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WETLANDS, Vol. 21, No. 2, June 2001, pp. 265–273 q 2001, The Society of Wetland Scientists

RELATIONSHIP BETWEEN HYDROLOGY AND ZONATION OF FRESHWATER SWALE WETLANDS ON LOWER HATTERAS ISLAND, NORTH CAROLINA, USA Richard D. Rheinhardt1,2 and Karl Faser1 1 Department of Biology East Carolina University Greenville, North Carolina, USA 27858 2 E-mail: [email protected] Abstract: Only a few relatively wide barrier islands support shallow freshwater aquifers. Rare, swale wetlands occur on islands where fresh water inundates, at least seasonally, low-lying troughs between interior dunes. Swale wetlands are dominated by emergent vegetation and submerged aquatic vegetation in the deepest areas and by woody shrubs in more shallow areas. On southern Hatteras Island, wetland shrubs have progressively invaded open water areas over the past 40 years, suggesting a change in hydrologic regime. To determine the relationship between vegetation cover type and length of saturation, water-level fluctuations over time were analyzed to tie boundaries of six wetland cover types to the duration of soil saturation at 20-cm depth. We found that areas dominated by herbaceous vegetation had significantly longer flooding regimes than areas dominated by shrubs (85–95% vs. 12–69% of the growing season, respectively). Only 22–25 cm elevation differences were found to separate emergent marsh from the various shrub cover types, suggesting that lowering the mean water level via drainage has likely been responsible for shrub swamps replacing emergent marsh in swales. Although succession from open water to shrub swamp probably occurs naturally in the absence of drainage through the accumulation of organic matter, natural disturbances such as wildfire and storm-driven surges of saline water would have periodically re-set succession. Therefore, managing for long-term maintenance of freshwater swale wetlands on barrier islands should include (1) eliminating or controlling drainage through constructed ditches, (2) eliminating man-made barriers that prevent the transport of saline water into ponds during hurricanes and nor’easters, and (3) initiating a prescribed burning program to mimic the historic, natural fire regime. Key Words: tuations

barrier islands, fire, freshwater wetlands, marsh, shrub swamp, succession, water level fluc-

(Schafale and Weakley 1990). Although water-table measurements have never been tied to vegetation in swales, the conventional wisdom is that submerged aquatic plants and emergent marsh develop where length of saturation is longest, while flood-tolerant shrubs grow where length of saturation is shortest (i.e., at the interface between upland dunes and on hummocks in areas of open water). Although duration of saturation is undoubtedly the main factor controlling plant distribution patterns in wetland swales, other factors (e.g., competition and natural disturbances) are probably also important. Because fresh surface water is relatively rare on barrier islands, wetland swale communities are also relatively rare (Schafale and Weakley 1990). However, barrier islands that have sufficient fresh water to support swale wetlands were also among the first islands settled (and exploited) by humans, thus making unaltered wetland swale communities even rarer today. In order to preserve (via proper management) the few swale wetlands remaining, it is imperative to learn how

INTRODUCTION Most barrier islands along the Atlantic and Gulf coasts of North America are low, narrow strips of sand with little or no available fresh groundwater (Leatherman 1980, Stalter and Odum 1993). However, a few relatively wide barrier islands support shallow freshwater aquifers (Heath 1988, Odum and Harvey 1988). These unconfined, freshwater lenses float atop more dense saline water (Art et al. 1974) and often inundate, at least seasonally, low-lying swales (troughs between sandy ridges). Several types of freshwater wetlands occur in interdunal swales, depending in part upon their duration of soil saturation. The North Carolina Natural Heritage Program calls herbaceous and open water swales ‘‘Interdune Ponds’’ because they are flooded for most of the year in most years (Schafale and Weakley 1990). Locals call these freshwater marshes ‘‘Sedges’’ because sawgrass (Cladium jamaicense Crantz) and other Cyperaceae are prevalent in them. Areas dominated by shrubs have been classified as maritime shrub swamp 265

