Floristic composition of Alabama piedmont floodplains across a gradient of stream channel incision.
Riparian areas exist at the interface of aquatic and terrestrial environments, and the ecological functions of these habitats are extensive. Riparian zones serve to buffer aquatic environments from overland sedimentation and pollution (Hill, 1996), provide habitat and refugia for organisms (Gregory et al, 1991), act as sinks for carbon (Mitsch and Gosselink, 2007), and are often sites of high biodiversity within the landscape (Naiman and Decamps, 1997). Depth to seasonal high water table, along with streamside elevation and soil texture, are principal drivers in riparian plant distribution; these factors are dependent upon the hydrologic regime of the watershed (Poff et al, 1997; Tabacchi et al, 2000). Wetland and riparian plants are adapted to the saturated soils and high water tables that typify these environments, although frequency, duration, and magnitude of overbank flooding events may also significantly affect riparian vegetation assemblages (Mitsch and Gosselink, 2007; Goebel et al, 2012).
A landscape feature typical of riparian environments is the floodplain, an area of relatively flat land bordering a stream or river that may be inundated by flow during flooding events (Wolman and Leopold, 1957). Floodplain function is reduced when channel morphology prevents connection between the stream and floodplain surface, leading to alterations in flood frequency, soil morphology, and groundwater dynamics. Disconnections between stream and the floodplain surface can often be attributed to channel incision, a condition best defined as the lowering of the base level of a stream or river (Leopold et al., 1964). Channel incision is a common symptom of anthropogenic disturbance of fluvial systems, with features readily identified in the field: unnaturally high banks, a high degree of mass wasting, and slumping of soil and vegetation into the active channel (Simon and Rinaldi, 2006). Channel incision results from an imbalance in the sediment load and sediment transport capacity of a fluvial system (Leopold et al, 1964; Schumm et al, 1984; Simon and Rinaldi, 2006). Entrenchment of the active channel is often an inherent process in landscape evolution, such as valley development, but the exceedance of certain geomorphic thresholds due to anthropogenic disturbance may lead to disequilibrium and channel response in the form of incision (Schumm et al, 1984).
Stream channel incision results in a decrease in floodplain-stream connectivity, therefore reducing the exchange of water, nutrients, sediments, and organisms (Opperman et al, 2010). Reduced soil moisture is also symptomatic of disconnected floodplains, as channel incision leads to decreased flood frequency and the lowering of water tables throughout the riparian soil system (Junk et al, 1989; Steiger and Gazelle, 1998; Amoros and Bornette, 2002). Alterations to the flood regime may lead to shifts in disturbance dynamics and topographical heterogeneity, therefore affecting floodplain species composition and diversity (Steiger and Gazelle, 1998; Ward et al, 2002; Steiger et al, 2005; Opperman et al, 2010). Incision is also linked to high seasonal variability in floodplain water tables that results in significantly lower groundwater levels in comparison to nonincised, reference systems (Groffman et al, 2003; Schilling et al., 2004; Hardison et al, 2009). Soil conditions in disconnected floodplains may also differ significantly from reference counterparts, as alluvium from upland erosion may result in thicker accumulation of sediments over older stable surfaces (Trimble, 2008).
In the southeastern United States, the Piedmont physiographic region is an area that has undergone extensive transformation, particularly in its stream and riparian areas. Before European settlement hickory and oak species were the dominant components of Piedmont forests (Oosting, 1942; Nelson, 1957; Golden, 1979; Cowell, 1995). The earliest descriptions of the Alabama Piedmont come from colonial-era explorers and naturalists, whose descriptions of the region point to a landscape dominated by late-successional forests and "clear, salubrious streams flowing over stone" (Harper, 1943). William Bartram and his contemporaries noted the clear streams in the Southern Piedmont region, lending credence to the idea that fluvial systems of this region were not inundated with sediment before colonial expansion (Trimble, 2008). Precolonial explorers characterized floodplain soils of the southern Piedmont as dark in color and "rich" (Harper, 1943), indicating negligible rates of overland sediment deposition (Trimble, 2008).
