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Introduction

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Compartmentalization of the Everglades ecosystem through the installation of levees, canals, and other water-control structures during the late 19th and 20th centuries has altered the distribution of native plant communities throughout the system. Historically (or pre-drainage), most of the central Everglades was occupied by a ridge and slough landscape, characterized by elongate, dense stands of sawgrass separated by open water sloughs (Fig. 1) and scattered tree islands (McVoy, 1999). The ridge and slough landscape now is restricted to Water Conservation Area 3A (WCA 3A) and Everglades National Park (Fig. 2), with other areas occupied almost entirely by broad expanses of sawgrass or monospecific stands of cattails. These changes have been attributed at least partly to 20th century changes in water management practices as well as nutrient enrichment of surface waters from agricultural activities in the north. In 2000, the Comprehensive Everglades Restoration Plan was enacted to restore the natural hydrology and communities of the Everglades wetland ecosystem. This report presents paleoecological data from Everglades ridge and slough sites that addresses: the age of the features; long-term variability in community composition; stability of sawgrass ridge and slough size; and the response to 20th century changes in hydrology. These data are necessary to accurately predict the response of the ridge and slough plant communities to planned changes in water management.

aerial photograph showing ridge and slough landscape
Figure 1. Ridge and Slough Landscape. Photo courtesy of Thomas Smith III (USGS). [larger image]
The Everglades is a freshwater, nutrient-limited, subtropical wetland covering an area of approximately 6,000 km2 (Davis and others, 1994) in southern Florida. The system consists of a mosaic of vegetation communities, including tree-islands, mangrove forests, cypress domes, marl prairies, sawgrass marshes, sawgrass ridges, and sloughs (Davis, 1943; Loveless, 1959; Davis and others, 1994). The ridge and slough landscape, the focus of this research, consists of dense sawgrass (Cladium) stands oriented approximately northeast to southwest, parallel to flow, separated by open-water sloughs dominated by waterlily (Nymphaea) (Figure 1). Their distribution, as well as the distribution of all plant communities within the Everglades, is controlled by water depth, hydroperiod (which Lodge 2005 states is the average annual duration of continuous flooding), substrate type, and fire regime (Kushlan, 1990).

map of Greater Florida Everglades and Core Locations
Figure 2. Map of Greater Florida Everglades and Core Locations. The above map illustrates the compartmentalization of the Greater Everglades into Water Conservation Areas. The historical extent of the ridge and slough landscape is represented by the light blue shading (modified from McVoy, 1999). The core collection locations are marked with a yellow circle. [larger image]
In the natural Everglades system, rainfall and the overflow of water from Lake Okeechobee dictated the hydrologic patterns. Water flowed southward from Lake Okeechobee along a gentle slope of 3 cm/km (Kushlan, 1990), eventually reaching Florida Bay and the Gulf of Mexico through Shark River Slough and, to a lesser extent, Taylor Slough. The late 19th through the early 20th century mark the beginning of the intensive drainage efforts to render parts of the Everglades usable for agricultural and urban development (Light and Dineen, 1994). The construction of four drainage canals (the North New River, Hillsboro, Miami, and West Palm Beach) drained 607,100 ha. Alterations continued with the completion of the Tamiami Trail in 1928 (Light and Dineen, 1994), greatly reducing freshwater flow across the wetland. Even more extensive compartmentalization began in the 1950's, when three Water Conservation Areas (WCA) (Figure 2), which are a series of canals and levees, were constructed to control flooding within the northern Everglades (Light and Dineen, 1994). Each WCA was designed for specific purposes regarding the containment and release of water; therefore, hydrologic conditions (i.e. hydroperiods) within each WCA differ. This alteration of the natural hydroperiod resulted in extreme changes of the natural landscape. For example, in Water Conservation Area 2, a combination of altered hydroperiod and nutrient enrichment resulted in the replacement of the discrete ridge and slough vegetation environments by a homogenous sawgrass landscape (McVoy, 1999).

To restore a more natural distribution of plant and animal communities within the Everglades ecosystem, the U.S. Federal and Florida State governments enacted the Comprehensive Everglades Restoration Plan (CERP). CERP aims to achieve flow patterns similar to the historic (or pre-drainage) hydrologic regime through modification and removal of existing water-control structures. For restoration to be successful, it is critical to understand the factors controlling the distribution of specific wetland communities. This requires an understanding of the origin of individual community types, how they have responded to past climatic changes, and how they responded to 20th century hydrologic changes. Such data are needed to determine whether the system has the resilience to return to pre-drainage conditions.

We designed this study to determine how vegetation in the ridge and slough landscape has changed during the 20th century and to assess the stability of ridges and sloughs over centennial to millennial time scales. Results are presented from three transects of cores collected in relatively pristine ridges and sloughs of Water Conservation Area 3A (Fig. 1). The following questions will be addressed through this research: 1) Can sawgrass ridges and sloughs be distinguished from one another in the palynologic record; 2) Have these two environments always been discrete; 3) How have they responded to natural climate variation; and 4) How have they responded to 20th century compartmentalization?

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