Everglades Tree-Island Response to Hydrologic Change
Debra A. Willard, William H. Orem, Christopher Bernhardt
U.S. Geological Survey, Reston, VA
Charles W. Holmes
U.S. Geological Survey, St. Petersburg, FL
The stability of Everglades tree islands is influenced strongly by hydrologic
parameters, including water depth, hydroperiod, and seasonality of flow. Based
on analysis of pollen from sediment cores, we have reconstructed changes in
vegetation and tree island size in response to hydrologic changes of the last
century.
Transects of sediment cores were collected on 29 tree islands throughout the
Everglades: five in Arthur R. Marshall Loxahatchee National Wildlife Refuge,
two in Water Conservation (WCA) 2A, eleven in WCA 3A, two in WCA 3B, four
in Shark River Slough, and five in Taylor Slough, Florida. The tree islands are
geologically old features, with mature tree-islands present in the northern and
southern Everglades for at least 2,000 and 1000 years, respectively. Even before
classical tree-island formation, sites now occupied by tree-island heads were drier
than surrounding marshes and sloughs; this finding is consistent with the
hypothesis that topographic highs in the underlying limestone were favorable sites
for tree-island development.
The response of tree-island vegetation to 20th century hydrologic changes varied
with location within the greater Everglades ecosystem. On strand islands in
Loxahatchee NWR, slight increases in abundance of shrubby taxa (holly, wax
myrtle) and weedy species occurred in the early 20th century. After 1960, pollen
of trees and shrubs increased at least fivefold, resulting in the dominance of holly
and bays now seen on these islands. On a “drowned” tree island in WCA 2A, the
abundance of tree-island taxa decreased greatly whereas water lily pollen
increased during the 1960’s; this finding is consistent with sustained flooding
after construction of the Central & South Florida (C&SF) Project. The
subsequent replacement of waterlily by sawgrass in the 1970’s reflects altered
water management practices, which incorporated periodic drawdowns into the
management scheme. Overall, the vegetational pattern indicates the loss of treeisland vegetation after sustained high water for 10-15 years.
Sites in WCA 3A and B have insufficient sedimentation rates to resolve 20th
century changes; however, assemblages from sediments deposited during the last
500 years show little change.
On and around tree islands in Shark River Slough in Everglades National Park,
pollen of marsh taxa disappeared almost entirely ~1930, when initial canal and
road construction was completed. After 1960, replacement of taxa characteristic
of relatively wet conditions with tree-island taxa suggests that the tree island head
expanded southward as fresh-water flow was reduced to Shark River Slough.
Our results indicate that tree-island response to future hydrologic response will
not be uniform within the Greater Everglades ecosystem. Restoration pre-20th
century hydroperiods and water depths to Shark River Slough is likely to result in
a decrease in size of tree-island heads, whereas drowned tree islands in WCA 2A
may see a gradual increase in size and restoration of previous tree-island plant
communities. The distinctive composition of Loxahatchee strand islands also is a
recent vegetational feature, and changes in biodiversity at those sites are likely as
hydrology is altered. Our results highlight the variability of tree-island
composition within the Everglades and indicate that no single performance
measure should be devised to evaluate the success of tree-island restoration
throughout the Everglades.
Contact: Debra A. Willard, U.S. Geological Survey, 926A National Center,
Reston, VA 20192, Phone: 703-648-5320, Fax: 703-648-6953,
dwillard@usgs.gov, Oral, Hydrology and Hydrological Modeling