Row crop agriculture dominated the Piedmont region during the 19th and early 20th centuries, but a large-scale shift in land use after the Great Depression led to the conversion of large swaths of arable land to secondary succession, timberlands, and residential development (Cowell, 1995). Alluvial material from cultivated uplands accumulated in the streams and riparian corridors of the Piedmont region, resulting in high-degrees of stream degradation and geomorphic channel alterations in headwater and low-order streams (Bennett, 1931; Happ, 1945; Ruhlman and Nutter, 1999; Trimble, 2008). Land-use changes in the latter half of the 20th century led to the gradual reduction of eroded material entering the region's floodplains and waterways, therefore allowing these alluvial systems to equilibrate to the current sediment regime (Trimble, 2008). A consequence of the increased downcutting of streams was incision, channel enlargement, and decreased overbank flooding events in low-order streams in the Southern Piedmont.
Groffman et al. (2003) coined the term "hydrologic drought" to describe the alterations in riparian habitats due to anthropogenic changes in stream morphology, specifically channel incision, relating shifts in riparian plant community composition and structure due to lowered groundwater tables. A similar study in the Coastal Plain of North Carolina found that Channel Incision Ratio was a reliable surface indicator of floodplain water table decline (Hardison et al, 2009). Hardison et al. (2009) observed significant differences in groundwater depth between incised and reference streams, although no analysis of floristic dynamics was presented. A consequence of hydrologic drought is likely a lack of hydrophytic species present in the floodplains and riparian habitats, therefore an analysis of vegetation composition in regards to stream channel incision is needed to fully elucidate hydrologic drought and its effects.
The relationship between groundwater and vegetation communities has been demonstrated in a variety of habitat types and environments, but few studies exist which examine plant community responses to the groundwater changes induced by channel incision, and no such studies were found for the Piedmont eco-region. The objective of this study was to test the concept of "hydrologic drought" in the field and determine if a shift in vegetation assemblages could be quantified along a gradient of stream channel incision. We expected a correlation between hydrophytic plant prevalence and magnitude of channel incision, with higher abundances and densities of OBL and FACW species associated with low BHR values and high groundwater depths.
This study was conducted in the Piedmont region of Alabama, an area comprised of 12 counties in the East-Central portion of the state. The Alabama Piedmont represents the southwestern terminus of the Piedmont eco-region, a 100-160 km wide physiographic province that extends from Alabama to southern New York (USDA-NRCS, 2006). Mean annual precipitation ranges from 110-150 cm and mean annual temperatures range from 12 to 18 C (USDA-NRCS, 2006). Floodplain soils of the Alabama Piedmont are comprised of alluvium and colluvium derived from metamorphic rocks (Golden, 1979). Representative soils of Piedmont floodplains include the Toccoa (coarse-loamy, mixed, active, nonacid, thermic Typic Udifluvent), Chewacla (fine-loamy, mixed, active, thermic Fluvaquentic Dystrudept), and Cartecay (coarse-loamy, mixed, semiactive, nonacid, thermic Aquic Udifluvent) series (USDA-NRCS, 2013). Happ (1945) characterized the alluvium in modern Piedmont floodplains as brown to reddish-brown, similar to topsoil in upland environments, and often overlaying a former soil surface.
Ten low-order streams along a continuum of channel incision were selected for this study (Fig. 1). Site selection was nonrandom and based upon establishing a gradient of BHR values for the study. Watershed land cover was largely consistent across all sites with low impervious surface cover (<5%) and dominant land use best characterized as a matrix of secondary growth hardwood forests, pine plantations, and pasture. Stream sites had unconstrained valleys, negligible upstream alterations (e.g. channeling), and minimal exotic invasive species. Floodplain width was a primary determinant of site suitability, as adequate distance between valley walls and transect placement was critical to reduce confounding interactions between study sites and hillslope groundwater seeps.
