U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004–3095

September 2004

Tree Islands of the Florida Everglades—

Long-Term Stability and Response to Hydrologic Change

Tree islands are important centers of 

biodiversity in the Florida Everglades; 

they have two to three times the plant and 

animal diversity of the surrounding wetlands. This high diversity is due primarily 

to their higher elevation relative to the 

adjacent wetlands (fig. 1). In the natural 

Everglades system, water levels fluctuated seasonally with rainfall, and tree 

islands were the only sites that escaped 

flooding during the wet season. These 

seasonally dry sites provided refugia and 

nesting sites for animals and allowed tree 

and shrub communities to flourish.

Major changes to the natural Everglades hydrology occurred after severe 

flooding from hurricanes in 1947 and 

1948. As a result of those storms, millions of acres of land were under water 

for several months. Shortly thereafter, 

the Central and Southern Florida (C&SF) 

Project for Flood Control was implemented to protect urban areas from flooding. Beginning in 1952, the C&SF Project 

constructed three Water Conservation 

Areas (WCAs) that hold surface water to 

augment water supply in the east and to 

maintain water discharge to Everglades 

National Park in the south. Construction 

of the Water Conservation Areas resulted 

in hydrologic fragmentation of the greater 

Everglades ecosystem and disrupted seasonal freshwater flow through the system. 

Consequently, wading bird populations 

were reduced, and it is estimated that half 

the tree islands were lost or degraded.

In an effort to increase water supplies 

to restore the Everglades to a more natural state while still meeting other regional 

water needs, Congress authorized the 

Comprehensive Everglades Restoration 

Plan (CERP) in 2000. Research is underway to ensure that restoration targets 

reflect the natural predrainage hydrology 

and ecology.

Ongoing research by the U.S. Geological Survey (USGS) is documenting 

the distribution of predrainage wetland 

plant communities and their response to 

specific hydrologic and environmental 

changes. Of particular interest are the 

long-term stability of tree islands and 

their tolerance for hydrologic variability. USGS researchers are using paleoecological techniques on tree islands 

throughout the Everglades to determine 

how long the tree islands have existed, to 

identify geological conditions necessary 

for tree-island formation, and to document changes in community composition 

through time. The resulting spatial and 

historical information will improve predictive models of ecosystem response to 

specific CERP implementation plans.

Everglades Tree-Island 

Communities

Tree islands consist of distinct clusters 

of trees, shrubs, and ferns interspersed 

throughout sawgrass marshes and waterlily sloughs of the Everglades landscape 

(fig. 2). Three types of tree islands occur 

in the Everglades (fig. 3): (1) fixed tree 

islands, (2) strand tree islands, and (3) 

pop-up, or battery, tree islands. These differ in community composition, distribution within the Everglades, and origin.

Fixed tree islands.—Fixed tree islands 

are teardrop shaped, and their long axis 

runs generally north to south, roughly 

parallel to flow. Many fixed tree islands 

developed over topographic highs in 

the underlying limestone. The head of 

the island overlies the highest point and 

is covered by semitropical hardwoods, 

upland plants that cannot tolerate prolonged flooding, and many ferns. The 

near tail of the tree island is directly 

downstream of the head and includes 

a community with dense ferns, shrubs, 

aquatic plants, and semitropical hardwoods that can tolerate limited flooding. 

The far tail contains a mixture of floodtolerant ferns, shrubs, sedges, and other 

marsh plants. Surrounding the entire tree 

island is a transitional zone with dense 

shrubs, ferns, and aquatic plants.

Strand tree islands.—Strand tree 

islands are elongate features found primarily in Arthur R. Loxahatchee National 

Wildlife Refuge (NWR). The central 

part of strand tree islands typically is 

dominated by dahoon holly and ferns and 

is flooded for short periods during the 

year. Their margins contain a mixture of 

shrubs and aquatic plants.

Pop-up, or battery, tree islands.—

Pop-up, or battery, tree islands are small, 

generally circular tree islands; they are 

very common in Loxahatchee NWR. 

