Environmental gradients and identification of wetlands in north-central Florida

WETLAND& Vol. t6. No. 4, December 1906, pp. 512--523 ~5 1996, The Society' or Wetland Scientists

ENVIRONMENTAL GRADIENTS AND IDENTIFICATION OF WETLANDS IN NORTH-CENTRAL FLORIDA

Mary M. Davis +, Sleven W. Sprecher ~, James S. Wakeley ~, and G. Ronnie Best-' Environmental Laboratory

U.S. Army Engineer Waterways Experiment Station Vicksburg, MS 39180-6199 USA

National Wetland Research Center U.S. Geological Survey

Lafayette, LA 70506 USA

Abstract: Vegetation composition, soil morphology, and hydrology were characterized along wetland-to- upland gradients at six forested sites in north-central Florida to compare results of Federal wetland delineation methods with 3-5 yr of hydrologic data. Wetland and non-wetland identifications were supported by hydrology data in eight of nine plant communities. Lack of hydric soil indicators and hydrophytic vegetation in two upland communities (scrub and mixed mesic hardwoods) agreed with a deep water table. Six wetland communities (cypress dome, cypress strand, bayhead, cypress/bayhead, red maple/oak swamp, and cedar swamp) with field indicators of wetland hydrology, hydrophytic vegetation, and hydric soils were inundated or had water tables at or near the ground surface at least 5% of the growing season in most years. Flatwoods communities, however, occurred at intermediate positions on the moisture gradient and could not be consistently identified as wetland or upland communities. Identification of flatwoods as wetlands depended on wetland delineation method and was not usually supported by hydrologic measurements. In the flatwoods community, soil properties aad vegetation composition were correlated with the mean and standard deviation of water-table depths, as well as the depth continuously exceeded by the water table at least 5% of the growing season in most years. Various hydrologic parameters need to be considered in addition to the 5% exceedence level currently used in Federal wetland delineation guidance when characterizing wetland conditions in low-gradient areas such as flatwoods.

Key Words: environmental gradients, flatwoods, Florida, hydrology, hydrophytic vegetation, hydric soils, plant communities+ soil morphology, wetland delineation

I N T R O D U C T I O N

The current Federal regulatory standard for wetland delineation is the Corps of Engineers Wetlands Delin- eatiol£ Manual (Environmental Labora tory 1987; hereafter+ the 1987 Manual) , which uses a three-parameter test requiring evidence o f hydrophyt ic vegetation, hydric soils, and wetland hydrology. While vegetat ion composi t ion and soil morpho logy can be readily sampled and characterized, quantitative understanding o f the hydrologic regime requires long-term moni tor ing o f surface inundation and ground-water levels. There- fore, wetland hydro logy decisions are usually based on observable hydrologic indicators (e.g., water marks, drill linc~, oxidi,ccd rhizosphcres) that convey little in- format ion about the frequency, duration, or t iming of inundation or soil saturation.

Upper limits o f the three wetland parameters may align closely and wetland boundaries be readily ap-

parent in areas where topographic and hydrologic gradients are steep and wetland boundaries abrupt. In areas such as north-central Fforida, however, where t(~- pography is relatively flat and hydrologic gradients are prolonged, hydrophyt ic vegetat ion and hydric soil l im- its may not coincide. Applicat ion o f wetland vegetation and soil standards in these areas is problematic . Refinement o f the standards is hampered by the scar- city of studies of vegetation and soil changes along measured hydrologic gradients (see Carter et al. 1994).

Two methods are c o m m o n l y used for e,caluating whether vegetation is hydrophyt ic . In the first method, described in the 1987 Manual , a plant communi ty is considered to be hydrophyt ic if more than 50% of dominant species nave a wetland indicator status of obligate (OBL), facultative wetland (FACW), or facultative (FAC) on regional lists o f plant species that occur in wet lands (Reed 1988). The second method is the prevalence index (Federal ln teragency Commit tee

512

Davis et al., W E T L A N D I D E N T I F I C A T I O N IN N O R T H - C E N T R A L FLORIDA 513

for Wetland Delineation 1989; hereafter, the 1989 Manual), which is a weighted average indicator status of all species in the community (or in a sample), where OBL = l, FACW = 2, FAC = 3, facultative upland (FACU) = 4, and upland (UPL) = 5 and weighting is by abundance or frequency. Both the 1989 Manual and the National Food Security Act Manual (Soil Conser- vation Service 1994) consider a plant community to be hydrophytic if the prevalence index is less than 3.0. Although the 1989 Manual considers these two methods to be equivalent, they can produce different re- suits. The differences need to be evaluated with long- term hydrologic data.

Hydric soils are identified in the field primarily by low-chroma (gray or grayish) colors in the subsoil of loamy and clayey soils and accumulation of organic matter in the upper horizons of sandy soils (Environ- mental Laboratory t987, Federal Interagency Com- mittee for Wetland Delineation 1989). Sandy soils do not usually have sufficient iron content to produce the colors seen in finer textured soils. Consequently, by_ dric soil indicators in sandy soils rely on accumulations of organic matter. Current lists of hydric soil morphological indicators in sandy soils are based mainly on the accumulated experience of soil scientists and mappers (Hurt and Brown 1995); few of these indicators have been tested against long-term hydrologic records (Segal et al. 1995).

The purpose of this study was to investigate the relationships among vegetation, soils, and hydrology along moisture gradients in north-central Florida. Spe- cific objectives were to (1) determine the relative positions of hydrophytic vegetation, hydric soil, and wet- hand hydrology boundaries based on readily observable field evidence, (2) examine the distribution of plant communities and selected soil properties in relation to hydrologic gradients, and (3) compare the results of recent Federal wetland delineation methods in Florida with 3-5 yr of hydrologic data. Flatwoods, low-relief, pine-dominated plant communit ies that are common in Florida landscapes, were of particular interest in this study due to often-ambiguous and conflicting indicators of wetland conditions.