MATERIALS AND METHODS
Bank height ratio (BHR) was used to quantify the degree of channel incision at all study sites. Bank height ratio is a geomorphological diagnostic tool that uses bankfull depth and bank height to quantify channel incision in a fluvial system (Rosgen, 1996). Bank height ratio was assessed at all stream sites using the following protocol:
(1) Determine a stable upstream riffle, (2) measure depth from thalweg to bankfull (Dmax), (3) measure depth from thalweg to top of bank (DTOb). and (4) divide DXOB/Dmax. Bankfull was identified in the field according to the criteria outlined by Leopold et al. (1964) whereby bankfull is defined as the point at which floodwaters filling the stream channel would first flow onto the floodplain during an overbank event or the elevation of "incipient flood stage" in a stable nondegraded stream. In severely incised streams a field determination of bankfull depth is often unreliable; therefore a consistent measure of Dmax was unattainable in a small sample of the study sites. Therefore Dmax was calculated by taking geomorphic data from 21 streams of the Alabama Piedmont, creating a regional Dmax curve, and plotting against watershed size; this plot was used to generate a function for predicting bankfull (y = 0.581n(x)+2.248, R" = 0.82) (Brantley et al, 2013). This method was used to determine BHR at three highly incised study sites (Forest Eco, Bird, and UT Coon creeks). Watershed boundaries were delineated using digital elevation models in ArcGis 10.0.
Vegetation data were collected after leaf out in May and June of 2012. Five plots were placed at intervals of 50 m along a 250 m stream segment. Plots were established perpendicular to stream channels with dimensions of 5 X 20 m (0.01 [ha.sup.2]). Plants were identified to the species level, and three layers of vegetation were assessed according to diameter at breast height (DBH) and height above soil surface: canopy layer, shrub, and ground flora/herbaceous vegetation. All trees and shrubs in each transect were measured for DBH, counted, and identified to the species level. Stems greater than 2.5 cm at DBH were ranked as canopy-level species; stems less than 2.5 cm at DBH but greater than lm above soil surface were ranked in the shrub class. Woody stems and herbaceous vegetation <1 m in height were identified and ranked according to species abundance and cover class. The herbaceous and woody ground-flora component were assessed using 1 [m.sup.2] quadrats placed at 5 m intervals in the 0.01 ha transects; each quadrat was placed equidistant from transect margin, and the final quadrat in the transect straddled the terminal transect boundary. A total of 25 herbaceous/ground flora surveys were conducted at each site using the 1 [m.sup.2] quadrats (250 across all sites). Cover class was estimated visually using the Braun-Blanquet scale, in which a cover percentage is assigned to individual plants based on logarithmic scale: 1: 0-2%, 2: 3-10%, 3: 11-25%, 4: 26-50%, 5: 51-100% (Braun-Blanquet, 1965).
All species were categorized according to their wetland indicator status: obligate upland (UPL), facultative upland (FACU), facultative (FAC), facultative wetland (FACW), and obligate wetland (OBLW) (US Army Corps of Engineers, 2012).
Importance values ([IV.sub.200] and [IV.sub.300]) were calculated for canopy, shrub, and herbaceous-level species with data pooled from each transect to provide importance values per stream. Importance values of canopy species ([IV.sub.300]) were calculated using the following formula:
[IV.sub.300] = Relative density of species X + relative frequency of species X + relative dominance of species X
Importance values of shrub species were calculated on an [IV.sub.200] scale, in which the sum of relative frequency and relative density was determined as follows:
[IV.sub.200] = Relative density of species X + relative frequency of species X
The importance values of herbaceous and ground-flora were also assessed on an [IV.sub.200] scale, with relative cover class percentage used in conjunction with relative frequency. Prevalence Indices (PI) were calculated for each forest layer across all sites.
Prevalence Index = [SIGMA]AiWi/[SIGMA]Wi
Where Ai = abundance of species i, Wi = Wetland Indicator Category value for species i (UPL = 5, FACU = 4, FAC = 3, FACW = 2, OBL =1), and i = species.
Prevalence Indices are weighted averages that combine relative abundance and wetland indicator status categories in order to determine the prevalence of hydrophytic vegetation at a particular site (Peet et al., 1988). Mean PI scores of the canopy and understory strata at each site were generated by calculating PI of the sample layer in each transect and averaging among the 5 transects. The PI of each 1 [m.sup.2] quadrat was calculated and averaged among the 25 quadrats sampled per site in order to determine a PI score for the herbaceous/ground flora layer of each study site.
Prevalence index scores <3.0 typically indicate that the site has a dominant component of hydrophytic vegetation, therefore representative of a wetland-type habitat (Peet et al., 1988, U.S. Army Corps of Engineers, 2012).
