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projects > development and stability of everglades tree islands, ridge and slough, and marl prairies > abstract

Impacts of 20th Century Hydrologic Change on Everglades Tree Islands

Debra A. Willard1, Christopher E. Bernhardt1,2, Charles W. Holmes3 and Marci Marot3
1Eastern Earth Surface Processes Team, U.S. Geological Survey, Reston, VA, USA
2Department of Geology, University of Pennsylvania, Philadelphia, PA, USA
3U.S. Geological Survey, St. Petersburg, FL, USA

Tree islands are centers of biodiversity within the Florida Everglades, and restoration and preservation of these features is an important component of Everglades restoration plans. Within this extensive wetland system, significant variability exists in tree-island community composition, size, and shape, and ongoing research is designed to better understand the factors influencing tree-island distribution, formation, and development. This research integrates analysis of sediments deposited before the 20th century with analyses of recent sediments to understand factors influencing tree-island formation, development, and stability and to evaluate the impacts of specific 20th century land-use changes on Everglades tree islands.

We use pollen assemblages from 15 tree islands throughout the greater Everglades ecosystem to reconstruct the timing of tree-island formation, patterns of development, and response to specific climatic and environmental stressors. These data indicate that fixed (teardrop-shaped) and strand tree islands developed well before substantial human alteration of the system, with initial tree-island vegetation in place between 3500 and 500 cal yrBP, depending on location in the Everglades wetland. Tree-island development appears to have been triggered by regional- to global-scale climatic events that correspond to drought intervals documented in Central and South America and periods of southward displacement of the Intertropical Convergence Zone. The records indicate a coherence of climate patterns in both subtropical North America and the Northern Hemisphere Neotropics.

Water-management practices of the 20th century altered plant communities and the size of tree islands throughout the Everglades. Responses include loss of tree-islands due to artificially long hydroperiods and deep water, expansion of tree islands after flow reductions, and significant changes in forest composition. These data provide evidence for the rapidity of tree-island response to specific hydrologic change and facilitate prediction of the response to future changes associated with Everglades restoration plans.

Contact Information: Debra A. Willard, US Geological Survey, 926A National Center, 12201 Sunrise Valley Drive, Reston, VA 20192 USA, Phone: 703-648-5320; Fax: 703-648-6953, Email:

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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