Tropical hardwood hammock facts for kids

Mahogany Hammock Trail, a path through a tropical hardwood hammock in Everglades National Park.

Tropical hardwood hammocks are special kinds of forests found in South Florida and the Everglades. They are like dense, green islands where the tree branches grow so close together that they form a thick roof, called a canopy. Most of the trees and shrubs in these forests are from the West Indies. They are either evergreen (meaning they keep their leaves all year) or semi-deciduous (meaning they lose some, but not all, of their leaves).

You can find many tropical hardwood hammocks on the Miami Rock Ridge, in the Florida Keys, along the northern shores of Florida Bay, and in the Pinecrest area of the Big Cypress Swamp. These hammocks are home to unique plants found nowhere else. They are also vital for many West Indian plant species that reach the northernmost part of their range here. Many animals, including nine species protected by the Endangered Species Act, also live in these hammocks.

Even though many hammocks are now protected, especially outside the Florida Keys, they are still in danger. Building new homes and businesses in the Keys is a big threat. Hammocks have also been hurt by being completely destroyed, turned into farmland, or taken over by non-native plants and animals. People collecting plants and animals, human-caused fires, and changes to the water flow (called hydrology) also cause problems. A lot of effort is now going into restoring damaged hammocks and controlling these non-native species. There are also chances to create new hammocks or care for existing ones in developed areas.

Where Do Hammocks Grow?

You can find large areas of tropical hardwood hammocks in Everglades National Park and Biscayne National Park. They are also common throughout the Florida Keys and in Big Cypress National Preserve. Smaller hammocks exist along the Atlantic coast from Miami-Dade County up to Martin County. Similar forests are found in the Bahamas and the Greater Antilles. Many coastal hammocks on barrier islands in South Florida are very much like these communities.

What Makes a Hammock Special?

A tropical hardwood hammock is a forest where the tree tops form a dense, closed roof. It is filled with many different kinds of evergreen and semi-deciduous trees and shrubs. Most of these plants originally came from the West Indies. These hammocks are also important homes for ferns and orchids from the West Indies.

Unlike some other Florida habitats, tropical hardwood hammocks do not need fire to survive. However, fires can sometimes burn into them under certain conditions. The soil in these hammocks is mostly made of organic material, like dead leaves and plants, which sits right on top of the rock. The soil is usually moist but rarely flooded with water.

There are five main types of hammocks:

• Rockland hammock islands: These are found on limestone rock, often within or at the edges of pine rockland or marl prairie areas. You can see them on the Miami Rock Ridge and in Big Cypress National Preserve.
• Keys rockland hammock: These grow on limestone rock and are the main type of forest in the Florida Keys.
• Coastal berm hammock: These form on ridges created by storms, found in the Sand Keys (west of Key West), the Florida Keys, and along the northern shores of Florida Bay.
• Tree island hammock: These are small islands of trees found in the Everglades marsh and the surrounding marl prairie.
• Shell mound hammock: These grow on elevated mounds made of old mollusk shells and other items left by ancient people.

Tropical hardwood hammocks also include more open areas called coastal rock barrens and sinkholes. Coastal rock barrens are rare and found in tiny spots in the Florida Keys. Sinkholes are natural holes in the ground found in areas with karst limestone, mostly in hammocks on the Miami Rock Ridge.

While not many plant species are found only in tropical hardwood hammocks, these areas are very important for West Indian species. This is where the northernmost parts of their ranges extend into South Florida. The height of the tree canopy changes depending on the ground and the weather. On the Miami Rock Ridge, a mature hammock's canopy can be about 18 m (59 ft) tall or less. In the Florida Keys, the canopy is usually 9 to 12 m (30 to 39 ft) tall. The lower layers of shrubs and herbs in a hammock are usually sparse, mostly made up of young trees and shrubs.

The edges of hammocks are very important. This is where the hammock meets other areas like pine rocklands or wetlands. Many hammock plant species only grow in these edge areas, called ecotones. Gaps within the hammock, often created by storms like hurricanes, are also important. These gaps allow new, pioneer plant species to grow in the open spaces.

Coastal rock barrens have two types. Upland coastal rock barrens are open, flat rocky areas with sparse, low-growing plants that like dry conditions. Wetland coastal rock barrens are affected by high tides and are home to common wetland plants and coastal shrubs.

Soils, Water, and Climate in Hammocks

Tropical hardwood hammocks grow on limestone, sand, and shell. The ground is moist and usually does not flood. Hammocks on limestone rely on the water underground to keep the air humid, especially in sinkholes. The hammock's rounded shape and dense edges help block wind, which keeps the air moist. The thick canopy also helps keep temperatures steady, reducing how much the soil heats up during the day and cools down at night.

Rockland hammocks are found on raised limestone areas, often near limestone sinkholes. Coastal berm hammocks grow on ridges of marine debris left by storms, usually within mangrove or salt marsh areas. In the Keys, these hammocks can also be found facing open water. Shell mound hammocks grow on elevated mounds of shells and old garbage left by Native Americans. Coastal rock barrens are flat, rocky surfaces right next to the coast or inside islands in the Florida Keys. Sinkholes are round or cone-shaped holes with steep limestone walls, found in karst rockland areas. Any soil present is made of calcareous marls (a type of soil with clay and calcium carbonate) and organic material. This soil is found on the surface, in solution depressions, and in cracks in the limestone.

Elevations on the Miami Rock Ridge vary. They can be over 7 m (23 ft) above sea level near Biscayne Bay and less than 2 m (6.6 ft) in the Long Pine Key area of Everglades National Park. The average height is about 3 m (9.8 ft), and the ridge is 6 to 16 km (3.7 to 9.9 mi) wide. The limestone formations in the Keys are much lower, from 1 to 2 m (6.6 ft) above sea level. However, some high spots, 4 to 5 m (13 to 16 ft) above sea level, are found in Key West and Lignumvitae Key.

