Hydrology, vegetation, and landscape distribution of depression wetlands on the Francis Marion National Forest Final Report 11/16/2005 (updated 3/1/2006) Diane De Steven1 and Charles A. Harrison2 USDA Forest Service Southern Research Station Center for Bottomland Hardwoods Research, Stoneville, MS 2 Center for Forested Wetlands Research, Charleston, SC
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Author for contact: Dr. Diane De Steven, USDA Forest Service, Southern Hardwoods Lab, P.O. Box 227, Stoneville, MS 38776; (662) 686-3602; e-mail:
[email protected]
Summary. On the Francis Marion National Forest, depression wetlands are a significant part of the plant community diversity, and they also provide critical habitat for sensitive wetland animals such as pond-breeding amphibians. Both vegetation and habitat suitability for wetland fauna depend strongly on wetland hydrological behavior. Therefore, understanding what properties may regulate hydrology can provide a more informed basis for management. We used a structured study approach to monitor depression hydrologies across defined landscape settings, to characterize depression vegetation, and to identify landscape or site properties that correlate with hydrologic and vegetation diversity. Nineteen wetlands across the Forest were monitored for changes in pond stage over a two-year period spanning both wet and dry years. Vegetation types of 27 wetlands varying in size and location were characterized. We compiled observations on wetland soils, and also on impacts from recent upland prescribed burns. Many factors interact in complex ways to shape wetland hydrology and plant communities, but some general patterns emerged. The principal findings were that: •
Depression wetlands do not exhibit a single hydroperiod pattern. Inherent differences among wetlands are partially predictable from landscape setting, soil type, and size.
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Depression wetlands are diverse at a Forest-wide (landscape) scale, as reflected in different hydrologic patterns and different natural vegetation types. The natural variety of hydrologic types is likely important to the meta-population dynamics of pond-breeding amphibians.
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There is not a uniform fire regime for all depression wetlands, because fire effects differ naturally according to hydrology, landscape setting, fire season, and climate conditions.
Our findings also suggested that soil landscape settings, a potential refinement of Forest Ecological Units, can provide a useful framework for ecologically-based integration of wetland and upland management.
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BACKGROUND AND OBJECTIVES Management goals for the Francis Marion National Forest (FMNF) include providing for forest diversity and sustainable wildlife habitat, conserving unique areas, maintaining an ecologically sound distribution of plant communities, and incorporating ecological approaches in management (USDA Forest Service 1996). One element of these goals is to identify and maintain wetland plant communities within the Forest for their diversity and habitat values. Depression wetlands (Carolina bays and others) are an abundant wetland type on the FMNF. In wetland science, wetlands are classed by their hydrodynamics — i.e., the primary water source (rainfall, overland flow, or ground water) and the pattern of water movement (e.g., flowing, stagnant, or tidal) (Brinson 1993). As a group, depression wetlands are a hydrodynamic category typified by: 1) lack of inlets and outlets, 2) supply by rainfall, possibly supplemented by ground water, and 3) non-flowing “vertical” dynamics (changes in ponding depth). By definition, they are embedded within uplands and so are geographically “isolated” from other wetlands and water bodies (Sharitz 2003). As depressions, they function in similar ways ecologically, but among them there is great variety in size, form, and biological communities. Management may be improved by understanding the distribution and causes of this variety. Following recent loss of regulatory jurisdiction over “isolated” wetlands, depression wetlands are now at greater risk of loss nationwide and, effectively, have become a threatened ecosystem (e.g., Southern Environmental Law Center 2004). Notably, they are critical habitats for sensitive semi-aquatic fauna such as pond-breeding amphibians; habitat suitability for various species is strongly influenced by wetland hydrology and other properties. Public lands such as the FMNF could become essential refuges for these threatened wetlands, thus necessitating a more informed basis for ecological stewardship and management. Studies of Upper Coastal Plain depressions on the Savannah River Site (De Steven & Toner 2004) revealed correlated patterns for the distribution of wetland types in relation to hydrologic, landscape, and other site properties. Such patterns can provide an ecological framework for conservation and management. We proposed a similar effort for the FMNF as a comparable managed land area in the Lower Coastal Plain, where less topographic relief and higher regional water tables could result in different correlated patterns. We identified and utilized “soil landscape settings” across the FMNF to plan a structured study of depression hydrology, soils, and plant communities. We also compiled observations on effects of upland prescribed burns. Our overall goal was to develop a better understanding of factors that affect wetland diversity and habitat suitability for depression pond fauna. This report summarizes the preliminary findings from hydrologic monitoring and vegetation surveys to address these objectives: •
Describe depression wetland distribution across FMNF landscape settings
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Characterize wetland hydrologic patterns and determine to what extent hydrology is predictable from wetland location or other site features such as soil type
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Develop a simple functional system for classing depression vegetative communities and assess what environmental site properties influence depression vegetation types
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METHODS Wetland identification and selection. We used a representative sampling approach to describe depression wetland distribution on the FMNF and to choose study sites: •
In a GIS, published NRCS soil maps were used to create a generalized map of principal soil/landform associations (“landscape settings”) across the FMNF (Fig. 1 and App. 1).