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they respond to natural and anthropogenically-induced fluctuations in environmental conditions. We assumed that freshwater wetland swale communities on barrier islands are maintained by fluctuations of the freshwater lens, which controls the timing, frequency, and length of soil saturation. The objectives of this study were to determine how various wetland cover types in swales are related to period of saturation and use this information to predict how alterations to hydrologic regime would affect succession from open water to shrub swamp or vice versa. This information is essential for managing remaining barrier island swale wetlands and restoring those that have already been anthropogenically altered. STUDY AREA Swales This study was conducted on lower Hatteras Island, a part of the Outer Banks of North Carolina, USA (Figure 1). Lower Hatteras Island is a deltoid-shaped section approximately 12-km long by 5-km wide at the widest point. Most of the southern two thirds of lower Hatteras Island is publicly owned and managed by natural resource agencies (Cape Hatteras National Seashore and North Carolina Coastal Reserve), including most freshwater wetlands on the island. Most of the northern one third of the island is privately owned and has been or is being developed for residential and commercial use. Lower Hatteras Island formed on a Pleistocene headland 12,000 years ago and has since prograded southwestward in response to a slowly rising sea level, prevailing winds, and longshore currents (Riggs et al. 1995). Because this portion of the island remained anchored to the headland as it accreted sand, it has been expanding in width and now is quite wide for a barrier island. The accreted sands have formed a series of somewhat parallel, east-west trending, high sandy ridges (relic dunes) alternating with low-lying swales. The highest relic dunes, which support maritime forests, run along the center axis of the island in an east/ west direction. A series of freshwater swales (interdunal ponds and maritime shrub swamps) also lie along this east/west axis at the base of the relic dunes. Forested dunes of various heights border the freshwater swales on Hatteras Island. Where the transition from swale to upland is steep, only a few tens of centimeters in elevation separate swale wetlands from xeric maritime forests. Along the borders of some interdunal ponds, a low sill occurs at the edge and maritime shrub wetlands inter-digitate among low dunes beyond the pond margin. When water levels are high, the pond and shrub wetlands connect via surface water. Wetland

shrubs also occur along upland/wetland borders; others occur on raised hummocks throughout both the interior of ponds and on low sills. The largest interdunal pond on Hatteras Island, called Jennette Sedge, begins near the far eastern (widest) end of the island and continues westward approximately 3.5 km. West of Jennette Sedge, the ponds get progressively smaller and more narrow and are crossed by low dunes in several locations. At about 8.2 km west of the eastern end of the island (near Frisco, NC), the island is too narrow to support surface water. Climate and Hydrologic Regime Weather on Hatteras Island is ameliorated by its close proximity to the Gulf Stream. Winters are milder and summers cooler than nearby mainland locations; mean January temperature is 7.9o C (46.2o F) while the July mean is 25.8o C (78.4o F) (National Oceanic and Atmospheric Administration 1995). Mild winter temperatures mean that the growing season lasts for more than 60% of the year: 227 days (March 29 to November 11) (Tant 1992). Annually, Hatteras Island receives an average of 141 cm (55.5 inches) of precipitation (National Oceanic and Atmospheric Administration 1995). Most precipitation occurs in July, August, and September during hurricane season. In fact, hurricanes are common on or near Hatteras Island due its oceanward position relative to other barrier islands of the Outer Banks. From 1806 to 2000, 18 hurricanes hit Hatteras or passed near enough to cause flooding and overwash. Thirteen of those 18 hurricanes occurred in August or September (Stevenson 1989, pers. obs.). Nor’easters, extratropical coastal storms caused by low pressure disturbances, are frequent from September through February. Like hurricanes, nor’easters bring large amounts of rain and often cause extensive flooding via tidal surge. However, nor’easters occur more frequently (at least once per year) than hurricanes and tend to last longer (generally 3–5 days). Thus, nor’easters probably contribute more to Hatteras’ annual average precipitation budget and frequency of overwash than do hurricanes. Water-table fluctuations are primarily determined by input from rainwater and output through creeks and man-made ditches, evapotranspiration (ET), and ground-water discharge at the edges of the island. Interdunal ponds usually flood to more than one meter in depth in autumn and winter when ET is minimal and coastal storms (nor’easters and occasional hurricanes) deposit large amounts of rain. Submerged aquatic vegetation and emergent marsh plants thrive during this period, particularly during the early growing season (April through June) when atmospheric

Rheinhardt & Faser, HYDROLOGY AND ZONATION OF SWALE WETLANDS

Figure 1.

