Entering the park from the northeast, you are on a road traversing the
pineland-and-hammock “ridge.” This elevated part of the South Florida
limestone bedrock, which at the park entrance is about 6 feet above sea
level, is the driest zone in the park. Pine trees, which will grow only
on ground that remains above water most of the year, thrive on this
rockland.
There is another condition essential to the survival of the pine forest
in this region—fire. We usually think of fire as the enemy of forest
vegetation; but that is not true here. The pines that grow in this part
of Florida have a natural resistance to fire. Their thick, corky bark
insulates their trunks from the flames. And strangely enough the fire
actually seems to help with pine reproduction; it destroys competing
vegetation and exposes the mineral soil seedlings need. If there has
been a good cone crop, you will find an abundant growth of pine
seedlings after a fire in the pinelands.
What would happen if the pinelands were protected from fire? Examine a
pine forest where there have been no recent fires. You will note that
there are many small hardwood (broadleaved) trees growing in the shade
of the pines. These hardwoods would eventually shade out the
light-demanding pine seedlings, and take over as the old pines died off.
But under normal conditions, lightning-caused fires sweep at fairly
frequent intervals through the pineland. Since the hardwoods have little
resistance to fire, they are pruned back.
Before this century, fires burned vast areas. The only barriers were
natural waterways—sloughs, lakes and ponds, and estuaries—which retained
some water during the rainless season when the rest of the glades and
pinelands dried up. Old-timers say that sometimes a fire would travel
all the way from Lake Okeechobee to the coastal prairie of Cape Sable
(_see_ page 2). In the pine forest, any area bypassed by these fires for
a lengthy period developed into a junglelike island of hardwoods. We
call such stands “hammocks,” whether they develop in the pine forest or
in the open glades. On the limestone ridge, the hammocks support a
community of plants and animals strikingly different from the
surrounding pine forests.
[Illustration: PINE AND HAMMOCK RIDGE
(_elevation: 3 to 7 feet above sea level_)]
SAWGRASS GLADES
PINELAND
HARDWOOD HAMMOCK
PINELAND
1 SOUTH FLORIDA SLASH PINE
2 SAW-PALMETTO
3 COONTIE
4 SAW-PALMETTO AFTER FIRE
With the opening up of south Florida for farming and industry, man’s
works—particularly roads and canals—soon crisscrossed the region,
forming barriers to the spread of the fires. Suppression of fire by
farmers, lumbermen, and park managers also lessened their effect. Thus
the hardwoods, which previously had been held back by fire, tended to
replace the pines. And although the park was established to preserve a
patch of primitive subtropical America as it was in earlier centuries,
the landscape began to change.
Continued protection of the park from fire would in time eliminate the
pineland—a plant community that has little chance to survive elsewhere.
So, in Everglades National Park, Smokey Bear must take a back seat: park
rangers deliberately set fires to help nature maintain the natural
scene. Thus, as you drive down the road to Flamingo, do not be shocked
to discover park rangers burning the vegetation. The fires are
controlled, of course, and the existing hammocks are not destroyed.
When you visit the park take a close look at the pinelands community.
Notice, as you walk on the manmade trail through the pine forest, that
the ground on either side of you is extremely rough. The limestone
bedrock is visible everywhere; what soil there is has accumulated in the
pits and potholes that riddle the bedrock. The trees, shrubs, grasses,
and other plants are rooted in these pockets of soil.
The limestone looks rather hazardous to walk on—and it is. You must be
careful not to break through a thin shell of rock covering a cavity.
This pitted, honeycombed condition is due to the fact that the limestone
is easily dissolved by acids. Decaying pine needles, palmetto leaves,
and other dead plant materials produce weak acids that continually eat
away at the rock.
If a fire has passed through the pineland recently, you may notice that
while most of the low-growing plants have been killed, some, such as the
saw-palmetto, are sending up new green shoots. The thick, stubby stem of
the palmetto lies in a pothole, with its roots in the soil that has
accumulated there; even in the dry season the pocket in the limestone
remains damp, for water is never very far below the surface in this
region. When fire kills the top of the plant, the stem and roots
survive, and the palmetto, like the pine, remains a part of the plant
community.
A number of other plants of the south Florida pinelands have adapted to
the conditions of periodic burning. Coontie (a cycad, from the
underground stems of which the Indians made flour) and moon vine (a
morningglory) are among many you will see surviving pineland fires
severe enough to result in the death or stunting of the hardwood
seedlings and saplings.
Sometimes we forget that fire—like water, wind, and sunlight—is a
natural force that operates with the others to influence the evolution
of plants as well as to shape the landscape.
