Project Summary Sheet

Project: Evolution of Everglades Tree Islands

Web Site: http://sofia.usgs.gov/flaecohist/

Location (Subregions & Counties): Central Everglades; Broward and Miami-Dade Counties

Funding (Source): USGS Place-Based Studies

Principal Investigators: Debra A. Willard, dwillard@usgs.gov, 703.648.5320; William H. Orem, borem@usgs.gov, 703.648.6273

Project Personnel: Charles Holmes, cholmes@usgs.gov, 727.803.8747x3056

Other Supporting Organizations: South Florida Water Management District, Everglades National Park, Loxahatchee National Wildlife Refuge, and Florida Fish and Wildlife Conservation Commission

Associated / Linked Projects: South Florida Ecosystem History: Terrestrial and fresh-water ecosystems

Overview & Status: This project aims to: determine geologic and hydrologic controls on tree-island formation, development and sustainability; identify tree island community responses to natural and anthropogenic environmental changes; determine the role of tree islands in the geochemical budget of nutrients; and investigate the use of sediment phosphorus as a tracer of historic bird populations in the Everglades. To date, cores have been collected on 23 tree islands in Loxahatchee NWR, WCA 2A, 3A, 3B, and Everglades National Park (ENP). During FY02, additional sampling is planned in Taylor Slough, western WCA 3A, and in sawgrass ridges and sloughs. The latter sites represent a new direction of the project to determine whether sawgrass ridges were initial sites of tree island formation, evaluate their stability and longevity, and calculate differences in natural vs. 20^th century peat accretion rates in ridges and sloughs. Completion of initial charcoal analyses in FY02 will provide data on natural fire frequency.

Completed analyses indicate that: 1) Everglades tree islands have existed for at least 3,000 years, reaching maturity ~1,000 to 2,000 years ago; 2) tree islands developed on sites drier than surrounding wetlands; 3) in ENP, hardwood hammocks became larger after water diversion early in the 20^th century; 4) sustained high water levels (~20 years) in WCA 2A decreased tree-island size and altered plant communities; 5) plant community composition on Loxahatchee strand islands has changed strikingly since ~1950; and 6) phosphorus content of tree islands is extremely high. These results imply 1) tree-island restoration efforts should concentrate on sites previously occupied by tree islands or in marshes analogous to those preceding tree island formation (possibly sawgrass ridges); 2) water management practices of both the early and mid-20^th century altered tree island size and community composition; and 3) presence of certain forms of phosphorus may be proxies for the presence of wading bird populations.

Needs & Products: Modeling efforts to evaluate impacts of CERP projects on tree islands and fresh-water marshes require an understanding of the relationship between hydroperiod/flow/fire and tree-island maintenance and species composition. Products will include data on the impact of natural long-term droughts, multi-decadal intervals of altered flow rates and water depths, and altered water quality on tree island sustainability. Data from sawgrass ridges and sloughs will clarify the natural stability of the features, the role of fire in their maintenance, and natural vs anthropogenic peat accretion rates in both features.

Application to Everglades Restoration: Products support development of CERP performance measures to 1) determine appropriate hydroperiods, water depths, and water quality to maintain existing tree islands and restore degraded tree islands; 2) determine role of fire in tree-island maintenance and loss; 3) establish appropriate hydroperiods to restore “natural” distribution of sawgrass ridges and sloughs; and 4) determine role of fire in maintaining the ridge and slough system.