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Location of study area on Hatteras Island. Study was located near wells (dots) on Jennette Sedge.

temperatures rise. In particularly wet years, flooding is so deep (. 1.5 m) that open water remains in ponds throughout the summer and persists in adjacent, lowlying shrub swamps as well. In fact, many interdunal ponds have been flooded long enough in most years that a peat layer more than 40-cm thick occurs in some places (Brown 1983 and unpublished data of senior author). In contrast, during dry periods or particularly dry years (e.g., summer of 1992) the water table drops more than 0.5 m below the bottom of swales for several months during summer. The water table on Hatteras Island manifests itself as a freshwater lens, which floats atop underlying, saline water (Heath 1988). This freshwater lens has an elongated, convex shape oriented along the longest (east/west) axis of the island, with the highest portion of the lens located on the eastern (widest) end of the island and near the center (Anderson 1999, Anderson et al. 2000). The shape of the lens is due to the general shape and orientation of lower Hatteras Island: narrow

in the north/south direction, elongated in the east/west direction, and wider at the eastern end than the western end. As a result, freshwater swale wetlands are concentrated in the center and eastern end of the island where the water table is highest. Water-table elevation slopes downward from the island center toward the Atlantic Ocean (on the south) and Pamlico Sound (on the north). Therefore, fresh ground water is discharged into the ocean and brackish fringe marshes on the Sound. The shape and orientation of the fresh-water lens means that water-level curvature along any north/south trending line can be assumed to be essentially identical to any parallel line nearby (i.e., within 100 m). This relationship was crucial for designing this study in that we could determine vertical distance to the water table for any point near a well line by knowing (1) the water-table elevation along the north/south well line, (2) the ground-surface elevation of the point in question, and (3) the north/south position of the point rel-

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ative to the north/south-oriented well line. With this information, period of soil saturation for any location could be determined by knowing the behavior of the well line over time. METHODS Water levels were recorded from two strings of monitoring wells located approximately 5-km apart (Gregory and Morgan 1996). Both well strings were oriented in a north/south transect from the middle of Jennette Sedge northward toward Pamlico Sound. However, because subsequent analysis revealed the same results from both study sites, results from only one of the study sites (Figure 1) are provided here. We recognized 6 wetland cover types (described below) associated with freshwater swales. Upper and lower boundaries of 5 of these cover types were fairly distinct, so elevations of both boundaries were surveyed using a laser level (providing 100 sample points). The upper and lower boundaries of the SAV (Submerged Aquatic Vegetation) cover type were indistinct, so this type was surveyed at 10 points only. Each cover type boundary was surveyed in such a way that no pair of survey points (upper and lower boundary) was located closer than 5 m from any other pair of surveyed points. All points were surveyed within 25 m of the well line to ensure that water-table elevations at the surveyed points could be tied to water-table fluctuations along the well line. To do this, we laid a meter tape along the well line from well #1 northward to wells #2, #3, #4, and #5. Well #1 (the most southerly well) was located in the interdunal pond (in open water); wells #2, #3, #4, and #5 were located progressively more northward and spaced approximately 50–60 m apart. Wells #2 and #3 were located in shrub-dominated wetlands, and well #4 and #5 were located in maritime forest (upland) (Figure 2). For each surveyed boundary point, we determined its position relative to the meter tape stretched along the well line by using a compass to sight 90o from the tape to the measured point. Thus, for each surveyed cover-type boundary, its position was recorded relative to its position northward of well #1 (located in the interdunal pond). All surveyed locations south of well