The pineland, like other plant communities, has its own community of
animals. Some of its residents, such as the cotton mouse, opossum, and
raccoon, are found in other communities of the park, too.
Some of the pineland animals, however—pine warbler, reef gecko, and
five-lined skink, for example—are particularly adapted to this
environment. These lovers of sunlight are dependent, like the pine
forest, on the occasional natural or manmade fires that hold back the
hardwood trees.
The pine rockland is quite different from the other plant-and-animal
communities you will see as you drive through the park: it is the only
ecosystem you can explore on foot in any season. Other parts of the park
are largely flooded during the wet season. Elevated boardwalks have been
provided in some of these areas to enable you to penetrate them a short
distance from the road.
As you will see, fire plays an important role in some of the other
Everglades communities, too.
Tree-island Glades
(_elevation: 1 to 3 feet above sea level_)
[Illustration: Tree-island Glade]
BAYHEAD
WILLOW HEAD
SAWGRASS
PINE
DWARF CYPRESS FOREST
ALLIGATOR HOLE
CYPRESS HEAD
[Illustration: PIG FROG
one-third life size]
[Illustration: GREEN TREEFROG
one-third life size]
[Illustration: SQUIRREL TREEFROG
color variation]
Beyond the pinelands the road, having descended some 2 feet from the
park entrance, brings you into the true everglades—the river of grass,
or, as the Seminoles call it, Pa-Hay-Okee (grassy waters). To the eye,
the glades look like a very flat, grassy prairie broken by scattered
clumps of trees. During the dry season (winter) it is in fact a
prairie—and sometimes burns fiercely. The dominant everglades plant is
sawgrass (actually not a grass but a sedge). The tree islands develop in
both high and low spots of the glades terrain. In this unbelievably flat
country, small differences in elevation—measured in inches rather than
feet—cause major differences in the plantlife: tropical hardwoods on the
“mesas,” and swamp trees in the potholes.
A spot in the glades where the limestone base is elevated just 2 feet
will be occupied by a small forest of tropical hardwoods and palms—a
“hammock” much like those of the pinelands. A low spot—just a few inches
below the general level of the limestone base—will remain wet even in
the relatively rainless winter when the sawgrass becomes tinder dry.
This sloughlike depression will support a stand of baldcypress, called a
“cypress head.” Other tree islands, called bayheads and willow heads,
develop in many places where soil and peat accumulate.
Step from the sawgrass glades into one of these hammocks or heads; you
will find yourself in another world. You cannot know the park until you
have investigated these plant-and-animal communities so distinct from
the surrounding marsh yet so much a part of it. As you drive through the
park, look for the trails provided to give you easy access into the
interior of the tree islands.
Also characteristic of the glades are the sloughs—channels where the
glades water, generally a thin, seemingly motionless sheet, is deeper
and has a noticeable current. The sloughs support a rich plantlife and
attract a variety of animals, particularly during the dry season when
the water level drops below the shallow glades bottom. Animals that live
in the glades when they are under water must migrate or estivate (_see_
glossary) if they are to survive the rainless months. Many migrate to
the sloughs, the best known of which is Taylor Slough, where the
elevated Anhinga Trail enables you to walk over the water and observe
the wildlife.
Fire is an important factor in the ecology of the tree-island glades,
just as it is in the pineland. Here, too, artificial barriers such as
canals and roads have hindered the spread of natural fires. There is
some evidence that tree islands were scattered more thinly over the
sawgrass prairie a half-century ago, when a single fire might wipe out
scores of them and destroy much of the bed of peat that provided a
foothold for them. A bird’s-eye view of the glades region today shows
many tree islands that have been established in recent decades. But park
rangers are now utilizing controlled fires in the glades as well as in
the pineland. This tends to prevent new tree islands from taking hold,
and thus helps maintain the natural everglades landscape.
Driving over the glades toward Florida Bay, you come to a sign reading
“Rock Reef Pass—Elevation 3 Feet.” The road then traverses the so-called
dwarf cypress forest. The forest is an open area of scattered, stunted
baldcypress growing where marl (which, unlike peat, does not burn) has
accumulated in small potholes dissolved in the limestone. These marl
potholes provide a foothold for the dwarf cypresses in an area that is
spotted with cypress heads containing much larger trees. Many of the
dwarf cypresses are more than 100 years old, while tall cypresses in the
heads may be less than 50 years old. These anomalies can be attributed
to varying soil depths and water levels and to the effects of fire.
Before you reach the limit of the fresh-water marsh you will come to a
side road leading to Mahogany Hammock. (A good foot trail makes it easy
to explore this hardwood jungle island.) Just beyond, you will notice
the first red mangroves. Small and scattered in this zone, they are a
signal that you are approaching a strikingly different plant-and-animal
community, the mangrove swamp.
