0.31 0.18 −0.12 0.13 −0.06 −0.01 0.03 −0.19 0.04 0.02 −0.13 −0.03 −0.01 −0.18 −0.22 0.02 0.03 0.04 −0.10 −0.11 −0.14 −0.14
−0.06 0.01 −0.07 0.08 0.09 −0.11 −0.01 0.19 −0.07 0.12 0.02 0.01 0.21 −0.14 −0.16 −0.15 −0.10 0.03 0.02 −0.08 −0.02 −0.04 −0.18
Steud. Hypericum crux-andreae (L.) Crantz Hypericum fasciculatum Lam. Hypericum microsepalum (T. & G.) Gray ex Wats. Ilex coriacea L. Ilex glabra (L.) A. Gray Ilex myrtifolia Walt. Itea virginiana L. Leucothoe racemosa (L.) Gray Liriodendron tulipifera L. Lyonia lucida (Lam.) K. Koch Magnolia virginiana L. Myrica cerifera L. Myrica heterophylla Raf. Nyssa biflora (Walt.) Cory Persea borbonea (L.) Sprengel Peiris phillyrefolius (Hook.) Small Pinus elliottii Engelm. Pyrus arbutifolia (L.) Aiton Quercus minima (Sarg.) Small Quercus pumila Walt. Stillingia aquatica Chapm. Taxodium distichum (L.) L.C. Rich. Vaccinium arboreum Marsh. Vaccinium corymbosum L. Vaccinium darrowii Camp Vaccinium myrsinites Lam.
affected herbaceous species distributions. In headwaters of stream drainages in longleaf pine savannas, local hydrology can change abruptly if elevations above impermeable layers of clay are steep (Clewell 1986). Since elevation gradients were steeper in Louisiana, this may explain why herbaceous species or-
dination patterns were more tightly grouped for Louisiana than Florida. On a cautionary note, however, site characteristics independent of ground level elevation may have affected herbaceous species distributions in our study areas. For example, steepness of elevation gradients was confounded by geographic re-
73 Table 7. Relationships using Pearson correlation between environmental variables, dimension 1 (Hdim1) and 2 (Hdim2) sample scores from NMMDS performed on Florida herbaceous species abundance data, and dimension 1 (SDim1) and 2 (Sdim2) sample scores from NMMDS performed on Florida shrub/tree species abundance data. SOM, Clay, Sand = surface soil organic matter, clay and sand content respectively; EL = relative ground level elevation; Moist = surface soil moisture.
SOM Clay pH Sand Moist EL
Hdim1
Hdim2
Sdim1
Sdim2
SOM
Clay
pH
Sand
Moist
EL
0.11 −0.11 0.28 0.11 0.77 −0.61
0.02 −0.21 0.05 0.01 −0.16 0.29
−0.39 −0.37 0.41 0.38 0.58 −0.65
0.58 0.31 0.03 −0.07 0.48 −0.32
1 0.42 −0.49 −0.24 0.24 −0.06
1 −0.10 −0.22 0.07 0.03
1 0.24 0.29 −0.39
1 0.13 −0.29
1 −0.63
1
Table 8. Summarized surface soil characteristic and ground level elevation data for each of three Florida community types. Community Type Pine savanna
Clay (%) Elevation (m) Moisture (%) Soil Organic Matter (%) pH Sand (%)
Seepage bog
Titi/cypress swamp
Mean n=12
SE
Mean n=10
SE
Mean n=10
SE
4.09 0.52 5.37 2.47 4.10 93.65
0.92 0.10 2.29 0.15 0.06 0.58
4.23 0.33 8.47 2.42 4.24 93.53
0.64 0.04 2.48 0.13 0.06 0.58
3.16 0.04 23.78 2.38 4.58 94.58
0.89 0.02 2.88 0.51 0.20 0.58
gion. In addition, the floras were generally different between Louisiana and Florida. Only 48 of the 280 herbaceous species encountered were found in both regions, and, of those 48 species, 14 were common (i.e., overall frequency ⭓ 20%) in Louisiana and eight were common in Florida. Thus, it is possible that some suites of Florida species possess broader tolerances to changes in edaphic conditions than those in Louisiana. Soil characteristics may have influenced the distributions of herbaceous species, but more in Louisiana than Florida. In Louisiana, some surface soil properties (texture, organic matter and pH) were associated with local differences in species composition at different elevations. In Florida, soil properties other than moisture varied little along elevation gradients, aside from a few subtle differences in texture and pH. Differences in soil texture, organic matter, and pH were not strongly associated with differences in abundances of herbaceous species, supporting Clewell (1986) regarding the relatively unimportant nature of these soil characteristics in Florida longleaf pine flatwoods. Thus, greater differences in elevation (and hence water table depth), coupled with differences in surface soil moisture and other characteristics, may