In order to associate channel incision and relative floodplain water levels, groundwater monitoring wells were installed 10 m from the floodplain-stream boundary at three sites along each stream site in transects previously delineated for vegetation analysis. Well depth varied among sites (0.63-2.1 m), with maximum depth often dependent on water table height and proximity to bedrock. Maximum well depth for most sites was 20-30 cm below water table elevation at time of installation. Wells were constructed from 10.16 cm diameter slotted PVC pipe and encased in geotechnical fabric. Pressure transducers (Solinst Levelogger Gold) were placed in one well per site to provide a continuous log of groundwater levels over the year-long study period (March 2012-March 2013). Transducers were placed in well sites most representative of overall geomorphic condition of stream site. Groundwater levels were also monitored at all sites on a monthly basis.
Linear and nonlinear regressions were performed to determine level of association between vegetation data and independent variables (BHR, mean groundwater depth). Nonmetric multidimensional scaling (NMDS) was used to test community composition of herbaceous/ground flora to measured environmental variables. Analysis was based on species importance values, with rare species (less than five individuals) omitted from examination. The global NMDS procedure was performed using the 'metaMDS' function in the 'vegan' package; data were square root transformed and Wisconsin standardized prior to analysis. Vector fitting using the 'envfif function was then performed to determine possible significant relationships between composition of the herbaceous layer and tested environmental variables (BHR, median groundwater depth, stem density). P values obtained from the NMDS procedure were derived from 1000 permutations. All statistical analyses were conducted in R v. 2.15.2 (R Development Core Team 2012) with a levels = 0.05.
GEOMORPHOLOGY AND HYDROLOGY
Bank height ratios varied among study sites and ranged from 1.0 at the least incised, reference-type streams to over 5.0 at the most incised study site (Table 1). The highest degrees of incision were recorded at the UT Coon and Forest Eco sites, with BHR values of 3.1 and 5.2, respectively. Mean rainfall of the study area, calculated as mean precipitation across the 10 study sites for the study period, was 100.17 [+ or -] 4.59 cm (NCDC, 2013). This value is markedly lower than the 30 y average annual precipitation (134.26 cm) for Wadley, Alabama, a municipality located in the central portion of the Alabama Piedmont, (NCDC, 2013). Groundwater depths varied throughout the study period (March 2012-March 2013) and among study sites (Table 1). Linear regression of BHR and mean annual groundwater depth indicated a significant relationship (P = 0.003, [R.sup.2] = 0.66) (Fig. 2). Average floodplain width across all study sites was 106.54 [+ or -] 5.92 m.
A total of 94 unique plant species were identified across 10 study sites. The dominant ranopy-level species among all sites were Liquidambarstyraciflua (10 sites, mean [IV.sub.300] 53.7 [+ or -] 7.7), Liriodendron tulipifera (10 sites, mean [IV.sub.300]: 37.10 [+ or -] 6.4), Carpinus caroliniana (9 sites, mean [IV.sub.300]: 50.8 [+ or -] 11.7), Acer rubrurn (9 sites, mean [IV.sub.300]: 17.4 [+ or -] 3.9), and Quercus nigra (7 sites, mean [IV.sub.300]: 40.9 [+ or -] 18.1).
The most common shrub species among the study sites were Smilax rotundifolia (10 sites, mean [IV.sub.200]: 27.4 [+ or -] 6.9), Carpinus caroliniana (7 sites, mean [IV.sub.200]: 28.2 [+ or -] 5.4), Halesia Carolina (6 sites, mean [IV.sub.200]: 25.9 [+ or -] 9.9), Vitis rotundifolia (6 sites, mean [IV.sub.200]: 11.7 [+ or -] 2.8), and Quercus nigra (5 sites, mean [IV.sub.200]: 28.3 [+ or -] 14.1). The composition of ground flora species was highly variable among study sites, with pteridophytes (Athyrium filix-femina, Woodwardia areolata) and Arundinaria gigantea dominating at low degrees of bank incision. Graminoids and forbs with FACU and FAC wetland indicator status tended to increase in abundance as degree of bank incision increased.