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The soils map (Fig. 1) suggested three upland landscape settings (Table 1) where depression wetlands could potentially be found (excludes the large expanses of swampy hydric soils). These soil landscapes are correlated with underlying surface geology (App. 1). Table 1. Features of FMNF upland landscape settings where depressional wetlands occur. Soil landscape setting Feature
Sandy
Loamy
Clayeya
Physiography
ridges
flats
terrace (?) flats
barrier beach
backbarrier lagoon
riverine/backbarrier
Predominant upland soils
deep sands
sands with loamy subsoils
loams with clay subsoils
Typical upland soil series
Cainhoy, Witherbee, Chipley (Echaw)
Bonneau-Norfolk, Goldsboro-Lynchburg
Duplin-Craven, Lenoir-Wahee
Pleistocene landforms
a
This setting is not distinguished in FMNF Ecological Units (currently classed with sandy ridges/side slopes)
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In the GIS, we examined false-color IR aerial photography (flight year 1995) covering an area equivalent to one 7.5 quadrangle for each upland soil landscape (see App. 2 for topographic quads surveyed). All visible depressional wetland features greater than ~0.2 acre were delineated, and areas calculated in the GIS. Apparent man-made ponds were excluded.
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From the resulting list of 187 sites, we selected 19 focal wetlands for hydrology monitoring, and 27 for vegetation survey (App. 2). The wetlands were distributed across soil landscape settings and were chosen to represent a range of sizes (0.5–22 ac); choice was also subject to the constraints of occurring on National Forest land and reasonable access from a road.
Though not a complete assessment for the entire FMNF boundary, this sampling was representative of the three landscape settings and included most uplands in National Forest ownership (see App. 1) except the Bethera-Cordesville area. We were unable to sample the few very largest wetlands. Most were either highly disturbed, on private lands, or dominated by impenetrable vegetation. We had permission to create access into two Carolina bay pocosins, but we could not complete this owing to subsequent resource constraints. We supplemented the report with published literature on pocosins when possible. 3
Figure 1. Generalized soil associations on the Francis Marion National Forest (legend explanation in Appendix 1). (low-resolution graphic) 4
Hydrology monitoring. Staff gauges (Type M stream gauge) for monitoring pond stage (surface water depth) were installed in 19 wetlands during March−April 2003. Each wetland had one gauge placed at its deepest center point. For easier access in large wetlands, 1–2 additional gauges were placed at shallower depths (closer to the wetland edge) and calibrated to the center gauge. Gauges were checked bi-weekly, and water level readings were translated to pond stage at the wetland center. Monitoring of gauged sites was conducted over two years (Apr 2003−Apr 2005), after which it had to be discontinued for administrative reasons. For 8 additional wetlands, we measured pond stage at a marked deep point periodically during 2004, and then used data from the gauged wetlands to interpret hydrologic behavior. Rainfall data were obtained from a meteorological station (#25) maintained within the Santee Experimental Forest. These data were supplemented by rain gauges (US Weather Bureau type) placed in 3 locations across the FMNF in June 2003 (Macedonia area, Cainhoy/Northampton area, Honey Hill/Echaw Creek area); rainfall accumulations were recorded bi-weekly until May 2005. In general, the year 2003 was wetter than normal and 2004 was drier (see Fig. 3, p. 8). We used cluster analysis to assign a “hydrologic index” value to each wetland based on its maximum pond stage, variability in pond stage, and annual hydroperiod (% of the year ponded >0 m), particularly in the drier year of 2004. The index ranged from 1 (shallowest, most variable and shortest hydroperiod) to 5 (deepest, least variable and longest hydroperiod). We focused