STUDY AREAS

Six forested sites (Table 1) were selected that were part of earlier, ecological studies of Florida plant communities conducted in cooperation with the Center for Wetlands, University of Florida, and funded by the Florida Institute of Phosphate Research (Davis et al. 1991, Segal et al. 1995). Site selection was based on the presence of a natural hydrologic regime, relatively mature condition of the forest, and the long-term ac-

cessibility needed to monitor water-level fluctuations over several years.

Each site contained wet plant communit ies with obvious indicators of surface-water hydrology (e.g., standing water, buttressed tree trunks, lichen lines, water-stained leaves, drift lines) and upslope communit ies that did not have surface indications of hydrology. More specific communi ty types were identified at each site based on visual assessments of dominant vegetation and surface hydrology (Table 1). Following ter- minology of Wharton et al. (1977) for wet communities, cyp re s s d o m e s are depress iona l , s t i l l -water swamps that are dominated by Taxodium ascendens. Cypress strands are also dominated by T. ascendens and have surface water moving in multiple channels or sheet flow through the forest. Bayheads are depressional wetlands with tree canopies that can be dominated by Gordonia lasianthus, Magnolia virginiana, or Persea palustris. Cypress/bayheads are depressional, still-water swamps with tree canopies codominated by cypress strand and bayhead species. The red maple/ oak (Acer rubrum and Quercus spp.) and cedar (Cha- maecyparis thyoides) swamps occur along nonatluvial blackwater creeks and spring runs, respectively.

The three remaining community types occurred upslope of the wet communities. Flatwoods are the most common plant community in Florida landscapes. They are characterized by a relatively open overstory of pines, low shrub stratum, and a herbaceous layer that varies in abundance (Abrahamson and Hartnett 1990). Mesic mixed-hardwood forests are characterized by a diverse array of overstory and understory tree species but few species of herbs (Plait and Swartz 1990). Scrub is a xeromorphic shrub community dominated by a layer of evergreen oaks. with or without a pine overstory, that occupies well-drained, infertile sandy soils (Myers 1990). Ecotones were identified as areas with a plant species composit ion intermediate between or structurally different I¥om either of the adjacent upslope or wet plant communities.

METHODS

A line transect was established at each site from an upslope community, through the ecotone, if present, and into the wet community. Transects started from a random point at least 30 m upslope from the edge of the wet communi ty and were oriented perpendicular to its boundary. Relative elevations were surveyed every 2-3 m along each line except when microtopographic features such as hummocks or stream channels required more frequent measurements. Location of the upland extent of surface-water indicators was noted at each transect.

514 W E T L A N D S , Volume 16, No. 4, 1996

Table 1. Chm-acterization of study sites in north-central Florida.

Sile Name Hydrologic and Location Record Plant Communities and Selected Domimmt Species Soils Classification

Highland SR Highland Hammock State Park, Highlands County

Ocala NE Ocala Na- tional Forest, Marion County

Orange Co., Orange County Landfill, Or- ange County

Osccola NE Osceola National Forest, Ba- ker County

Tiger Creek. Tiger Creek Nature Pre- serve, Polk County

Mar 1 9 8 6 Flatwoods: Pinus etliottii Engelm., Lyonkt lucida Dec 1989 ; (Lam.) Koch, flex glabra (L.) Gray, Panic,m ab- 42 obser, cissum Swallen.

Bayhead: Gordonia lasianthus L,, Persea palustris (Raf.) Sarg., Magnolia virginiana L., Woodwardia virginica (L.) Smith.

Feb t985-Dec Scrub: Pinus clauxa (Chapm. ex Engelm.) Vasey ex 1988; 42 Sarg., Quercus chapmannii Sarg., Serenoa repet~.s obser. (Bartr.) Small.

Flatwoods: Pinus taeda L., G. lasianthus, S. repens. L. tucida. I. glabra, Osmunda cinnamomea 1,.

Cedar Swamp: Chamaecyparis rhyoides ~L.) BSR, Sabat palmetto (Walt) Lodd. ex Schuttes, llex cassine L_

Feb 1985-Jan 1989; 38 obsen

Flatwoods: Pinus serotina Michx., L gtabra, S, repens, L. lucida, Myrica c~rifera L., Panicum sp.

Cypress/Bayhead: Taxodium ascendens Brongn,. G. las&~nlhus, M. t:~rifera, L. lucida. L cassine.

Feb 1985-Dec Flatwoods: P. etliottii, L glabra, L, lucida, Vaccin- 1989; 45 ium myrsinites Lain. obser. Cypress Strand: T. ascendens, P. palustris, P. elliot-

tii. L glabra, L. lucida

Flatwoods: coarse-loamy, xi- Hceous, hyperthermic MoL lic OchraqualJs

Bayhcad: fine-brainy, siliceous, hyperthermic l'vpic Argiaquol[s

Scrub: Sandy, siliceous, hv- perthermic Entic H~lplohu- hinds

Flatwoods: N/A

Cedar Swamp: .randy or sandy-skeletal, siliceous, ~lvsic, hyperthermic "Ferric Medisaprisls

Flatwoods: sandy, siliceous. hyperthermic Typic Hap- taquods

Cypress/Bayhead: dysic, hyperthermic llvpic Medi- sapri~ts

Flatwoods: sandy, siliceous, thermic, Ultic Haplaquods

Cypress Strand: loamy, siliceous, thermic Gro.~.~aren- ic Paleaquugts

Mar 1986-Feb Mesic Mixed Hardwoods: Quercus laur(folia Michx., Mesic Mixed Hardwoods: 1990; 24 Carya glabra (Mill.) Sweet, Q. virginiana Mill., S. sandy, siliceous, hyperth- obser, repens, ermic Grosxarenie Entic

Red Maple/Oak Swamp: Acer rubrum L., Q. lauri folia. S. palmetto, Saururus cernuus L., Blechnum serrulatum L.

Withta, SE Withlacoo- Apr 1985-Jan chee State Forest, 1989:35 tlernando County obser.