VEGETATION COMPOSITION (PREVALENCE INDICES)
Prevalence index (PI) scores were used to assess abundance of hydrophytic vegetation in the canopy, shrub, and herbaceous strata at each study site (Table 2). A significant linear relationship was observed between mean PI scores of herbaceous/ground flora plants and BHR ([R.sup.2] = 0.64, P = 0.005) (Fig. 3). In contrast no significant relationship was found for shrub and canopy layer PI scores and BHR (Table 3). Mean PI scores of the herbaceous/ ground flora across all sites also showed a significant level of association with mean groundwater depth ([R.sup.2] = 0.52, P = 0.028) (Fig. 4). There were no significant relationships between mean groundwater depth and PI scores of the shrub and canopy strata (Table 3).
Percent composition of FACW/OBL species in the ground flora layer ranged from 27.16 to 0% along the gradient of stream incision. A significant relationship (P < 0.001) was observed between BHR and percent composition of FACW/OBL in the herbaceous/ground flora layers, with increasing BHR typically indicating lower prevalence of FACW/OBL species across all sites ([R.sup.2] = 0.55). In contrast linear regression analysis of percent FACW/ OBL and mean groundwater depth did not indicate a significant relationship between the two parameters (P = 0.58).
NMDS ordination of the herbaceous species data yielded a 2 dimensional solution at a final stress of 0.06 after 100 iterations. Axes 1 and 2 of the NMDS plot suggest a gradient of sites largely described by BHR and median groundwater depth (Fig. 5). Pteridophytes and forbs with FACW/OBL designations are associated with sites exhibiting reference-type geomorphic conditions, i.e. low BHR values (Woodwardia areolata, Osmunda regalis). In contrast incised sites often contain species typical of dry' upland settings (Rubus argutus, Prenanthes altissima). Results of the vector fitting analysis indicate significant correlations between species composition and BHR and median groundwater depth; no significant relationships between stem density and species composition were observed (Table 4).
The results of this study suggest that channel incision in low-order streams of the Alabama Piedmont has led to functional shifts in herbaceous/ground flora vegetation structure and composition. Bank Height Ratio was strongly correlated with the distribution and composition of riparian herbaceous/ground flora layers. Prevalence index scores of the canopy, shrub, and herbaceous stratum show variation in the abundance of hydrophytic vegetation across the sites, but only herbaceous/ground flora PI values were significantly related with BHR and mean groundwater depths. These findings are similar to other studies that note the herbaceous/ground flora layer is most responsive to changes in soil hydrology due to greater plant turnover rates (Naiman et al., 2005; Goebel et al., 2006). Because forest strata respond to disturbance and microtopography in different ways, compositional patterns may differ by forest layer (Glenn-Lewin, 1977). Herbaceous species are more sensitive to small-scale temporal and spatial changes in soil moisture and topographical heterogeneity than shrub or canopy layers, and therefore exhibit rapid response to environmental gradients (Lyon and Sagers, 1998; Lite et al, 2005; Steiger et al., 2005).
Examination of the PI for each forest stratum indicates the herbaceous/ground flora layer was more responsive to changes in groundwater levels and BHR. Prevalence index scores of the least incised study sites (BHR = 1.0-1.11) are below 3.0 indicating a prevalence of hydrophytic vegetation in the herbaceous/ground flora layer (Peet et al, 1988; US Army Corps of Engineers, 2012) (Table 3). Studies conducted in the western US report compositional shifts of the herbaceous layer from wetland-obligate plants to dry site upland-adapted species in response to declines in groundwater (Allen-Diaz, 1991; Law et al, 2000; Dwire et al, 2006; Hammersmark et al, 2009). In a comparable study conducted in the Midwestern U.S., Goebel et al. (2006) observed changes in the composition of riparian ground-flora were strongly related to elevation above bankfull. Prevalence index scores of the herbaceous/ground flora layer generally increased with increasing BHR, indicating a functional shift from hydrophytic plant species at low degrees of incision towards an herbaceous/ground flora layer comprised of plants more typical of upland drier settings. These findings are in line with similar studies that have examined riparian vegetation distribution and may substantiate the idea of "riparian hydrologic drought" as defined by Groffman et al. (2003). The significant decline in FACW/OBL species in response to increased BHR is another indication of a functional shift from hydrophyte-dominated to FAC and FACU vegetation assemblages. Mean groundwater depth was not found to be a significant indicator of FACW/OBL herbaceous species. The p-value from this analysis (0.058) was not statistically significant, although a biologically significant relationship is probable.