#1 were recorded as 0.0 distance northward from well #1 because the water table is essentially flat along a distance of at least 200 m south of well # 1 (Anderson 1999), particularly when the pond is flooded. By measuring the elevation of cover-type boundaries and their locations northward from well #1, we were able to tie those boundaries to water-table fluctuations along the well line. Water-table elevations at points between wells were interpolated using a FORTRAN program developed by D. Evans. Well data were evaluated only for days during the growing season in which there were simultaneous records from all five wells. Since data were not available for the entire growing season for all three years, duration of saturation among each cover class was compared only for the period of record. Differences among cover-type boundaries in mean period of saturation were determined using a Model I Analysis of Variance (ANOVA) followed by a Tukey multiple comparison test to examine differences between means. Surveyed cover types and boundary conditions were defined as follows: (1) Submerged Aquatic Vegetation (SAV) consisted of aquatic floating and submerged vascular plants (Potamogeton spp./Nymphaea odorata Aiton community) rooted on the bottom of the interdunal pond. SAV were either completely submerged or partially floating on the pond surface. The range in elevation for SAV was determined by surveying the elevation of the substrate (between tussocks of emergent plants). Because we could not distinguish between upper and lower boundaries for SAV, only 10 points were surveyed for this cover type. (2) Emergent Marsh (EM) consisted of emergent vascular plants rooted on the bottom of the interdunal pond, dominated by Cladium jamaicense, Echinochloa walteri (Pursh) Heller, Typha spp., Polygonum punctatum Ell., Lippia lanceolata Michaux, Hydrocotyle verticillata Thunberg, and Boehmeria cylindrica (L.) Swartz. The bases of emergent plant tussocks were measured at the lower elevation boundary of (EML) at various locations throughout the interdunal pond. Each upper elevation boundary (EMH) was determined by surveying emergent marsh tussocks located closest to →

Figure 2. Cross-section of well alignment relative to topography and water-level elevations for three dates during the 1993 growing season. Vertical lines show well locations; thick portions denote water-table elevations) and the line connecting wells denotes the water table. The topographic profile drawn for the Maritime Forest (right of Well 3) is inaccurate in that Wells 4 and 5 were located atop high dunes, which were much higher than 4.0 m above mean sea level (msl). Note the high stand of water in early spring and a low water table in late summer (from April 1 to August 31). Note also the abrupt (1-day) rise in the water table from August 31 to September 1 following a 20-cm rainfall event.


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270 the pond edge, some of which were beneath overhanging ecotone shrubs. (3) Shrub Hummock (SH) consisted of wetland shrubs and small trees (primarily Salix nigra Marshall, Myrica cerifera L., and Cornus stricta Lam. growing on islands of higher elevation (hummocks) within the interdunal pond. Each lower boundary of shrub hummock (SHL) was determined by surveying elevations at hummock edges where herbaceous vegetation gave way to shrubs. Each upper boundary (SHH) was surveyed at the highest elevation on each hummock island. Ten distinct hummock islands were surveyed. (4) Ecotone Shrub Swamp (ESS) consisted of hydrophytic shrubs and small trees (primarily Myrica cerifera L. and Persea borbonia (L.) Sprengel) that comprised ecotone vegetation between the interdunal pond and adjacent uplands. The upper boundaries (ESSH) were surveyed at the most upgradient locations nearest the upland/wetland ecotone where hydrophytic shrubs were rooted (usually at the toe-of-slope along the ecotone). The lower elevation boundaries (ESSL) were defined by the most pondward (downgradient) extent where hydrophytic shrubs were rooted. (5) Maritime Backswamp (MBS) consisted of hydrophytic shrubs and small trees (primarily Persea borbonia, Cornus stricta, and Myrica cerifera) that grew in swales interdigitating among interior dunes but with surface-water connections to the interdunal pond during periods of high water. Upper elevation boundaries (MBSH) were surveyed at the upgradient position of hydrophytic shrubs at the wetland/upland ecotone. The lower elevation boundaries (MBSL) were defined by the most downgradient location where the hydrophytic shrubs were rooted. (6) Backswamp Hummock (BSH) consisted of hydrophytic shrubs and small trees located on hummocks within backswamp swales. Backswamp Hummocks were floristically similar to Maritime Backswamps (Persea borbonia, Cornus stricta, and Myrica cerifera), but woody vegetation occurred on hummocks rather than along the wetland/upland ecotone. Upper elevation boundaries (BSHH) were surveyed at the top of the hummocks; lower elevation boundaries (BSHL) were surveyed at the lowest elevations where shrubs were rooted. Total time during which the water table was at ground level (0.0 m) and below ground level by 0.1 m and 0.2 m was determined for each surveyed point. However, because the total time in which the water table exceeded these three elevations followed the same pattern relative to cover-class boundaries, we chose to discuss water-table behavior relative to only one elevation:2 0.2 m. We chose this depth because the top 0.2 m of the root zone encompasses the portion of the root zone most likely to be stressed by saturated