Haphgmmods Red Maple/Oak Swamp:

sandy, ~iliceous. tOT~erth ermie Typic Haplaquods

Flatwoods: Q. laurifolia, I. glabra, S. repens. M. cer- Flatwoods: fine, montmoril- ifera, L. lucida, Befaria racemosa Vent. lonitic, h37wrthermic Tppic

Cypress Dome: 1" ascenden.~. Lacnanthes earolini- Afl~aqua!~' ann (Lain.) Dandy, W. virginica, Hyptis alata Cypress Dome: fine-loamy. (Raf.) Shinners. siliceous, hype rthermic

Histic Ochraquults

Hydrologic Monitoring

Monitoring wells were established at three points along each t r ansec t - -a t the upslope end, in the ecotone, and at the lowest elevation in the wet community. Distances between wells ranged from 35 to 104 m. Wells were constructed of 3,2-cm-diameter schedule 40 t 'V(: pipe, t ipped with a 30-era PVC well screen. Each well was installed in an augered hole, and soil was loosely backfilled around the pipe. Soils were generally sandy and highly permeable. Therefore, measurements reflected the unrestricted ground-water sur-

face, commonly called the water table. Wells were installed I - 4 m deep to maintain contact with ground- water during most dry periods and extended above surface water levels. Water levels were measured in each well monthly for 3-5 yr at each site between 1985 and 1989 (Table 1).

Wc obtained precipitatitm recorOs anti frequency analyses from the U.S.D.A. Natural Resources Con- servation Service (NRCS) Water and Climate Center (WCC), Portland, OR to determine the extent to which precipitation was normal during the years of hydro-

Davis et al,, W E T L A N D IDENTIFICATION IN NORTH-CENTRAL FLORIDA 515

logic monitoring on our study sites. The WCC calcu- lates precipitation frequency tables for individual weather stations by fitting the most recent three-decade ( 1961-1990) period of annual or monthly precipitation totals to a two-parameter gamma distribution. Precip- itation during any period of interest is considered to be "normal" if it falls within the 30th to 70th percen- tiles of the fitted rainfall distribution for this 30-year record.

According to the 1987 Manual (Environmental Lab- oratory 1987: 34, 36, 50) and recent guidance for application t~f the manual (6 March 1992, memorandum from U.S. Army Corps of Engineers Headquarters to District offices), an area has wetland hydrology if it is inundated or saturated to the surface continuously for at least 5% of the growing season in most years; such areas are identified as wetlands if they also meet hydric soil and hydrophytic vegetation requirements. The manual does not clearly specify at what depth the water table must be for saturation to be "to the surface." The NRCS, formerly the Soil Conservation Service. specifies that surface saturation occurs whenever the water table is within 15 cm of the ground surface in sands and 30 cm of the surface in other soils (Soil Conservation Service 1994). On these study sites, 5% of the growing season was from 15-18 days depending on local climatological data (USDA Natural Resources Conservation Service, Water and Climate Center, Port- land, OR).

The 5% water-table exceedence level was calculated at each site as the highest water level continuously reached or exceeded for 5% of the growing season in most years based on daily water levels interpolated between monthly readings. Error in estimating the 5% exceedence level using this method would be greatest where water-level fluctuations are frequent and large. Errors are not expected to be great because water-level fluctuations in these wetlands generally follow season- al ground-water levels (e.g., Mitsch and Gosselink 1993: 73). with rapid increases following rainfall events that usually drain to more stable levels within days (Heimburg 1984). As it is not likely that monthly water-level measurements occurred at peak water levels, interpolating between monthly readings probably overestimated the depth to the 5% exceedence level and resulted in conservative estimates of hydrologic boundaries. Growing seascm limits were defined by the dates of >28 °F ( - 2 °C) air temperatures for the average year reported for weather stations near the field sites by the WCC.

Exceedence values, mean water-table depths, and standard deviations of water-table measurements were calculated every 2-3 m along each transect using water-table readings interpolated between wells. The limits of wetland hydrology along each transect were es-

timated as the zone in which the 5% exceedence level was within 15 cm of the surface for sandy soils and within 30 cm for organic soils (hereafter referred to as the 5% exeeedence level +15 or +30 cm, respectively).

Soil Sampling and Analysis

Soils were sampled at two levels of detail using pits and cores. Soil pits were dug to standard soil survey depths next to ground-water wells to identify soils to series and to characterize hydric soil features (Segal et al. 1995). In addition, 40-cm-deep pits were dug at intermediate points along the same transects to characterize hydric soil features in each plant community. All pits were described in terms of standard soil survey characteristics (Soil Conservation Service 1991) and the following hydric soil features used by the Florida Soil Survey Staff (Hurt and Brown 1995): dark surface (hereafter referred to as dark A horizon), mucky texture, and muck. These hydric soil indicators differ in relative amount of organic matter to mineral soil content. "'Dark A horizon" is the uppermost A horizon >-- 10 cm thick of organic-rich sand with Munsell value/ chroma of 3/1 or darker; "mucky texture" is a layer >5 cm thick with 5-12% organic carbon by mass located within 15 cm of the mineral soil surface; and "muck" is a layer of highly decomposed organic matter on the mineral soil surface. Five other hydric soil features (stripped matrix, stratified layers, organic bod- ies, hydrogen sulfide, and depleted matrix) used by the Florida Soil Survey Staff were observed too infrequently to analyze.

Soil cores were extracted with a 5-cm inner diameter soil probe to at least 30-cm depths at I -5 m intervals along the transects at points where vegetation had been characterized. Thicknesses of muck, mucky sand, or dark A morphology were measured at each point. Based on these features, hydrie soil boundaries were identified on each transect (Soil Conservation Service 1992). Spearman rank correlations were used to investigate relationships between soil morphological characteristics and water-table measurements.