Prevalence index scores of the overstory and shrub layers did not exhibit significant levels of linear association with BHR or mean groundwater depth. The PI scores of the canopy layer ranged from 2.93 to 3.47 across the 10 study sites, with no distinct linear trend of increasing PI score in accord with BHR or groundwater depth. Quadratic analysis of these factors did not indicate a significant association at [alpha] = 0.05, but examination of these graphs seem to indicate a threshold of BHR (approximately 2.1) at which woody hydrophytes cease to dominate. Similar studies in western North America (Shafroth et al., 2000; Guilloy-Froget et al., 2002) and in floodplains of major river systems (Bravard et al., 1997; Steiger and Gazelle, 1998; Darst and Light, 2008) have observed canopy-level shifts in woody species composition due to water table decline, but no studies have demonstrated these changes in low-order streams of the eastern U.S. In his examination of Alabama Piedmont tree communities, Golden (1979) reported small stream bottoms were populated by tree species that were otherwise typical of other Piedmont habitats: mesic uplands (Quercus alba), moist coves (Liriodendron tulipifera, Fagus grandifolia), and large bottomlands (Liquidambar styraciflua, Acer rubrum). Floodplain soils of this region are generally characterized as moderately well- to well-drained, therefore these habitats often favor woody species adapted to upland settings that would otherwise be ill-suited for riparian areas (Golden, 1979). These findings are in concurrence with results of this study that a mosaic of tree and shrub species adapted to variable habitats populates floodplains of Piedmont low-order streams. Therefore, a prevalence of hydrophytes in the shrub or overstory may not be a natural characteristic of these communities.
Nonmetric multidimensional (NMDS) analysis showed the distribution of sites in species space along the continuum of channel morphology (Fig. 5). Study sites with low BHR values exhibit herbaceous/ground flora vegetation dominated by FACW/OBL species. Fern species with FACW/OBL (Woodwardia areolata, Osmunda regalis) designations are aligned with sites exhibiting reference-type geomorphic conditions. In addition these sites have a high abundance of woody, hydrophytic species (Magnolia virginiana, Sambucas nigra). Results of the ordination analysis also reveal a strong association between upland-type woody species (Quercus alba, Cercis canadensis, and Comus florida) and sites with high BHR values which suggests high degrees of channel incision and lowered water tables are favorable to vegetation adapted to dry settings (e.g., low flood frequency, low soil moisture). The distribution of ground flora in the ordination space show compositional shifts in floodplain vegetation are associated with channel incision and groundwater declines, emphasizing the hydrogeomorphic origin of the observed changes in floodplain floristic composition. Although no known studies relate ground flora composition specifically to channel incision, these findings are in line with comparable studies that examined vegetation assemblages in relation to fluvial landforms (Osterkamp and Hupp, 1984; Hupp and Osterkamp, 1985) and elevation above bankfull height (Lyon and Sagers, 1998; Geise et al., 2000; Goebel et al, 2012).
Our results suggest BHR is an accurate predictor of water table depth in floodplains of the Alabama Piedmont. Similarly, a study in stream floodplains of the Coastal Plain of North Carolina reported that a comparable diagnostic of incision, Channel Incision Ratio, was an accurate predictor of groundwater levels (Hardison et al., 2009). The results of this study, along with Hardison et al. (2009), substantiate the groundwater component of the "hydrologic drought" concept (Groffman et al., 2003). Riparian zones affected by hydrologic drought are expected to exhibit high seasonal variability in groundwater levels (Groffman et al, 2003). In the study period of March 2011-March 2012, groundwater variability of the incised locations was higher relative to the reference floodplain conditions. Groundwater levels of sites, exhibiting BHR values <1.5, deviated 0.1-0.12 m from the yearly average. In contrast groundwater fluctuations at the incised sites often ranged 0.25-0.30 m from the 2011-2012 average. Hardison et al. (2009) observed similar patterns of groundwater fluctuation between reference and incised streams. Drainage area may influence the magnitude of channel incision; sites with the highest BHR values were found to have the smallest catchment areas. This observation may be attributed to certain site-specific land-use factors that historically affected these streams (i.e., intense soil removal in upland areas) or a greater depth to bedrock relative to other study sites.