WETLANDS, Volume 21, No. 2, 2001 conditions. Therefore, we define duration of saturation as the total amount of time the water table was above 0.2 m below ground level. RESULTS Well data were available for 485 days from 1993– 1995: 227 days in 1993, 97 days in 1994, and 161 days in 1995. This was 70% of the growing season period for all 3 years. During this period, precipitation was 8.05 cm below normal in 1993, 18.49 cm above normal in 1994, and 15.82 cm above normal in 1995 (National Climatological Data Center 1993, 1994, 1995). The water level in interdunal ponds fluctuated more than 1 m during the study. The most rapid fluctuations occurred immediately following large rainfall events (e.g., Figure 2b to 2c). After a period of high rainfall during the growing season, the water table tended to fall slowly in response to ET. However, when rainfall occurred near the end of the growing season, when ET was minimal, the water table did not drop until the following spring. Thus, late summer hurricanes and fall nor’easters tended to quickly pump up the water table, and interdunal areas remained inundated throughout the subsequent fall, winter, and early spring months. As expected, soils of Submerged Aquatic Vegetation (SAV) and Emergent Marsh (EM) in the interdunal pond were saturated longest, approximately 95% of the time for SAV and 85% (EMH) to 90% (EML) for Emergent Marsh (Figure 3). In contrast, soils of Shrub Hummocks (SH), located throughout the pond interior, were saturated 40% (SHH) to 68% (SHL) of the recorded period. The tops of Shrub Hummocks (SHH) were at least 55 cm higher than the bottom of the pond but varied widely in maximum elevation. Soils of Ecotone Shrub Swamps (ESS), rooted along the pond/upland ecotone (pond edge), were saturated from 30% (ESSH) to 58% (ESSL) of the time. Thus, soils at the lower boundaries (ESSL) were saturated for only 10% less time than the lower Shrub Hummock boundary (SHL). However, the lower boundary of Ecotone Shrub Swamp varied widely in duration of soil saturation due to the wide range in elevation over which hydrophytic ecotone shrubs grew. Although this cover type occurred at a higher elevation (and thus was saturated for less time) than Emergent Marsh, limbs of ecotone shrubs overhung the pond 5–6 m in many places, thus shading areas where emergent vegetation would have otherwise been able to grow. Soils of Maritime Backswamps were saturated 12% (MBSH) to 60% (MBSL) of the period. Although soil in the upper boundary of Maritime Backswamp was saturated for the shortest duration relative to the other


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Figure 3. Duration of saturation (mean and 95% confidence intervals) at 0.2-m depth over the period of record for boundaries of 11 wetland cover types. Letters A through E above bars denote which boundary conditions significantly differ (P,0.01) between one another in mean period of saturation (i.e., no significant difference could be found among types with the same letter). Abbreviations: SAV5 Submerged Aquatic Vegetation, SH5 Shrub Hummock, EM5 Emergent Marsh, ESS5 Ecotone Shrub Swamp, MBS5 Maritime Backswamp, BSH5 Backswamp Hummock; sub-scripted letters: L5 low boundary, H5 high boundary.

cover types, duration of saturation for the lower boundary was similar to that of the lower boundary of other shrub cover types. In contrast, soils of Backswamp Hummocks (BSH), located within Maritime Backswamps, were saturated between 47% (BSHH) and 69% (BSHL) of the period. Thus, the lower boundary (BSHL) was saturated for approximately the same amount of time as the lower boundary of Shrub Hummocks (SHL), located in the interior of the interdunal pond. The ANOVA test showed that inundation and soil saturation differed significantly among cover types (P,,0.0001; Power .98%). A Tukey multiple comparison test further revealed that SAV and Emergent Marsh (EM) did not vary significantly in duration of saturation (Figure 3). However, both of these cover types were saturated for significantly longer than were any of the shrub cover types. In addition, the lower boundaries of shrub cover types (SHL, MBSL, and BSHL) did not differ significantly in period of saturation from one another. However, the upper boundary of Backswamp Hummocks (BSHH) did not differ significantly from the lower boundary of Ecotone Shrub Swamps (ESSL) in duration of saturation. Backswamp Hummocks (BSHH) also were unusual in that they were saturated for significantly less time than were the upper boundaries of Maritime Backswamps (MBSH) or Ecotone Shrub Swamps (ESSH).