Mangrove Swamp
(_elevation: sea level to 1 foot above sea level_)
[Illustration: PRAIRIE SEDGES]
BAY
JAMAICA DOGWOOD
GUMBO-LIMBO
MAHOGANY
PRAIRIE SEDGES
BUTTONWOOD
WHITE-MANGROVE
OSPREY NEST
BLACK-MANGROVE
RED-MANGROVE (very dense growth)
ESTUARY
[Illustration: BONEFISH Comes in with the tide to feed on crabs and
mollusks in shallow water]
[Illustration: FLORIDA HORN SHELL Lives in shallow water and feeds
upon algae and other aquatic plants]
[Illustration: ’COON OYSTER A small (1½″) oyster that lives attached
to the roots of mangroves]
The southward-creeping waters of the glades eventually meet and mingle
with the salty waters of the tidal estuaries. In this transition zone
and along the gulf and Florida Bay coasts a group of trees that are
tolerant of salty conditions, called “mangroves,” form a vast, watery
wilderness. Impenetrable except by boat, it occupies hundreds of square
miles, embracing both the shifting zone of brackish water and the
saltier coastal waters.
Several kinds of trees are loosely called “mangroves.” The
water-tolerant red mangrove grows well out into the mudflats and is
easily recognized by its arching stiltlike roots. Black-mangrove
typically grows at levels covered by high tide but exposed at low tide,
and it is characterized by the root projections called pneumatophores
that stick up out of the mud like so many stalks of asparagus growing in
the shade of the tree. White-mangrove has no peculiar root structure and
grows, generally, farther from the water, behind the other trees.
Sometimes all three are found in mixed stands.
This mangrove wilderness, laced by thousands of miles of estuarine
channels (called “rivers” and “creeks”) and broken by numerous bays and
sounds, is extremely productive biologically. The brackish zone is
particularly valuable as a nursery ground for shrimp. The larvae and
young of these marine crustaceans and of other marine animals remain in
this relatively protected environment until they are large enough to
venture into the open waters beyond the mangroves.
[Illustration: THE MANGROVE SWAMP AND THE COASTAL PRAIRIE]
The shrimp represent a multi-million-dollar industry, and the
sports-fishing business of the area is said to exceed that by far. Both
would suffer if any damage occurred to this ecosystem. The greatest
danger is the alteration in the flow of fresh waters from the glades and
cypress swamps that occurs when new canals are built and land is drained
for cultivation or development. The flow carries with it into the
estuaries organic materials from the rich glades ecosystem; these
supplement the vast quantities of organic matter derived from the decay
of red mangrove leaves. Thus, a reduction in the amount of
nutrient-laden fresh water flowing into the mangrove region will affect
the welfare of the ecosystem, and indirectly the livelihood or
recreation of many persons.
The productive zone of brackish water varies in breadth according to the
flow of fresh water. In the wet summer it moves seaward as the flow of
fresh water from the glades pushes the tides back. In the drier winter
the bay and gulf waters move inland and the brackish zone is quite
narrow. The drainage and canal-building operations in south Florida can
be extremely disruptive here, since too little, or too much, fresh water
flowing into the estuaries can interfere with their productivity.
Natural disasters such as hurricanes can also bring about great changes
in the mangrove ecosystem. Yet biologists do not necessarily view the
destruction of mangroves by hurricanes as catastrophic. The hurricanes
have been occurring as long as the mangroves have grown here and are
part of the complex of natural forces making the region what it is.
Fire does not seem to be a problem in the mangrove wilderness. The trees
themselves are not especially fire-resistant, but it is not uncommon to
see a glades fire burn to the edge of the mangroves and stop when it
runs out of fine fuel.
The mangrove wilderness is a mecca for many park visitors. Sportsmen
take their motorboats into the bays and rivers to challenge the fighting
tarpon. Bird lovers seek the roosts and rookeries of herons and wood
storks. Canoeists, the only ones able to explore the secret depths, are
drawn by the spell of labyrinthine channels under arching mangrove
branches. Here, in a wilderness still thwarting man’s efforts at
destruction, one experiences a feeling of utter isolation from the
machine world.
But the relentlessly rising sea of the past 10,000 years has belittled
drought, fire, hurricane, and frost as it slowly inundated this land 3
inches each hundred years. In compensation, the mangrove forest adds
peat and rises with the sea. The sawgrass marshes retreat, and the
mangrove ecosystem prevails essentially unchanged.
[Illustration: APPLE MUREX
A carnivorous mollusk that feeds on oysters.]