Vegetation Sampling and Analysis

Percent cover of each plant species was estimated in each of five strata (Federal Interagency Committee for Wetland Delineation 1989) within and above a 1.5 × 0.5 m plot at 1-5 m intervals along each transect during May 1993. Plots were spaced more closely in areas that were transitional between plant communities. A total of 208 plots was sampled at the six sites. Species that occurred in more than one stratum were recorded multiple times. In addition, total percent coy-

5 1 6 W E T L A N D S , V o l u m e 16. N o . 4 , 1 9 9 6

A. OCALA NAT IONAL F O R E S T

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F i g u r e I. C h a r a c t e r i s t i c s of vege t a t i on , soils , and h y d r o l o g y a l o n g e levat ional gradients at six n o r t h - c e n t r a l F lo r ida s tudy ~i t~ : . Ro lc l l i n e ~ a l o n g t h e X - a x i s ot-" e a c h g r a p h i n d i c - a t e t h ~ e x t e n t o f h y d r o p h y t i c v c g c t a t l u n b y d o m i n a n c e r lxt io, h y d r i c :~oil

indicators, and wet land h y d r o l o g y field indicators. B o l d l ines ex tend into the wet c o m m u n i t i e s b e y o n d the end o f the measurements to indicate that indicators o f wet land condi t ions did not stop at the end o f the transect. See assoc iated Results sect ions for further explanat ions o f graphs_

Davis e t al., W E T L A N D IDENTIFICATION IN NORTH-CENTRAL FLORIDA 517

E, O R A N G E C O U N T Y

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Figure 1. Continued

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LINE DISTANCE, m

er estimates were made for each species in each plot. Determinations of hydrophytic vegetation were made at each plot based both on the indicator statuses of dominant species (hereafter termed the dominance ratio) and the prevalence index weighted by percent coy- e l .

Distributions of dominant species in flatwoods plots were examined relative to hydrologic conditions (mean depth to water table, standard deviation of water-table depths, and depth to 5% exceedence level) with canonical correspondence analysis using CANOCO soft- ware (Ter Braak 1986, Ter Braak and Prentice 1988. Palmer 1993).

RESULTS

Hydrology

Dates of hydrologic measurements and numbers of growing season observations at each site used to cal- culate hydrologic variables are given in Table 1. Nine National Oceanic and Atmospheric Administration weather stations near our six study sites provided 39 station-years of precipitation data during the monitoring period. Data for 27 station-years (69%) fell within the normal rainfall range. Only one station (near the Orange County site) reported two consecutive years of higher-than-normal precipitation, and none reported consecutive years of lower-than-normal precipitation.

Mean depth to the water table +_- 1 standard deviation and depth to the 5% exceedence level relative to ground surface are shown in the lower graph of Figure 1 for each site. The " + 1 5 cm'" or " + 3 0 cm" label on the ground surface indicates the first point upslope of the wetland at which the 5% exceedence level was 15 or 30 cm below the ground surface, respectively. The portion of the transect downslope of this label was saturated to the surface or inundated for at least 5% of the growing season in most years during the period of study. There is no " + 1 5 cm" label on the Osceola NF transect because the 5% exceedence level was within 15 cm of the ground surface along the entire transect. The bold line along the X-axis of the same graph indicates the extent of wetland hydrology field indicators (e.g., water marks, water-stained leaves) that were observed in the field. These indicators were used to determine the extent of wetland hydrology in the absence of measured hydrology.

Hydrologic regimes varied with slope position, water source, local topography, and soil texture. Means and standard deviations of water-table depths were generally greater in wells at upslope locations than in the wet communities (Figure I). Water levels were most stable at Ocala NF and least stable at Withlacoo- chee SE The Ocala NF transect was situated at the base of deep sand hills that provided stable inputs of ground-water; the transect ended at a spring-fed

518 WETLANDS, Volume 16, No. 4, 1996

stream. The Withlacoochee SF site lacked deep organic soils that hold water and retard drainage and had a very small watershed in which rainfall was the primary water source. Water tables at the other sites had intermediate ranges of fluctuation. These sites were characterized by thick organic soils in the wet communities and received water in various proportions from rainfall, ground-water, and surface flows front upstream areas.

One or more of the following features coincided in the landscape where the 5% wate>table exceedence level intersected the ground surface along each transect (Figure I): (1) a downward break in topography, (2) the reIative elevation at which wetland hydrology field indicators became apparent, (3) shifts from upslope to wet plant community types, or (4) increased depths of muck. These features coincided at the 5% exceedence level +30 cm point on the Tiger Creek transect (Figure 1 c). Five-percent exceedence levels were consistently within 15 or 30 cm of the ground surface in all wet communities and were usually at or above the surface.

The tneasured wetland hydrology boundary did not correspond well with wetland hydrology field indicators. Wetland hydrology field indicators were located downslope of measured wetland hydrology boundaries as determined by the 5% exceedence level + 15 or +30 cm elevation at 5 of the 6 sites (Figure 1). In fact, the 5% exceedence level + 15 cm elevation consistently extended beyond the edge of the wet community into either the ecotones or upslope communities. The only exception was Ocala NE which had the steepest topographic gradient, where wetland hydrology lield indicators coincided with the 5% exceedence level + 15 c m .

Soils

Soils of the study areas consisted of either fine sandy mineral and/or sapric organic materials, except for two Alaquods (upsiope locations on Withlacoochee SF and Osceola NF transects) that were fine sandy to 56 and 94 cm, respectively, and loamy below. Soils were either Haplohumods, Alaquods, Humaquepts, or Saprists (Table 1).

Distribution of Hvdric Soils. Organic nmtter accumulations were found in 127 of 181 cores. The distributions of the organic hydric soil indicators are given in the central graph of Figure 1 for each site. Bold lines along the X-axis indicate tim extent of llydric soils based on the presence of muck or -->10 cm of dark A morphology or >-5 cm of mucky texture. Usu- ally only one kind of organic accumulation was present in a core, although several cores at Osceola NF

and Withlacoochee SF had a muck layer on top of mucky sand or dark A horizons.