DISCUSSION Although each vegetation cover type differed somewhat in elevation and the length of time its soils were saturated, shrub cover types were more similar to one another in the length of time their soils were saturated than they were to SAV or Emergent Marsh. This supports the conventional wisdom that differences in length of flooding or soil saturation (58–68% for shrub cover types vs. 85–95% for herb cover types) separate shrub-dominated and herb-dominated wetland communities. The fact that the upper boundary of Backswamp Hummocks did not differ from the lower boundary of Ecotone Shrub Swamp in duration of saturation suggests that shrubs on the hummocks may still be in the process of increasing the elevation of the hummocks via the accumulation of organic matter. Perhaps shrubs on Backswamp Hummocks have not been able to grow as rapidly as shrubs on hummocks in interdunal ponds because they have to compete for light with Maritime Backswamp shrubs and shrubs located on nearby uplands, both of which tend to form canopies that overtop hummocks. The difference in mean elevation between the upper Ecotone Shrub Swamp boundary (ESSH) and lower boundary (ESSL) probably represents the elevation (25 cm) that mean water level would have to rise to convert shrub cover types to Emergent Marsh in the study

272 area. A rise of 25–30 cm would probably kill shrubs on hummocks, since the upper and lower boundaries of shrub hummocks (SHH and SHL, respectively) are approximately 22 cm apart in elevation, and backswamp hummock boundaries are approximately 23 cm apart. In contrast, a lowering of the mean water table would likely have the opposite effect (i.e., shrubs would be expected to invade interdunal ponds). A study by Berman et al. (1996) of aerial photography taken in 1955, 1970, and 1990 revealed that shrubs have been progressively invading Jennette Sedge since 1955, particularly at the eastern end of the pond. In calculating changes among cover types over this period, Rheinhardt and Brinson (1997) found that approximately 45% of emergent marsh and open water present in 1955 had been replaced by shrub swamp by 1990. A series of ditches, constructed during the 1930s prior to acquisition by the National Park Service, drain water from the eastern end of Jennette Sedge to Pamlico Sound. Other ditches drain the northern and southern parts of the island. A streamflow study conducted by the United States Geological Survey (http://water data. usgs. gov/ nwis- w/ NC/ data. components/ hist. cgi? statnum50208463120&bdatepmonth503&bdatepday5 18&bdatepyear51994&edatepmonth509&edatepday5 30&edatepyear51995&mode5graph&graphsize51.5 &dateformat50) found that the ditch draining Jennette Sedge removed 1022 to 1021 m3 s21 (0.4 to 4.5 cfs) from September through April in 1994 and 1995. Based on a study simulating the effects of this surface drainage, Anderson (1999) predicted that the water table in Jennette Sedge was about 30 cm lower during wet periods than it would have been if ditches were absent. If these drainage ditches have indeed been effective in lowering the water table of interdunal ponds, then such drainage may have partially contributed to the invasion of shrubs into the interdunal ponds. However, it is unlikely that excess drainage would have been the only alteration allowing shrubs to invade emergent marsh areas. Before the advent of fire suppression by humans on Hatteras Island, an accumulation of downed wood following hurricanes would have provided sufficient material to fuel lightning-sparked wildfires (Davison and Bratton 1987, pers. obs.). Also, aboriginal peoples probably set fires to improve hunting (Pyne 1982). Wildfires would have most likely occurred during summer droughts when interdunal ponds would have been without water. Fires burning across dried-out swales would have burned emergent grasses, underlying peat, and shrub hummocks in interdunal ponds. Thus, we expect that fire would have reset succession in ponds and could even have deepened ponds wherever peat burned. Today, wildfires are aggressively suppressed on Hatteras Island, and no major