Stream channel morphology has been shown to affect floodplain function in variety of ways. The disconnect between the stream and floodplain environments due to channel incision has the potential to disrupt the tightly coupled relationship between aquatic and terrestrial environments resulting in a cascade of diminished function throughout the riparian ecosystem. This disconnection can be seen in watersheds of the Alabama Piedmont, a region characterized by high degrees of stream incision. Results of this study demonstrate that floodplains in this region exhibit varied degrees of "hydrologic drought," as evidenced by compositional shifts in herbaceous/ground flora vegetation and lowered groundwater levels. Bank height ratio was an accurate predictor of floodplain groundwater levels, and the use of this technique could be extended to scientists, land managers, and stakeholders concerned with assessing the hydrogeomorphic status of a low-order stream. In addition BHR may provide a relatively simple diagnostic for evaluating plant species suitability for restoration efforts.
Acknowledgments.--Funding for this project was provided in part by the US Environmental Protection Agency, Region 4, Wetland Program Development Grant, and the USDA-NIFA 406 Southern Regional Water Program. Authors would like to thank Brian Folt, Tom Hess for assistance with field work and Greg Jennings, Jason Zink, and Zan Price for guidance on stream morphological determinations. This is contribution #707 to the Auburn University Museum of Natural History.
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SUBMITTED 20 JANUARY 2014
ACCEPTED 12 JUNE 2015
IAN P. TURNER (1), EVE F. BRANTLEY (2), and JOEY N. SHAW (3)
Department of Crop, Soil and Environmental Sciences, Auburn University, Alabama 36849
CHRISTOPHER J. ANDERSON (4)
School of Forestry and Wildlife Sciences, Auburn University, Alabama 36849
BRIAN S. HELMS (4)
Department of Biological Sciences, Auburn University, Alabama 36849
(1) Corresponding author present address: Civil & Environmental Consultants, Inc., 600 Marketplace Avenue, Suite 200, Bridgeport, WV 26330; Telephone: (304) 848-7125; Fax: (304) 933-3327; e-mail: email@example.com
(2) Present address: Department of Crop, Soil and Environmental Sciences, Auburn University, Alabama 36849; Telephone: (334) 844-4100; Fax: (334) 844-3945; firstname.lastname@example.org
(3) Present address: School of Forestry and Wildlife Sciences, Auburn University, Alabama 36849
(4) Present address: Department of Biological Sciences, Auburn University, Alabama 36849
TABLE 1.--Bank height ratio (BHR), watershed area ([km.sup.2]), and mean and median groundwater (GW) depths across all study sites. Values in parentheses indicate standard deviation Watershed area Median Watershed BHR ([km.sup.2]) Mean GW depth (m) GW depth (m) Rotton 1.0 2.59 0.51 [+ or -] <0.01 0.51 (0.05) Loombeam 1.0 1.27 0.31 [+ or -] <0.01 0.33 (0.11) Pile 1.1 6.09 0.93 [+ or -] <0.01 0.98 (0.15) Osburn 1.3 7.51 1.06 [+ or -] <0.01 1.10 (0.16) Little MS 1.8 7.10 0.70 [+ or -] 0.01 0.80 (0.21) Steams 2.2 4.22 1.50 [+ or -] 0.01 1.62 (0.25) Mitchell 2.4 5.00 1.44 [+ or -] 0.01 1.57 (0.23) Bird 2.8 1.89 1.22 [+ or -] 0.01 1.29 (0.20) UT Coon 3.1 0.60 1.19 [+ or -] 0.09 1.34 (0.33) Forest Eco 5.2 0.31 2.00 [+ or -] 0.02 1.98 (0.29) TABLE 2.