Debra A. Willard¹, Christopher E. Bernhardt^1,2, Laura A. Brandt³ and Marci Marot^4
1Eastern Earth Surface Processes Team, U.S. Geological Survey, Reston, VA, USA
²Department of Geology, University of Pennsylvania, Philadelphia, PA, USA
³A.R.M. Loxahatchee NWR, Boynton Beach, FL, USA
⁴U.S. Geological Survey, St. Petersburg, FL, USA

Tree islands are an important component of the Everglades landscape and are a key feature of the A.R.M. Loxahatchee National Wildlife Refuge. Changes in the spatial distribution and community composition of tree islands have occurred during the last century. There is evidence that tree islands in the north end of the Refuge, where it is drier than it was historically, are being invaded by shrubs, particularly along the edges and may be experiencing severe fires at a frequency greater than occurred pre-development. Vegetation in the southern Refuge, however, shows evidence for stress due to high water levels.

We analyzed pollen assemblages from sediment cores collected on tree islands and adjacent marshes to reconstruct vegetation history and hydrology during the last few centuries. We included sediments deposited before the 20^th century to provide a baseline for the pre-drainage distribution of vegetation throughout the Refuge. Three transects of cores were collected in the northern, central, and southern parts of the Refuge. A total of nine tree-islands and adjacent marshes were sampled to provide broad spatial coverage for the study. Chronologic control was provided by a combination of pollen biostratigraphy, lead-210, and radiocarbon dating.

Prior to the 20^th century, marsh cores from all three transects exhibit similar community composition, analogous to sawgrass marshes with moderate hydroperiod and water depth. During the 20^th century, however, spatial differences within the Refuge are evident. In the northern and central Refuge, tree island and marsh assemblages indicate the onset of drier conditions during the 20^th century. On tree islands, this change is characterized by a shift from pre-drainage communities to the bay-holly forests that presently dominate most Loxahatchee tree islands. In adjacent marshes, taxa characteristic of moderate hydroperiods became less common, and shrubs and weedy species became more abundant. This stands in contrast to sites in the southern transect. There, sharp increases in abundance of waterlily pollen in marshes indicate significantly wetter conditions during the 20^th century. Likewise, high water levels resulted in drowning of tree islands in the southern Refuge.

The correlation of altered species composition with water management practices during the past century provides insights into the tolerance of tree-island and marsh species for hydrologic change. Such data are critical to predict the response of wetland plant communities to different restoration strategies.

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: dwillard@usgs.gov

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

Back to Project Homepage