All 38 sample points in wet communities had hydric soils based on presence of muck (Figure 1); 25 of these were Histosols (muck >40 cm thick). Indicators of hydric soils were lacking from all sample points in mesic hardwood and scrub communities. The upslope extent of hydric soil indicators coincided with or extended beyond the upland extent of wet communities and ecotones. Several patterns of hydric soil indicators were evident: (1) thickness of muck was generally greatest at the low ends of the transects and decreased upslope. (2) layers of mucky sand and dark A morphology were usually found upslope of the layers of muck, and (3) hydric soil features in flatwoods were variable but usually poorly developed.

There was good correspondence between the presence of hydric soil indicators and measured wetland hydrology (5% exceedence within 15 or 30 cm of the ground surface) in the mesic hardwoods, scrub, and wet communities (Figure I). Determinations of hydric soils in these communities were straightforward. There was not good agreement, however, between hydric soils and wetland hydrology in flatwoods or in ecotones between flatwoods and wetter communities. Be- cause wetland delineation is most problematic in flatwoods, the rest of the soils discussion is limited to the flatwoods portions of the transects.

Corretationx betweet~ Hydric Soil Indicators and Hy- drology in Flatwoods. Hydric soil indicators extended upslope of measured wetland hydrology in 3 of the 5 flatwoods communities ~Highland SR Oeala NE and Orange Co.), and wetland hydrology was present without hydrie soil indicators in a fourth (Osceola NF). Hydric soil determinations in flatwoods agreed with measured wetland hydrology only in the Withlacoo- chee SF flatwoods community, where both were ab- sent.

Muck was associated with different hydrologic regimes than dark A and mucky sand horizons. Muck was recorded at three flatwoods sites. Within these fiat- woods communities, muck was significantly thicker at downslope portions of the transect where depths to mean water table and 5% exceedence level were shallow- and water-table standard deviations were small (Table 2), in contrast, dark A and mucky sand horizons occurred upslope of muck accumulations at four of the five flatwoods sites. At the four flatwoods sites where dark A or mucky sand horizons were recorded, these organic accumulations were thicker where mean water tables and 5% exceedence levels were relatively deep (except at Wilh]acoochee SF) and standard deviation of water-table depths Iarge (except at Highlands SP) (Table 2). Dark A and mucky sand horizons dimin-

Davis et at., W E T L A N D I D E N T I F I C A T I O N IN N O R T H - C E N T R A L F L O R I D A 5 t9

Table 2. Spearman correlation coefficients between soil features and hydrologic parameters within individual flatwoods sites where the soil feature was present.

Standard Depth to Depth to 5% Deviation

Soil Feature/ Mean Water Exceedence of Water- Flatwoods Site Table ~ LeveP Table Depth

Thickness of Muck Highland SP

(n = 21) -0 .75"** -0.67*** -0,76*** Ocala NF

(n - 18) -0.89*** -0.84*** 0.92*** Orange Co.

¢n - 25) 0.89*** 0.89*** 0.08 ns

Thickness of Dark A Morphology Ocala NF

(n = 18) 0.72*** 0.63** 0.79*** Osceola NF

~,n = 24) 0.45* 0.09 ns 0.42* Withla. SF

(n = 16) -0 .42 ns -0 .51" 0.48 ns

Thickness of Mucky Fine Sand Highland SP

(n = 21) 0.52* 0.52* -0.50* Osceola NF

(n - 24) 0.37 ns -0 .17 ns 0.52** t Depths were measured as positive distances below the ground surface. : *, **, *** indicate significance at P < 0.05, P < 0.01, and P < 0.l)[)l. respectively; ns = not significant (P > 0.05),

i shed and even tua l ly d i s appea red far ther ups lope , where dr ie r c o m m u n i t i e s occur red , as in the scrub c o m m u n i t y ups lope o f the f ta twoods at Oca la N F (F igure 1 a). These re la t ionsh ips sugges t that o rgan ic mat te r is more l ikely to accumula t e on the soil sur face as muck where wa te r tables are sha l lower and m o r e sta- b le and more l ike ly to be incorpora ted into the minera l soil as da rk A or mucky texture hor izons where wate r tables are deepe r o r more var iable .

A l though re la t ionsh ips among organic mat ter accumula t ions and hyd ro log i c pa r ame te r s were not as evident be tween f la twoods sites as wi th in sites, genera l pa t te rns o f muck accumula t ion in f la twoods were s im- ilar. M u c k layers t ended to occur in f la twoods c o m - muni t ies at si tes with re la t ive ly s table wate r tables and sha l low depths to mean water tables and 5% exceedence levels . M u c k d id not accumula te at Wi th l acoo - chee SF f la twoods, which was the f la twoods site wi th the greates t water - tab le f luctuation. In fact, on ly a thin l aye r of muck was present in the W i t h l a c o o c h e e S F cypress dome , which had the wides t f luctuat ions o f wate r tables and sha l lowes t muck o f all the wet c o m - muni t ies .

No s ingle hyd ro log i c pa rame te r was cons i s ten t ly

useful in p red ic t ing hydr ic soil condi t ions (Table 2). Th is ind ica tes that a l though 5% exceedence levels ag reed with hydr ic soil de t e rmina t ions in the wet tes t and dr ies t communi t i e s , degree o f f luctuat ion and mean depth o f wate r tables also inf luenced o rgan ic mat te r a ccumula t i ons in fine sandy soi ls o f the flatwoods sites.

Vegeta t ion

S ix ty -e igh t plant spec ies were r eco rded as dominan t on at leas t one o f the 208 plots . The major i ty o f dominant spec ies (44) were t rees and shrubs. D o m i n a n t herb spec ies were p r imar i ly perenn ia l ferns and grass- es. Al l ind ica to r status ca tegor i e s f rom O B L to UPL were represented .