--Mean and standard error for Prevalence Index (PI) scores of ground flora, shrub, and canopy layers across 10 study sites. Density of stems >2.5 cm at DBH (per ha) are also provided. Sites listed in order of increasing BHR Watershed Herbaceous PI Shrub PI Rotton 2.86 [+ or -] 0.05 2.71 [+ or -] 0.15 Loombeam 2.92 [+ or -] 0.05 2.95 [+ or -] 0.09 Pile 2.88 [+ or -] 0.07 3.03 [+ or -] 0.04 Osburn 3.05 [+ or -] 0.10 3.07 [+ or -] 0.05 Little MS 3.13 [+ or -] 0.02 3.08 [+ or -] 0.07 Steams 3.00 [+ or -] 0.08 2.73 [+ or -] 0.15 Mitchell 3.40 [+ or -] 0.04 3.54 [+ or -] 0.09 Bird 3.31 [+ or -] 0.05 3.34 [+ or -] 0.08 UT Coon 3.21 [+ or -] 0.08 3.51 [+ or -] 0.09 Forest Eco 3.38 [+ or -] 0.10 3.23 [+ or -] 0.10 Watershed Canopy PI Stem density Rotton 3.07 [+ or -] 0.16 2620 Loombeam 3.00 [+ or -] 0.26 3840 Pile 2.93 [+ or -] 0.09 1660 Osburn 3.17 [+ or -] 0.05 2300 Little MS 3.21 [+ or -] 0.06 3920 Steams 2.95 [+ or -] 0.11 1780 Mitchell 3.40 [+ or -] 0.10 8480 Bird 3.33 [+ or -] 0.10 2840 UT Coon 3.47 [+ or -] 0.04 1560 Forest Eco 3.20 [+ or -] 0.06 4700 TABLE 3.--Results of linear regression analysis between environmental variables (Bank Height Ratio, mean annual groundwater depth) and prevalence indices of the canopy, shrub, and ground flora strata Independent variable Forest layer [R.sup.2] P-value Prevalence indices BHR Ground flora 0.64 0.005 ** Shrub 0.27 0.12 Overstory 0.25 0.14 Mean GW depth Ground flora 0.52 0.03 * Shrub 0.13 0.21 Overstory 0.04 0.34 * Significance at [alpha] < 0.05 TABLE 4.--Results of two dimensional NMDS ordination of herbaceous/ ground flora species as related to key hydrologic and environmental variables Environmental variable NMDS1 NMDS2 [R.sup.2] P-value BHR 0.946 -0.322 0.885 0.003 ** Stem density 0.371 -0.928 0.383 0.19 Median GW depth -0.943 0.332 0.702 0.01 * ** Significance at [alpha] < 0.01 * Significance at [alpha] < 0.05 TABLE 5.--Herbaceous/ground flora species found at 10 floodplain sites of the Alabama Piedmont. Species codes used in NMDS plot Species name Species code Wetland indicator Acer rubrum Acerub FAC Arisaema triphyllum Aritri FACW Arundinaria gigantea Arugig FACW Asimina triloba Asitri FAC Athyrium filix-femina Athfix FAC Berchemia scandens Bersca FACW Bignonia capreolata Bigcap FAC Callicarpa americana Caiame FACU Carpinus caroliniana Carear FAC Cercis canadensis Cercan FACU Comus florida Corflo FACU Dicanthelium clandestinum Diccla FAC Duchesnea indica Diicind FACU Elephantopus tomentosa Eletom FACU Euonymus americanas Euoame FAC Halesia Carolina Halcar FAC Hexastylis arifolia Hexari FAC Illiceum floridanum Illflo FACW Ligustrum sinense Ligsin FACU Liquidambar styraciflua Liqsty FAC Lonicera japonica Lonjap FAC Magnolia virginiana Magvir FACW Mitchella repens Mitrep FACU Oplismenus hirtellus Oplhir FACU Osmunda regalis Osmreg OBL Parthenocissus quinquefolia Parqui FACU Polystichum acrostichoides Polacr FACU Prenanthes altissima Prealt FACU Quercus alba Quealb FACU Quercus nigra Quenig FAC Rhodendron canescens Rhocan FACW Rubus argutus Rubarg FACU Rudbeckia fulgida Rudful FAC Sambucus nigra Samnig FACW Scleria sp. Sclsp FAC Smilax bona-nox Smibon FACU Smilax rotundifolia Smirot FAC Toxicodendron radicans Toxrad FAC Ulmus alata Ulmala FACU Uvularia sessifolia Uvuses FAC Viola sp. Viosp FAC Vitis rotundifolia Vitrot FAC Woodwardia areolata Wooare FACW Xanthorhiza simplicissima Xansim FACW
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|Author:||Turner, Ian P.; Brantley, Eve F.; Shaw, Joey N.; Anderson, Christopher J.; Helms, Brian S.|
|Publication:||The American Midland Naturalist|
|Date:||Oct 1, 2015|
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