Pop-up tree islands are co-dominated by 

red bay and dahoon holly trees and ferns. 

Figure 1. Fixed tree island in the central Everglades. Photograph by Debra A. Willard.

Figure 2. Satellite image of the Florida Everglades. Tree islands are visible even in satellite 

imagery, and a very large tree island is marked 

by the arrow.
In their margins, shrubs and ferns are 

common, and the entire tree island typically is surrounded by a narrow strip of 

dense sawgrass with abundant ferns.

Everglades Tree-Island Pollen 

Assemblages

Pollen deposited in Everglades sediments (fig. 4) typically is not transported 

far from the source plant communities, 

and at least 11 types of Everglades wetland are distinguishable by using pollen 

assemblages (Willard and others, 2001). 

In the pollen record, fixed tree islands 

are distinguished by high percentages of 

fern spores for two main reasons. First, 

ferns are the most common ground-cover 

plants on most tree islands. Second, the 

most common trees on tree islands are 

insect pollinated, and so their pollen is 

not readily dispersed to the atmosphere, 

and relatively little is available for preservation in the sediment.

Present-day strand and battery tree 

islands from Loxahatchee NWR differ 

from fixed tree islands in dominance of 

holly pollen. Analysis of pollen and spores 

from transects across the tree islands into 

the adjacent marsh indicates that pollen 

from tree-island taxa is rare in adjacent 

marshes. Abundant occurrence of treeisland pollen in samples from sediment 

cores can be used to identify the timing 

of tree-island formation and to identify 

changes in community composition.

Analysis of Everglades 

Tree-Island Development

Until recently, the timing and cause of 

tree-island formation have been poorly 

understood. Estimates of the timing of 

initial tree-island development originally 

ranged from several thousand years ago 

to as recently as the last few decades. 

To better understand the age and origins 

of these features and the response of 

tree-island communities to natural climate variability and historic hydrologic 

changes, sediment cores were collected 

on and around 29 tree islands throughout 

the Everglades (fig. 5). These cores were 

dated by using radioisotopic techniques, 

including carbon-14 dating, which provides reliable dates from ~40,000 to ~300 

years ago, and lead-210 dating, which 

provides age models for the last century. 

These age models were paired with vegetational reconstruction based on pollen 

analysis from cores to identify the timing 

of tree-island formation and to determine tree-island response to hydrologic 

changes in the 20th century.

Tree-Island Trends in the 

Northern Everglades

The northeastern Everglades, including Loxahatchee NWR, contains the 

thickest and oldest peats of the Everglades. Dates from basal peat samples 

indicate that peat accumulation began 

as long as 6,000 calendar years before 

present (Gleason and Stone, 1994). 

Loxahatchee vegetation consists of a 

mixture of waterlily sloughs and sawgrass marshes with strand and battery 

tree islands scattered densely throughout. 

Ongoing USGS research in Loxahatchee 

NWR focuses on the strand islands that 

formed on shallow sites in a deeper 

marsh (Stone and others, 2002). Pollen 

assemblages indicate that tree-island taxa 

were established on some strand islands 

more than 1,100 years ago (fig. 5a); 

further sampling is necessary to determine how much earlier these features 

truly formed. All sites sampled indicate 

that holly did not become a dominant 

element in strand communities until the 

mid-20th century after construction of the 

C&SF Project. At that time, holly pollen 

increased more than fourfold, indicating 

a substantial effect of water-management 

structures on plant communities of strand 

tree islands in the Loxahatchee NWR.

Tree-Island Trends in the 

Central and Southern Everglades

Fixed tree islands are prominent 

features of the central and southern 

Everglades and contrast with Loxahatchee 

strand islands in their teardrop shape, 

concentration of trees and shrubs at the 

north (upstream) end of the island, and 

development of a dense, shrubby tail 

downstream from the head. Analysis of 

numerous fixed tree islands shows that 

tree-island heads formed in shallow water 

depths on topographic highs. Before trees 

and shrubs were present, the heads were 

covered by sawgrass marshes surrounded 

by waterlily sloughs. Tree-island development began about 1,000–2,000 years ago, 

depending on location within the greater 

Figure 3. Graphic representation of three 

tree-island types in the Florida Everglades. 