Distribution of Hydrophytic Vegetation. The distr i- but ion o f h y d r o p h y t i c vege ta t ion is r ep resen ted in the upper g raph o f F igu re I for each site. P reva lence in- d ices are g r a phe d re la t ive to d i s tance a long the transect; va lues less than 3.0 indica te h y d r o p h y t i c vegetation. Vert ical l ines mark the bounda r i e s o f plant communi t ies . Bold l ines a long the X-axes indicate the extent o f h y d r o p h y t i c vege ta t ion based on d o m i n a n c e ratios.

Eight o f the s a m p l e d p lan t c o m m u n i t y types had h y d r o p h y t i c v e g e t a t i o n d e t e r m i n a t i o n s tha t a g r e e d with the m e a s u r e d wet land hydro logy . Plant c o m m u - ni t ies with obv ious we t l and h y d r o l o g y field indica tors ( cypress strand, cypress dome , bayhead , cyp res s /bay - head, red m a p l e / o a k swamp, and ceda r s w a m p ) were loca ted be low the poin t at which the 5% cxcccdence level was wi thin 15 or 30 c m of the sur face and were hydrophy t i c based both on d o m i n a n c e ra t ios and prev- a lence ind ices (F igure l ) . Scrub and m i x e d mesic h a r d w o o d c o m m u n i t i e s were loca ted at e l eva t ions more than 15 cm a b o v e the 5% e xc e e dc nc c level and d id not have h y d r o p h y t i c vege ta t ion by e i ther measure . F l a t w o o d s p lant communi t i e s , however , were dis t r ib- uted over a wide range o f hyd ro log i c cond i t ions and d id not cons is ten t ly mee t hyd rophy t i c vege ta t ion criteria.

Vege ta t ion at three o f the five f la twoods t ransects was c lea r ly h y d r o p h y t i c based both on p reva lence in- d ices and on d o m i n a n c e rat ios (F igure l a, b, and d). A l o n g the W i t h l a c o o c h e e SF transect (F igure I f), prev- a lence ind ices in f l a twoods f luctuated a round the 3.0 threshold , whi le d o m i n a n c e rat ios ind ica ted hydro- p h y t i c v e g e t a t i o n , a l t h o u g h m a r g i n a l t h r o u g h o u t . There was a d i f fe rence , however , be tw e e n the two methods at Orange Co., where f la twoods vege ta t ion was gene ra l ly h y d r o p h y t i c based on p reva l ence index but mos t ly n o n - h y d r o p h y t i c by d o m i n a n c e rat ios a long the ma jo r i ty o f the t ransect (F igure I e).


2 ~

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I t . 2 O O I I I I L , -I 50 - 1 O0 -50 0 50 1 O0 1 SO 200 250 300

AXIS 1

Figure 2. Canonical correlation results for dominant flatwoods plant species at five north-central Florida study sties. Mean depth to water table (MEAN). depth to the 5% exceedencc level (5%), and standmd deviation of water-table depth (SD) were significantly correlated with Axis 1. Only species that occm'red al more lhan one site are labeled: Gaylussacia frondosa (GAYFRO), Gordonia lasianthus (GORLAS), llex glabra (ILEGLA), Lyonia lucida {LYOLUC), Myrica cerifera (MYRCER), Osmunda cimzamomea (OSMCIN), Persea palustris (PERPAL L Pinus elliottii (PINELL), and Serenoa repens (SERREP).

Across flatwoods sites, there was no consistent re- lationship between presence of hydrophytic vegetation, by either method, and measured wetland hydrology (Figure 1). Wetland hydrology did not supporl the hydrophytic vegetation determinations at Ocala NF and Highland SP flatwoods. Hydrophytic vegetation determinations were supported by wetland hydrology at Os- ceola NF flatwoods. Withlaeooehee SF and Orange Co, flatwoods had marginal hydrophytic vegetation determinations in areas where 5% exceedence levels were well below 15 cm from the surface.

As with analyses of soils and hydrology, there was a good correspondence between hydrophytic vegetation and measured hydrology in the mesic hardwoods, scrub, and wet communities; identification of hydrophytic vegetation was relatively straightforward in these communities. We will limit the rest of our vegetation discussion to the more problematic flatwoods p o r t i o n ~ o f t h e t r angec t~ .

Relationships bet~,een Dominant Vegetation and Hy- drology in Flatwoods. Figure 2 displays the ordina- tion diagram of dominant species scores and environmental vectors for flatwoods communities. The axes

can be thought of as results of multi-dimensional re- gression analyses; Axis 1 explained 52% of the variance among the species distributions, and Axis 2 explained 47% of the residual variance. The direction and length of the environmental vectors indicate the direction and strength, respectively, of correlations with species distributions. Positions of species on the graph indicate the relative hydrologic conditions where they were round in this study. For example, Gordonia lasianthus (GORLAS) and Persea palustris (PERPAL) were both found in areas with relatively shallow depths to the mean water table and 5% exceedence level, and with less water-level fluctuation. In contrast, Myrica cerifera (MYRCER) was usually found in dri- er areas with more variable water levels.

Mean water-table depth, standard deviation of water-table depths, and depth to the 5% exceedence level were all significant fac|ors (P < 0.01 ) in the distribution of flatwoods plant species. Correlations were strongest with the first axis for all hydrologic parameters. Water-table standard deviation was most strongly correlated (r = 0.72), followed by mean water-table depth (r --- 0.57) and depth to 5% exceedence level (r

Davis et al., WETLAND IDENTIFICATION IN NORTH-CENTRAL FLORIDA 521

Table 3. Comparisons of wetland boundary locations (meters from the upslope end of the transect) and communities identified as wetlands at six study sites as determined by different wetland delineation methods based on the 1987 and 1989 Manuals.