on pond stage dynamics as the process most relevant to wetland vegetation and fauna. Limited resources precluded installing shallow wells for studying water tables, but the existing pond stage data now offer a basis for choosing suitable wetlands for such study should the interest arise. With staff gauges already installed, surface monitoring can be easily resumed as needed. Soil sampling. Eighteen of 27 wetlands were sampled for soil properties in 2003, and 3 others in 2005. Planned sampling of the remaining wetlands was prevented by water-level rises; we hope to have those sites sampled at a later date when water levels drop sufficiently. Within each sampled wetland, two or three soil profiles were described and assigned to series by an NRCS Soil Scientist. The centermost profile was sampled for soil chemistry at 3 depths: surface (0–20 in. or less), subsoil (ca. 20–48 in.), and profile base (48–60 in.). Samples were analyzed by the Clemson University Soil Genesis Lab for pH, extractable macro- and micro-nutrients, total organic carbon (multiplied by 1.7 to estimate percent organic matter), and particle size distribution. Copies of all soil data were provided to the NRCS scientist and to the Soil Scientist in the Frances Marion-Sumter NF Supervisor’s Office, Columbia, SC. Vegetation sampling and wetland classification. We sampled vegetation composition at the whole-wetland scale with a line intercept method used in previous studies (De Steven & Toner 2004). In transects spanning the wetland long axis and two perpendicular axes, species occurrences in multiple strata were recorded in 1-m line segments at 10-m intervals along each transect (20-m intervals in the largest wetlands). This method yields species frequency as an abundance measure that is proportional to species cover, except for some overestimation of rare species abundance. A whole-wetland sampling approach introduces more data variability than plot-scale sampling, but provides a more general description for the wetland as an integrated system. It is not a complete species inventory. Fifteen of 27 wetlands were sampled in late summer 2003 and the remaining twelve in late summer 2004. Cluster analysis of species abundances was used to group the wetlands into vegetation types based principally on dominant species; the groups were cross-walked with National Vegetation Classification (NVC) alliances. 5
RESULTS Depression wetland distribution Over the surveyed area, we identified a total of 187 natural depression wetland features, of which nearly 50% were 10 acres (Fig. 2A). Our 27 study wetlands represented most size classes except the largest (see Methods). Wetland size and form have clear distribution patterns across the FMNF (Fig. 2B). On the less well-drained loamy and clay flats, depressions may be irregularly shaped and sometimes indistinct from other low swales or basins. Clay flats had relatively fewer wetlands, all of which were small; loamy flats had a greater range of wetland sizes but none >10 acres. The largest depressions — the distinctly oval Carolina bays that range from 10 to 200 acres (4–80 ha) or more — occur only on the sand ridges (principally the Cainhoy and Bethera ridges; cf. App. 1). Sand ridges also support many smaller depressions, including “limesink” subsidence features. Figure 2. Size distribution of depression wetlands identified in the FMNF survey. A. All wetlands (n = 187).
B. Distribution across landscape settings. 50
100
all wetlands study wetlands
90
40 number of wetlands
number of wetlands
80 70 0.5
30 area (ac) 20
50
10
20 10 0
0
50 wetland area (ac)
clayey loamy sandy soil landscape
Depression wetland soils Depression wetlands on the FMNF do not have a uniform soil type. Among the 21 wetlands sampled to date, soils ranged from deep sands (South Carolina Group 1) to sandy loams (Group 3) to clayey soils (Group 4) (cf. Clemson University CES 2001). The deep sands are sandy to 60” depth and some have a spodic (Bh) horizon, the loam soils have loamy or clayey horizons at depths of 30–40”, and the clay soils have clay subsoils starting within