Pop-up, or battery, tree islands are very small 

and have a narrow transition to the surrounding wetland. Strand tree islands are long 

and thin. They have relatively homogeneous 

woody vegetation on the head and a relatively 

narrow transitional zone. Fixed tree islands 

are teardrop shaped and have the tallest 

trees on the elevated head. Trees, shrubs, 

and flood-tolerant plants occupy the near 

tail, and shrubs, sawgrass, cattail, and other 

aquatic plants occur in the far tail. The entire 

tree island is surrounded by a relatively broad 

transitional zone.

Figure 4. Pollen and spores of plants commonly found on Everglades tree islands and 

in adjacent marshes. a, Thelypteris kunthii 

(maiden fern); b, Blechnum serrulatum (swamp 

fern); c, Cladium jamaicense (sawgrass); d, 

Nymphaea odorata (waterlily); e, Bursera 

simaruba (gumbo limbo); f, Melothria pendula 

(creeping cucumber); g and h, Cephalanthus 

occidentalis (buttonbush); i, Ilex cassine 

(dahoon holly); j, Myrica cerifera (wax myrtle); 

k, Sphagneticola trilobata (Bay Biscayne 

creeping-oxeye); l, Annona glabra (pond 

apple); m, Osmunda regalis (royal fern); n, 

Acrostichum danaefolium (giant leather fern); 

o, Ipomoea sagittata (glades morning glory).
Everglades ecosystem. Plant communities 

characteristic of mature tree-island vegetation developed 600–1,200 years ago.

After centuries of relative stability, 

hydrologic changes of the last 100 years 

have substantially altered the size and vegetational composition of fixed tree islands. 

In WCA 2A, water depths were kept 

at high levels for 13 consecutive years 

with no seasonal drawdowns. Sawgrass 

marshes and wet prairies were converted 

to deeper water sloughs, and many tree 

islands disappeared (fig. 6) (Light and 

Dineen, 1994). A sediment core collected 

in the modern tail of one drowned tree 

island, Treeceʼs Island, illustrates some of 

the paleovegetational patterns (fig. 5b). 

Pollen assemblages indicate that the island 

developed at least 1,900 years ago; from 

then until the 20th century, taxa characteristic of relatively dry tree-island heads 

dominated the assemblages. Increased 

abundance of waterlily pollen after 1960 

reflects the development of sloughs in 

deeper water while the tree-island community declined. After periodic drawdowns 

were incorporated into the water-management scheme in 1974, increased sawgrass 

pollen indicates a shift toward shallower 

water. These changes, and those in companion cores, reflect relatively rapid 

decrease in tree-island size and loss of 

trees due to sustained periods of deep 

Figure 5. Location of 29 tree islands sampled in the Everglades between 1998 and 2002. Acronyms: BCP, Big Cypress National Preserve; 

NWR, National Wildlife Refuge; WCA, Water Conservation Area. Graphs a–c show percent abundance of pollen vs. depth (in centimeters) at 

three sites: a, Strand Island, Loxahatchee NWR; b, Treece’s Island, WCA 2A; c, Manatee Hammock, Everglades National Park. Color coding 

in graphs: blue and red indicate fern abundance; blue = Osmunda regalis; red = other ferns; black = tree-island shrubs; green = marsh taxa.

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water as well as the subsequent wetland 

response to hydrologic modifications to 

restore a more seasonal hydroperiod to 

the area.

In contrast to WCA 2A, sites within 

Everglades National Park were affected 

by reduced freshwater flow in the early 

1930s. In a core collected in the tail 

of Manatee Hammock in Shark River 

Slough (fig. 5c), pollen assemblages 

indicate that mature tree-island vegetation was established about 500 years ago. 

Initial decreases in sheet flow around 

1930 resulted in replacement of marsh 

and slough taxa by tree-island taxa. Additional flow reductions after completion 

of the Water Conservation Areas in the 

early 1960s are correlated with twofold 

to sixfold increases in abundance of ferns 

characteristic of drier conditions. These 

increases suggest that diversion of water 

from sheet flow during the 20th century 

resulted in expansion of tree-island size 

in Shark River Slough.

