Abstract from the 2002 AGU Spring Meeting.

Pollen records in sediment cores from sites in the historic Everglades allowed us to document the natural variability of the ecosystem over the past 2,000 years and contrast it to 20th century changes in wetland plant communities. The natural system included extensive water-lily sloughs, sawgrass ridges, and scattered tree islands extending from Lake Okeechobee southward through Shark River Slough. Between 1000 AD and 1200 AD, weedy species such as Amaranthus (water hemp) became more abundant, indicating decreased annual rainfall, shorter hydroperiods, and shallower water depths during this time. After 1200 AD, vegetation returned to its pre-1000 AD composition. During the 20th century, two phases of hydrologic alteration occurred. Completed by 1930, the first phase included construction of the Hoover Dike, canals linking Lake Okeechobee to the Atlantic Ocean, and the Tamiami Trail. Reconstructions of plant communities indicate that these changes shortened hydroperiods and lowered water depths throughout the Everglades. The extent of water-lily slough communities decreased, and tree islands became larger in Shark River Slough. The second phase resulted from construction of canals and levees in the 1950s, creating three Water Conservation Areas. The response of plant communities to these changes varied widely depending on location in the Everglades. In Loxahatchee NWR, weedy and short-hydroperiod plant species became more abundant in marshes, and species composition of tree islands changed. In Water Conservation Area 2A, cattail replaced sawgrass in marshes with high nutrient influx; the ridge and slough structure of the marshes was replaced by more homogeneous sawgrass marshes; sustained high water levels for more than a decade resulted in loss of tree islands that had existed for more than 1,000 years. In Everglades National Park, the extent of slough vegetation decreased further. Near Florida Bay, the rate of mangrove intrusion into fresh-water marshes accelerated due to increased diversion of fresh water from the southernmost Everglades. Our reconstruction of pre-1930s vegetational distribution provides a scenario that may be a harbinger of Everglades vegetational response to decompartmentalization of the system as levees and canals are removed and restoration of a more natural (deeper water, rain-driven seasonality) hydrologic regime. Drowned tree islands provide the one exception; they were destroyed by a hydrologic regime that does not occur naturally, and it is unclear whether their recovery is possible.

¹dwillard@usgs.gov U.S. Geological Survey, 926A National Center, Reston, VA 20192 United States
²cbernhardt@usgs.gov U.S. Geological Survey, 926A National Center, Reston, VA 20192 United States
³cholmes@usgs.gov, U.S. Geological Surevy, 600 Fourth Street South, St. Petersburg, FL 33701 United States
⁴U.S. Geological Survey, 926A National Center, Reston, VA 20192 United States

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