Peat Stratigraphy and Long-Term Succession of Bayhead Tree-Islands in the
Northeastern Everglades
Peter A. Stone
SC Dept. Health and Environmental Control, Columbia, SC
Patrick J. Gleason
Camp Dresser & McKee, Inc., West Palm Beach, FL
Gail L. Chmura
McGill University, Montreal, Quebec, Canada
Tree-islands are distinct patches of forest surrounded by low-stature vegetation.
Though common worldwide, tree-islands in the Everglades are among the best
known. The northeastern Everglades hosts one of the main vast assemblages,
though it is now substantially diminished. Nearly all that remain are in the A.R.
Marshall Loxahatchee National Wildlife Refuge (CA1) where they are a dominant
landscape feature. Others, formerly extant in Conservation Area 2A (CA2A), are
now destroyed, mainly by impounded flooding. Here, we deal with those found
on relatively thick peat that seem unrelated to any local surface features on the
buried mineral substrate. These include seemingly all those in Loxahatchee NWR
and most those formerly in CA2A. (In CA2B and possibly southernmost CA2A,
the mineral-mound-focused tree-island predominated, similar to most tree-islands
in the middle and southern Everglades, excluding mainly the so-called SE “saline”
Everglades.)
This northeastern portion of the Everglades is characterized by an abundance of
marshes dominated or codominated by waterlilys, i.e., deeper marshes. Waterlilyrich peats in profiles below the marshes attest to the long prevalence of this
general conditionsince the Everglades peatland began, almost 5000 14C years in
the oldest areas. Sawgrass too is and has been common, occurring now in large
elongated strands down to small patches. Small patches of other shallower-marsh
marsh plants are common as well. Two main types of tree-island occur(red): (1)
roughly 100 large (to 1.5+ km long) elongated (“cigar” shaped in map outline)
forests principally of dahoon holly occupying low, seasonally flooded, peat ridges,
and (2) thousands of small (to a few 10s of meters in diameter) more-rounded
clumps of trees with redbay usually codominant, on distinct mounds of peat (ca. 1
meter high in surveyed examples).
Complete peat profiles obtained by coring were examined for peat-type
stratigraphy (main parent plant, shown anatomically in microscope thin section)
and for pollen stratigraphy. Pollen and several peat profiles are newly presented
here. All nine tree-islands (5 large, 4 small) had forest peat only as an uppermost
layer, with marsh peats below. All had initiated upon marsh surfaces somehow,
after 3000+ 14C years of marsh existence at their sites. Most showed a stage of
emergent marsh at their sites between the era of waterlily prominence and the
formation of the eventual tree-island. Sawgrass was most common as this
intermediate, but arrowhead occurred prior to two tree-islands (1 large, 1 small).
At the sites of present large tree-islands, spores (in the pollen analysis) show that
the later marsh stages were rich in ferns, which if they represent growth at the
immediate sites suggest shallower examples of sawgrass marsh (the sawgrass
shown by peat type). Waxmyrtle first prevailed as a woody colonist, only later
surpassed by the dahoon holly. It seems quite likely that the large tree-islands
colonized large individual sawgrass strands already growing on low ridges,
whereby the shaping and elongation occurred mainly in a previous stage. Some
distinct sawgrass strands in the western fringe area closely resemble the treeislands in size and outline. Either a shift to a less-wet condition (say, climatically
controlled) or a relative rise in local elevation (presumably by faster peat accretion
in the sawgrass strands) or even plausibly just a reduction in fire frequency may
have induced the replacement of sawgrass by bushes and then trees. Curiously,
one site showed a much earlier trend to woody vegetation (by presence of
waxmyrtle pollen), but this later reverted back to marsh dominance before finally
succeeding to a tree-island.
Most small tree-islands showed an intermediate stage of emergent marsh peat
(sawgrass or arrowhead). But one may have formed directly upon waterlily marsh
peat. Colonization of a floating peat island is a way this could have been
accomplished, but resinking of such peat mats very possibly explains the origin of
the more common shallow marsh stages of the other small tree-islands as well.
Pollen stratigraphy of one small tree-island showed a prominence first of
waterlily, then of arrowhead, then ferns, then waxmyrtle, then holly. It is difficult
to say whether the arrowhead and ferns were themselves instrumental or even
diagnostic of conditions instrumental to the later transition to bushes and forest
here. The peak of arrowhead pollen lay substantially below the beginnings of
waxmyrtle importance (ca. 230 cm vs ca. 170 cm, in a ca. 320 cm deep peat
profile). A small peak of buttonbush pollen at ca. 155 cm depth, the same depth
as the onset of holly pollen, may be significant. Buttonbush is a common colonist
on new floating peat islands.
The small tree-island is apparently the older type. To 1.5 meter thickness of forest
peat occurs, with several dated basal peats ranging roughly 8001300 14C years
old. Only thin forest peat is found in the large low tree-islands (ca. ¼ m), too thin
to reliably date by 14C methods (another dating method should be undertaken).
Peter Stone, Bureau of Water, SC Dept. Health and Environmental Control,
Columbia, SC 29201
Phone: 803-898-4151, Fax: 803-898-2893, stonepa@dhec.sc.gov