have contributed to the more discrete nature of herbaceous plant communities in Louisiana compared to Florida. We propose that local distributions of herbaceous groundcover plant communities in pine savannas should be related to ground level elevation and surface soil moisture, especially where there are subsurface impediments to downward movement of ground water. In agreement with Solbrig (1996), other edaphic factors, however, may not always have strong influences on groundcover plant communities at local scales. The importance of soil properties may be linked to geological history. Soils at the Louisiana study sites, which are located considerably farther from the Gulf Coast (150 km) than those in Florida (4 km), are older. Development and differentiation of soils in older Pleistocene terraces in Louisiana have occurred along steeper elevation gradients that are part of a rolling topography produced by appreciable weathering and erosion of original marine and fluvial deposits (Walker and Coleman 1987; Harcombe et al. 1993; Peet and Allard 1993; Platt 1999). Along the Florida Gulf Coast, sediments at higher elevations are more recent Pleistocene-aged bars of coarser sand that overlie old embayments of finer sand and clay, result-
74
Figure 4. Coefficients of variation used to represent the degree to which herb- and shrub/tree-defined plant communities were arrayed along localized topographic gradients in Louisiana and Florida: (1) upslope-hillside longleaf pine savannas, (2) mid-slope seepage bogs, and (3) lower-slope baygalls or titi/cypress swamps. Numbers above bars denote sample size.
ing in minimal topographic relief (Brown et al. 1990; Martin and Boyce 1993). Sediments are comprised of unconsolidated sands that have been periodically exposed and submerged, resulting in extensive reworking of soils (Walker and Coleman 1987; Stout and Marion 1993). As a result, these recent soils have not undergone a high degree of weathering and subsequent differentiation (Clewell 1986; Abrahamson and Hartnett 1990; Platt 1999). Differences in surface soil moisture occur along these elevation gradients, but are not as pronounced as in Louisiana. The degree to which herbaceous plant communities along elevation gradients in other sites are associated with other surface soil properties besides moisture may be a function of the geological history and the degree to which soils have differentiated. Broad distributions of trees and shrubs may be associated with alterations of fire regimes (Wharton et al. 1977; Coultas et al. 1979; Means and Moler 1979; Folkerts 1982; Wolfe et al. 1988; Ewel 1990; Platt and Schwartz 1990). In Louisiana, shrub/tree en-
croachment may have occurred mostly from baygalls into seepage bogs (Martin and Smith 1991). Moreover, we encountered numerous woody species with expanded distributions such as llex coriacea, Myrica heterophylla, Persea borbonia, and Pyrus arbutifolia that were abundant in both habitats. The Louisiana sites bear a history of mostly dormant season burning when conditions are almost always too wet for fires to burn down into hydric areas. At best, dormant season fires tend to topkill tree and shrub species (Olson and Platt 1995), often resulting in greater resprouting compared to natural fire regimes (Drewa et al. 2002). In Florida, shrub/tree encroachment may have occurred from upslope flatwoods and downslope titi/cypress swamps into mid-slope seepage bogs. Gaylussacia dumosa, Gaylussacia frondosa, llex glabra, Quercus minima, and Quercus pumila, abundant in upslope flatwoods, and Cyrilla racemiflora, Hypericum brachyphyllum, and Magnolia virginiana, common to titi/cypress swamps, were also abundant in mid-slope seepage bogs. Pyrus arbutifolia, Clethra
75 alnifolia, Gaylussacia mosieri, and Myrica cerifera were common in all communities, coinciding with previous observations (Folkerts 1982; Wolfe et al. 1988; Abrahamson and Hartnett 1990; Ewel 1990). Many old fire lanes typically occurred upslope from our Florida plots. They were once used to prevent fires in upland savannas from burning into downslope areas (that now contain our plots) for several decades after prescribed fire was first used on the St. Marks National Wildlife Refuge. Dormant season fires were more recently introduced into areas downslope from the fire lanes; these only infrequently burned into seepage bogs and titi/cypress swamps. These differences in fire management practices may explain encroachment of shrub/tree species from baygalls into mid-slope seepages of Louisiana and from