1987 Manual with 1987 Manual ~ Hydrologic Measurements 2 1989 ManuaP

Study Site Location Communities Location Communities Location Communities

Ocala NF 56 Cedar Swamp 54 Cedar Swamp 25 Flatwoods Cedar Swamp

Highland SP 98 Bayhead 94 6 Flatwoods Ecotone Bayhead

45 Red Maple/ 25 20 Red Maple/ Oak Swamp Oak Swamp

84 Cypress Strand 70 70 Ecotone Cypress Strand

52 Ecotone 45 28 Low Flatwoods Cyp./Bayhead Ecotone

Cyp./Bayhead 35 32 32 Cypress Dome

Tiger Creek

Osceola NF

Orange Co,

W'ithla. SF Cypress Dome

Low Ecotone Bayhead

Red Maple/ Oak Swamp

Ecotone Cypress Strand Low Flatwoods Ecotone Cyp./Bayhead Cypress Dome

Based on hydrophytic vegetation by dominance ratio, hydric soil indicators, and surface indicators of wetland hydrology, : Based on hydrophytic vegetation by dominance ratio, hydric soil indicators, and measured wetland hydrology. We did not use hydrologic data to override hydric soil decisions. -' Based on hydrophytic vegetation by dominance ratio or prevalence index and hydric soil indicators in the assumed absence of significant hydrologic modification.

= 0.50). Therefore, Axis 1 represents a hydrologic gradient primarily determined by water-level fluctuations and secondarily by average depth of the water table and depth to the 5% exceedence level (Figure 2). Wa- ter-table standard deviation was also correlated with Axis 2 (r = 0.46).

Wetland Boundary Determinations

Wetland boundary determinations at our study sites differed considerably depending upon (1) which manual (1987 or 1989) was followed, (2) which method (dominance ratio or prevalence index) for evaluating hydrophytic vegetation was used, and (3) whether direct hydrologic measurements were used to support wetland hydrology decisions.

Although the 1987 Manual and current guidance on its application assume that the user will employ con- siderable professional judgment and flexibility, a strict reading of the manual indicates that the wetland boundary should be the highest point on the gradient where evidence of all three parameters--hydrophytic vegetation, hydric soils, and wetland hydrology-- i s present. For the vast majority of wetland determinations, which are performed without benefit of direct hydrologic measurements or detailed soil and vegetation sampling, delineation of the boundary is based on readily observable indicators of each parameter.

If water-table measurements had not been available to us, field hydrologic indicators would have been the

critical parameter defining jur isdic t ional wet land boundaries at all six of our study sites, In each case, hydrophytic vegetation and hydric soil boundaries were upslope of the limits of hydrology indicators. In general, field hydrologic indicators were found only in the wettest community on each transect, where there was obvious evidence of surface flooding or ponding. Therefore, without the benefit of direct hydrologic evidence and by a strict interpretation of the 1987 Man- ual, all of the wet communities and only a portion of the Orange Co. ecotone would be identified as wetlands (Table 3 and Figure 1).

The 1989 Manual used a different rule for determining the wetland boundary. In the absence of evidence of significant hydrologic modification, the jurisdictional boundary was defined as the highest point on the gradient where evidence of hydrophytic vegetation (by either prevalence index or dominance ratio) and hydric soils was present. By this rule, wetland boundaries would extend (with some discontinuities) farther upslope at all sites than those determined under the 1987 Manual (Table 3 and Figure 1). In addition to the wetlands delineated under the 1987 Manual, the 1989 Manual would include flatwoods at Ocala NE Highland SP, and part of those at Orange Co. as wetlands. In all cases, evidence of hydric soils either extended as far upslope as the hydrophytic vegetation boundary or was the limiting factor in determining the wetland boundary.

With hydrologic data, one can evaluate wetland by-


drology criteria directly without having to rely on field indicators. Using the three-parameter test in the 1987 Manual and wetland hydrology defined as the 5% water-table exceedenee level within 15 or 30 cm of the surface depending on the soils, wetland boundaries at Ocala NE Highland SP, and Tiger Creek would be delineated at the limits of wetland hydrology: Orange Co. at the limit of hydrophytic vegetation and wetland hydrology; and Osceola NF and Withlacoochee SF at the limits of hydric soil morphology (Table 3 and Fig- ure 1). All of the flatwoods communities except the extreme lower end of the Orange Co. flatwoods would be outside jurisdictional wetland limits.

Clearly, the exact form of the rules for aggregating inlkwmation about vegetation, soils, and hydrology into a wetland determination, and whether or not direct hydrologic measurements are available, have a great in- fluence on jurisdictional wetland limits in north-central Florida, Differences in potential positions of wetland boundaries were least at Tiger Creek Preserve, where there was an obvious topographic break between upslope and bottomland communities, and greatest at Highland SP.

DISCUSSION

Communities at the extremes of the hydrologic gradients were readily determined to be either wetlands or uplands by both the 1987 and 1989 Manuals. Hy- drophytic vegetation, surface hydrologic indicators, and presence of muck consistently identified areas at the lowest relative elevations along our transects as wetland communities. The lack of all three parameters in mesic mixed hardwoods and scrub clearly identified these areas as upland communities. These determinations were supported by direct hydrologic measurements. Wetland indicators, however, were not consistently present in the flatwoods communities, nor was there good correspondence between wetland indicators and hydrologic data in flatwoods. Although flatwoods were frequently determined to be wetlands under the 1989 Manual, hydrologic data supported the 1987 Manual determination of most flatwoods areas as non- wetlands.

Flatwoods in Florida cncompass a variety of vegetation, soil, and hydrologic conditions. They are commonly found in low-relief landscapes (the Ocala NF transect w'as an exception) forming a matrix around wetlands (Abrahamson and Hartnett 1990). Depending on location in the hydrologic basin, water sources in flatwoods vary in importance among rainfall, intercep- tion of the fluctuating ground-water table, and lateral ground-water flow from upsiope areas. This study il- lustrates that flatwoods occur in a wide range of hy-

drologic conditions and that wetland determinations in flatwoods are very difficult without hydrologic data.