WETLANDS, Volume 21, No. 2, 2001 fires have occurred there since at least 1955 (Davison and Bratton 1987). Thus, wildfire suppression may have also partially contributed to the invasion of shrubs into Jennette Sedge. Prior to development of the eastern end of lower Hatteras Island and construction of an entrance road to the National Seashore, tidal surges associated with nor’easters and hurricanes probably flooded Jennette Sedge (particularly the eastern end) with saline water. Periodic storm surges would likely have killed wetland shrubs in Jennette Sedge, particularly shrubs on hummocks. Marsh vegetation would have tended to recover relatively quickly, but shrubs would have taken much longer to recover (van derValk and Davis 1978). We suspect that the frequency and amplitude of storm surges into Jennette Sedge, particularly those associated with nor’easters, have been largely impeded by roads and ditches built on the eastern end of the island between Jennette Sedge and the ocean (Figure 1). Therefore, we suspect that a reduction in the frequency of flooding by saline water (via storm surge) has probably also contributed to the invasion of shrubs into Jennette Sedge. The above suppositions regarding parameters that affect shrub invasion into interdunal ponds were based on our observations of seral stages between open water and shrub swamp. Based on these observations, we propose successional events that we feel occur in interdunal ponds on Hatteras Island and which may be applicable to other barrier islands as well. Succession in interdunal ponds Toward the end of the growing season (early fall), emergent vegetation begins to decay, forming matted clumps of various sizes and thicknesses (we often had to push such floating mats aside when moving between survey points). Driven by wind, these decaying mats drift slowly across the surface of an interdunal pond, and in the process, coalesce with other mats. In the spring, Salix sp. (willows) sprout on the floating mats and grow from seedlings into small saplings (we observed floating mats with a variety of willow age classes growing upon them). As water in ponds draws down during summer, organic mats begin to rest on pond bottom, first along the more shallow pond edges and later more toward the middle (deeper) areas. During particularly dry years, mats may rest on the bottom long enough for willow saplings to extend their roots through the mats and into the pond bottom. After the pond refloods, some willows remain rooted to the bottom and grow larger over time (willows are adapted to withstand long periods of soil saturation and flooding). As willows grow larger, they probably intercept and

Rheinhardt & Faser, HYDROLOGY AND ZONATION OF SWALE WETLANDS trap floating mats of organic matter in their stems and adventitious roots. In trapping organic mats, hummocks build around willows over time. The tops of hummocks occur at an elevation at or above the usual high water level (we observed more hummock formation at the edge of ponds than in the middle of ponds). Shrub hummocks eventually grow high enough to support other wetland shrub species (Myrica cerifera, Cornus stricta, Persea borbonia). Eventually, these other shrub species displace willows. Shrub islands continue to grow in size and shrub species richness until fire or saline water associated with a storm surge kills shrubs and resets the successional sequence. We observed remnants of hummocks with charcoal in only one or two places, leading us to believe that it had been a long time since fire had reset the successional sequence. Of course, the role of fire in resetting succession is speculative, but considering the observation of shrub invasion, it seems that open water areas in swales would not persist unless succession was periodically arrested. At this time, high dunes (particularly those on the south and west sides of the interdunal ponds) prevent wind-blown sand from burying the ponds. Therefore, the integrity of surrounding dunes should be protected from anthropogenic degradation to prevent burial by sands. Under current conditions, it is unlikely that interdunal ponds on Hatteras Island will be able to maintain themselves indefinitely. Rare interdunal swale ecosystems on barrier islands (including those on Hatteras Island) could be maintained over the long-term by (1) eliminating or managing artificial drainage from freshwater swales (to reduce the frequency of complete drawdown during droughts and increase flooding depths), (2) maintaining a natural fire regime with prescribed burns, and (3) re-establishing connections with ocean and/or sound waters so that periodic storm surges can reach interior swales. ACKNOWLEDGMENTS The U.S. National Park Service, Water Resources Division provided financial support for this study. Cape Hatteras National Seashore generously provided on-site accommodations throughout most of the study period. David Evans provided his well-monitoring data and an interactive FORTRAN program to determine period of saturation for locations between wells. LITERATURE CITED Anderson Jr., W. P. 1999. The hydrology of Hatteras Island, North Carolina. Ph.D. Dissertation. North Carolina State University, Raleigh, NC, USA. Anderson, W. P., Jr., D. G. Evans, and S. W. Snyder. 2000. The effects of Holocene barrier-island evolution on water-table ele-

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