both uplands and swamps into bogs of Florida. Herb and shrub/tree distributions in our study sites share similarities with savannas worldwide. Woody species distributions tend to be broader and more dynamic than herbs in neotropical savannas (Bourlière and Hadley 1983), including those found in Australia (Neldner et al. 1997), South America (Bate et al. 1982), and southern Africa (Solbrig 1996) especially in headwater areas (Tinley 1982). In addition, at the local level, soil moisture tends to be the most important edaphic factor overriding all other soil properties, as was determined for savannas of South Africa (Tinley 1982; Theron et al. 1984) and Sudan (Fournier and Planchon 1998). However, shrubs/trees tend to be less sensitive to localized changes in soil moisture along elevation gradients because they tend to be deeper-rooted than herbs and, thus, can more easily acquire subsoil water (Bate et al. 1982; Solbrig 1996). As in southeastern longleaf pine savannas, fire may be one of the few modifying factors that periodically reduces woody species encroachment in savannas all over the world (Bourlière and Hadley 1983). Herbaceous species may be more sensitive than shrubs/trees to localized changes in edaphic factors. Trees and shrubs tend to be more broadly distributed. They are typically animal dispersed and also often have underground rhizomes or root systems that can rapidly produce new aboveground stems and spread over longer distances than herbs, rendering them less responsive to subtle changes in edaphic factors (Olson and Platt 1995; Gile et al. 1997). In southwestern desert grasslands, for example, this morphological advantage explains why trees and shrubs have broader distributions than herbs during periods of fire suppression (Hennessy et al. 1983). The re-introduction
of growing season fires may be necessary to reduce shrub/tree distributions (Wright 1980). Several studies have described savanna communities using either a combination of herbs and woody species (Abrahamson et al. 1984; Peet and Allard 1993) or using mostly the latter (Marks and Harcombe 1981; Ewel 1990; Harcombe et al. 1993). These studies, however, were conducted at regional sampling scales where expanded distributions of trees and shrubs across more localized communities were not detected. At local levels, trees and shrubs may be better descriptors of evergreen shrub/tree zones where herbaceous species distributions tend to be broader. For the purpose of delineating plant communities across elevation gradients in headwaters of longleaf pine savannas, however, our results demonstrate that herbs are more useful than woody species.
Acknowledgements Many people helped with the field work: David Baker, Eliza Drewa, Mark Ford, Sue Grace, Charles Kwit, Paul Marples, Jed Redwine, Anita Stabrawa, as well as undergraduate students from Louisiana State University and Florida State University. Numerous people helped identify plant species, including Joan Bruser, Andre Clewell, Angus Ghoulson, Robert Godfrey, Barbara and Michael MacRoberts, Nelwyn McInnis, Mark Mayfield, Bruce Means, Carl Nordman, Roland Roberts, Latimore Smith, Keith Tassin, Lowell Urbatsch, Tom Wendt, and especially Susan Carr. Lodging in the field was provided by Mike Berg and Louisiana Wildlife and Fisheries, the Cabana family, as well as Mr. Joe White and the St. Marks National Wildlife Refuge staff of the US Fish and Wildlife Service. We are indebted to Wayne Hudnall, Department of Agronomy, Louisiana State University who made it possible to analyze soil samples in his laboratory with the assistance of Leah Boulton. We thank Susan Carr, Russ Chapman, Jim DeCoster, Julie Denslow, Jim Grace, Charles Kwit, Sarah Riley, and Jarrod Thaxton for helpful comments on earlier drafts and Jeff Glitzenstein, Charles Kwit, Peter Minchin, Donna Streng, and Maynard Hiss for helpful discussions of ideas. This study was supported by grant NG 95-029 (W.J. Platt and P.B. Drewa, co-P.I.s.) from the Florida Fish and Game Commission Non-game Wildlife Program, a graduate research grant in conserva-
76 tion biology from The Nature Conservancy (P.B. Drewa, P.I.), and a cost-share grant from the USDA Forest Service for work in the Kisatchie National Forest (KNF-94-18) (P.B. Drewa, P.I.).
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