The history of vegetation, hydroperiod, and salinity at 20 sites in the historic Everglades was reconstructed in order to understand the natural variability of the system and its response to past environmental changes. These sites are located in marshes and sloughs of the predrainage Ridge and Slough area, Taylor Slough, elongate tree islands in Water Conservation Area 3 (WCA-3), and the mangrove fringe along Florida Bay. Using the palynological record from these sites, we documented regional changes in hydroperiod and vegetational composition during the intervals known as the Medieval Warm Period (9th-14th centuries) and Little Ice Age (15th-19th centuries) as well as the changes on a more local scale in response to the 20th century alteration of the ecosystem.

Sediment cores collected in WCA-1, 2A, 3A, and 3B document the history of Ridge and Slough sites over the last 2,000 years. From about A.D. 0-800, each site was analogous to a modern slough, with abundant floating aquatic vegetation (such as Nymphaea and Utricularia), long hydroperiods, and relatively deep water. From about A.D. 800 to 1200, pollen of weedy annual plants (such as Amaranthus and the Asteraceae) dominated assemblages, indicating greater fluctuations of hydroperiod and water level, more frequent droughts, and/or greater fire frequency. After a return to wetter conditions and slough vegetation in the 13th and 14th centuries, most sites exhibited greater abundance of trees and shrubs, suggestive of drier conditions, during much of the 15th through 19th centuries. During the 20th century, all sites have undergone further shortening of hydroperiod; some also show responses to localized events, such as nutrient enrichment or construction. These changes are of a greater magnitude than those that occurred naturally.

Two tree islands in WCA-3B have been analyzed to determine their age and development and to understand controls on development of elongated, fixed tree islands. Analyses of cores from the island head, tail, and surrounding marsh indicate that conditions on the present tree islands were drier than the surrounding marsh as long ago as 4,000 yrBP. Cores from the present islands indicate that moderate-hydroperiod marshes with frequent fluctuation of hydrologic conditions occupied the sites while the surrounding region was covered by long-hydroperiod sloughs. Further field work will help establish whether this apparent hydrologic difference results from paleotopographic differences or differential permeability in the underlying limestone, or from some other factor. The initial phase of tree-island development consisted of shortening of hydroperiod, increased abundance of fern spores, first appearance of tree-island taxa (such as Salix, Cephalanthus, and tropical hardwoods), and elevation of phosphorus levels. Mature tree-island vegetation is represented in the pollen record by dominance of fern spores, and phosphorus levels are extremely high. The tree islands themselves are old features, with initial tree-island formation beginning at about 3,200 yrBP on Gumbo Limbo Island and 1,800 yrBP on Nuthouse Island. On the present island heads, the existence of moderate hydroperiod marshes, common fish bones, and high phosphorus levels prior to development of the tree islands suggests that the sites may have served as base colonies for wading birds prior to development of the tree islands. This information should be helpful in developing plans to artificially create tree islands at other sites in the Everglades.

Sites in the mangrove fringe along Florida Bay provide a long-term record of vegetation and salinity change. In the southernmost site, a change from freshwater to brackish conditions occurred about 700 yrBP. At that time, freshwater marshes were replaced by brackish marshes, and brackish species of foraminifers and molluscs were added to the previous suite of terrestrial and freshwater molluscs. Dwarf-mangrove assemblages became established there by the 17th century, with progressively later establishment in northern sites. During the 20th century, the replacement of brackish marshes by mangroves appears to have accelerated; the timing of this change coincides with a period of increased salinity fluctuation in Florida Bay and decreased freshwater flow from the Everglades.

These long-term records illustrate the natural variability of the system in response to climatic events during the last two millennia. Hydroperiod fluctuations are recorded at all sites, with relatively dry conditions occurring around 1,000 yrBP and during the last century. Disruption of hydroperiod during the 20th century has resulted in both shortening and altered seasonality of hydroperiod, affecting both the terrestrial Everglades system and the marine system in Florida Bay. Localized responses of vegetation to disturbance or nutrient enrichment provide useful measures of the response time of the system to changes and of the level of change that the system can tolerate. Such data are critical for prediction of future responses of the ecosystem to changes in environmental management.

This research has received critical support both in logistics and planning from a number of cooperating agencies, including South Florida Water Management District, Florida Game and Fresh Water Fish Commission, Florida Geological Survey, U.S. Army Corps of Engineers, Big Cypress National Preserve, Everglades National Park, and Loxahatchee National Wildlife Refuge.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)