Tree-Island Response to 

Everglades Restoration Strategies

Fixed and strand tree islands of the 

Everglades are geologically old features 

that first developed in response to natural climatic and topographic controls 

1,000–2,000 years ago (Willard and 

others, 2002). The distribution of these 

features was controlled by the topography of the underlying limestone; water 

depths were shallower than average over 

the high areas, which ultimately underwent sufficient seasonal drying for trees 

and shrubs to become established. Tree 

islands have been a distinctive component 

of the Everglades ecosystem for centuries 

to millennia, but altered water-management practices of the past century have 

changed their vegetational composition 

and size at varying scales.

Successful restoration and maintenance of tree islands require appropriate 

water depths and seasonally variable 

freshwater flow. Efforts to restore or create tree islands should focus on sites of 

drowned tree islands, because the requisite geologic and topographic conditions exist at those sites. Paleoecological 

evidence indicates that tree-island development took centuries; even with optimal hydrologic conditions, restoration of 

these features will not be rapid. Restoration goals should be set that are congruent with the pace of natural processes.

Although tree islands in some parts of 

the Everglades have disappeared because 

of overly long hydroperiods and deep 

water, those in other regions have become 

larger because of reduced water supplies. 

Therefore, restoration plans designed to 

improve Everglades tree-island health 

should incorporate regionally based 

performance measures; the plans would 

incorporate both differences in hydrology 

(past, present, and future) and inherent 

differences in species composition based 

on location within the greater Everglades 

ecosystem.

By Debra A. Willard

References Cited

Gleason, P.J., and Stone, Peter, 1994, Age, 

origin, and landscape evolution of the 

Everglades peatland, in Davis, S.M., and 

Ogden, J.C., eds., Everglades—The ecosystem and its restoration: Delray Beach, Fla., 

St. Lucie Press, p. 149–197.

Light, S.S., and Dineen, J.W., 1994, Water 

control in the Everglades—A historical perspective, in Davis, S.M., and Ogden, J.C., 

eds., Everglades—The ecosystem and its 

restoration: Delray Beach, Fla., St. Lucie 

Press, p. 47–84.

Loveless, C.M., 1959, A study of the vegetation of the Florida Everglades: Ecology, v. 

40, p. 1–9.

Stone, P.A., Gleason, P.J., and Chmura, G.L., 

2002, Bayhead tree islands on deep peats 

of the northeastern Everglades, in Sklar, 

F.H., and van der Valk, Arnold, eds., Tree 

islands of the Everglades: Dordrecht, The 

Netherlands, Kluwer Academic Publishers, 

p. 71–115.

Willard, D.A., Holmes, C.W., Korvela, M.S., 

Mason, Daniel, Murray, J.B., Orem, W.H., 

and Towles, D.T., 2002, Paleoecological 

insights on fixed tree island development 

in the Florida Everglades; I. Environmental controls, in Sklar, F.H., and van 

der Valk, Arnold, eds., Tree islands of the 

Everglades: Dordrecht, The Netherlands, 

Kluwer Academic Publishers, p. 117–151.

Willard, D.A., Weimer, L.M., and Riegel, W.L., 2001, Pollen assemblages as 

paleoenvironmental proxies in the Florida 

Everglades: Review of Palaeobotany and 

Palynology, v. 113, no. 4, p. 213–235.

Worth, D.F., 1988, Environmental response of 

WCA–2A to reduction in regulation schedule and marsh drawdown: South Florida 

Water Management District Technical 

Publication 88–2, 55 p.

For further information, 

please contact:

Debra A. Willard

U.S. Geological Survey

926A National Center

Reston, VA 20192

Telephone: 703–648–5320

E-mail: dwillard@usgs.gov

Figure 6. Comparison of distribution of plant communities in Water Conservation Area 2 before 

and after construction of the C&SF Project. Modified from Worth (1988).