Wetland hydrology is delined in terms of a single hydrologic criterion: the 50% chance of recurrcncc of inundation or saturation to the ground surface for at least 5% of the growing season (Environmental Lab- oratory I987), The repeated coincidence of the 5% exceedence level at the ground surface with several landscape features indicates that this hydrologic parameter corresponds to an ecologically significant elevation. Upland edges of tile wet communities generally occurred where the 5% exeeedence level emerged from upslope communities at a downward break in topography. This often coincided with sharp increases in depth of muck and a change in plant communities from higher to lower prevalence indices. Flatwoods did not occur below this elevation in the hmdscape. Since hydrophytic vegetation and hydric soils corn- monly occur in flatwoods and often lie > 15 to 30 cm above the 5e~ exceedence level, other factors must in- fluence the development of wetland indicators in these low-relief landscapes.

A wetland hydrology threshold based on the 50% chance of recurrence of continuous inundation or saturation for 59~ of the growing season is based on work done in bottomland hardwood forests of the south eastern United States {Clark and Benforado 1981). These forests are often located adjacent to gaged rivers and streams, and effects of frequency and duratinn of inundation on forest ecology are relatively well established (Wharton et al. 1982). In the Southeast, 5% of the growing season also roughly corresponds with the 14-day saturation period required for wetland hydrology by the Soil Conservation Service (1994).

In north-central Florida, application of the 5% exceedence level within 15 or 30 cm of the surface to delineate wetlands with steep drainage gradients or good hydrologic connections with streams generally worked well. For example, at Tiger Creek, the site with the steepest gradient, limits of hydric soils, hydrophytic vegetation, and wetland hydrology closely coincided. Our Ocala NF site was an exception to the good correspondence of wetland indicators in steep landscapes, because there was evidence of ground-water seepage above the 5% exceedence level. This sug- gests that more investigation is required in the use of the 5% exceedence level to determine wetland bound~ aries in slope wetlands.

Emphasis on the 50% chance of recurrence of in undation or saturation lbr 5% of the growing season to identify wetlands becomes problematic, however, in low-relief landscapes where drainage is relatively slow. With slow drainage, the lower limits of upland plant species distributions at a site can be determined with a single high-water event (Whitlow and Harris

Davis et aL, WETLAND IDENTIFICATION IN NORTH-CENTRAL FLORIDA 523

1979), leaving FAC and FACW species as dominants in the infrequently saturated areas above the wetland hydrology boundary. In addition, the presence of organic hydric soil indicators above the wetland hydrology boundary at most of our flatwoods plots indicates that less frequent and shorter periods of saturation may be sufficient to cause accumulation of organic matter within the soil. With a flat topography, a small change in water level affects a much greater area for a longer period of time than in areas where drainage gradients are steep. Thus, one result of small variation in frequency or duration of high-water events in low-relief landscapes is that the upland extent of hydrophytic vegetation and hydric soils becomes obscured. These wetland indicators may reach far upslope from the wetland hydrology boundary in distance but not necessarily in elevation. Others have shown that hydrophytic vegetation and hydric soil boundaries do not necessarily coincide in low-relief landscapes (Erickson and Leslie 1988, Allen et al. 19891, with hydrophytic vegetation often extending farther upslope than hydric soils (Carter et al. 1994).

This study indicates that the distributions of hydrophytic vegetation and organic hydric soil indicators in flatwoods of north-central Florida are influenced by a combination of hydrologic parameters, including mean depth and variability of the water table, as well as depth to the 5% exceedence level. Further investigation is required to determine the utility of hydrologic parameters in addition to the 5% exceedence level for delineating wetlands in low-relief landscapes.

ACKNOWLEDGMENTS

We thank Robert Tighe, Russell Pringle, and Freak Watts for their invaluable assistance in the field. Bar- bara Kleiss, R. Daniel Smith, Helen Light, Melanie Darst, and Debra Segal made significant improvetnents to the manuscript. This work was funded by the U.S. Army Corps of Engineers Wetland Research Program. Permission to publish this work was granted by the Chief of Engineers.

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Carter, V., ET. Gammon, and M.K. Garrett. 1994. Ecotnne dynamics and boundary determination in the Great Dismal Swamp. Ecolog - teat Applications 4:189-203.

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Erickson, N.E. and D.M. Leslie, Jr. 1988. Soil-vegetation correlations in coastal Mississippi wetlands. U. S. Fish and Wildlife Ser- vice, Office ot Biological Services. Washington, DC. USA. FWS/ OBS-89/3.

Federal Interagency Committee for Wetland Delineation. 1989. Federal Manual for Identifying and Delineating Jurisdictional Wetlands. U.S. Army Corps o1 Engineers, U.S. Environmental Protection Agency, 1.J.S. Fish and Wildlife Service, and U.S.D.A. Soil Conservation Service, Washington, DC, USA. Cooperative Technical Publication.

Heimhurg, K. 1984. Hydrology of North-Central Ftorida Cyprcss Domes. p. 72-82. In K.C. Ewel and H.T. Odum (eds) Cypress Swamps. University of Florida Press, Gainesville, FL. USA.

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Platt, W.J. and M.V~'. Swartz. 1990. Temperate Hardwood Forests. p. 194 229. In R.I. Myers and J.J. Ewel (eds.l Ecosystems of Florida. University of Flortda Press, Orlando. FL, USA.

Reed. RB., Jr. 1988. National list of plant species that occur in wetlands: 1988 national summary. U. S. Fish and Wildlife Ser- vice, Washington, DC, USA. Biological Report 88124t.

Segal, D.S., S.W. Sprecher, and F.C. Watts. 1995. Relationships between hydric soil indicators and wetland hydrology for sandy soils in Florida. U. S. Army Engineer Waterways Experiment S1ation, Vicksburg, MS, USA. Technical Report WRP-DE-7.

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Wharton, C,H., WM. Kitchens, E.C. Pendleton, and q2W. Sipe. 1982. The ecology of bottomtand hardwood swamp~ of the South- east: a community profile. U,S+ Fish and Wildlife Service, Office of Biological Services, Washington, DC, USA. FWS/OBS-81/37.

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Manuscript received 24 May 1995; revisions received 27 November 1995, 29 April 1996, and 29 May 1996; accepted 19 August 1996.

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Environmental gradients and identification of wetlands in north-central Florida