Rainfall in southeastern Florida averages over 163 cm (64 in) each year in the northwest part of Miami-Dade County. The rest of the county gets between 122 and 142 cm (48 and 56 in) annually. The upper Florida Keys receive about 127 cm (50 in), and the lower Keys get about 102 cm (40 in). Most of this rain (75 percent) falls between June and October. The rain usually soaks into the ground quickly, creating layers of fresh or slightly salty water under many hammocks.

Plants and Animals of the Hammock

Plants of the Hammock

Tropical hardwood hammocks are mostly filled with plants from tropical areas. Many of these are found only in hammocks in the Florida Keys within the U.S. Some West Indian species, like gumbo limbo and strangler fig, are common in South Florida. However, many others are very rare and are listed as threatened or endangered by the State of Florida. Hammocks on the Miami Rock Ridge are also important homes for many rare epiphytic orchids (orchids that grow on trees), bromeliads, ferns, and peperomias. These can also be found in the Big Cypress and other regions.

Trees

Shrubs

Plants and Flowers

Animals of the Hammock

Many animals protected by the government depend on or use tropical hardwood hammocks. For example, the Florida panther lives in hammocks in the Big Cypress region. The eastern indigo snake is found in hammocks all over South Florida, as well as in other areas like sandhills. The Key deer lives only in pine rocklands and tropical hardwood hammocks on Big Pine Key.

The Key Largo cotton mouse and the Key Largo woodrat are found only in tropical hardwood hammocks on Key Largo in the upper Florida Keys. The Stock Island tree snail has historically only been found in hammocks on Stock Island and Key West.

The endangered Florida bonneted bat is the largest bat in Florida. Its numbers are not well known, but it was once thought to be common on Florida's east coast. However, it has only been seen there once since 1967. On Florida's west coast, only one group of eight bats has been reported. These bats like to rest under Spanish roof tiles, in royal palm leaves, and in tree holes made by woodpeckers like the red-cockaded woodpecker (Picoides borealis) and the pileated woodpecker (Dryocopus pileatus). Most of these bats were found in heavily forested areas. Tree cavities are rare in South Florida, so there is a lot of competition for them.

How Hammocks Grow and Change

Tropical hardwood hammocks usually do not burn often. Scientists are still learning exactly how fire affects them. If a fire does happen, how quickly the hammock recovers depends on the fire's strength. If the fire burns away a thick layer of organic matter that protects tree roots, recovery is slower. But if this layer is not consumed, the tropical hardwood trees grow back quickly. The canopy can close up again in 40 years or less. Fire is a very important part of the South Florida ecosystem. Without fire, hammocks would only spread into pine rocklands if humans caused it.

Threats to Hammocks

Harmful Invasive Species

Non-native plant species have greatly affected tropical hardwood hammocks. These plants, also called exotic or invasive species, have caused a lot of damage, especially to hammocks on the Miami Rock Ridge. In some places, these non-native plants now make up half of all the plants in hammock areas on the Ridge. Vines like Gold Coast jasmine (Jasminum dichotomum), air-potato (Dioscorea bulbifera), and nephthytis (Syngonium podophyllum) have destroyed many hammocks. Non-native trees and shrubs, such as Brazilian pepper (Schinus terebinthifolius), are a problem in hammocks throughout South Florida, even in untouched areas of the Everglades. Coastal berm hammocks along Florida Bay have been severely impacted by the sprawling vine-like shrub latherleaf (Colubrina asiatica). Recent maps of the Florida Keys show that about 2,833 hectares (7,000 acres) of suitable hammock habitat has been taken over by non-native plants like Australian pine, Brazilian pepper, and latherleaf. Areas that have been disturbed near Keys hardwood hammocks are often full of these non-native plants, which quickly spread and push out the natural plant communities.

Non-native animals have also harmed tropical hardwood hammocks. Introduced species found in South Florida rocklands include 7 types of mammals, about 30 birds, 4 amphibians, and 25 reptiles. Animals like the armadillo (Dasypus novemcinctus), black rat (Rattus rattus), fire ant (Solenopsis invicta), and hog (Sus scrofa), as well as domestic cats (Felis domesticus), have all been found in South Florida hammocks. Black rats and fire ants hunt the endangered Stock Island tree snail. Fire ants may also cause more deaths among the Key Largo woodrat.

Natural Events and Their Impact

Hurricanes and other natural disturbances are a normal part of the South Florida ecosystem. However, they can have bad effects when hammocks are broken up into smaller pieces or when non-native plants have spread. This was clearly seen after Hurricane Andrew. This hurricane had winds over 233 km/h (145 mph), with gusts up to 322 km/h (200 mph). When species become rare, like the Schaus swallowtail butterfly, extreme weather events such as hurricanes, freezes, and droughts can become serious threats. In September 1998, Hurricane Georges caused big changes to tropical hardwood hammocks in the lower Keys. Roads and other breaks in the forest allowed strong winds to cause more damage, knocking down or breaking trees. Besides wind damage, the storm surge from Hurricane Georges flooded the Cactus Hammock on Big Pine Key, destroying the plants growing under the trees.

Protecting and Restoring Hammocks

To protect tropical hardwood hammocks for a long time, it is very important to work with landowners in Miami-Dade County and the Florida Keys. In 1979, Dade County started a program called the Environmentally Endangered Lands Covenant Program. This program lowers taxes for owners of tropical hardwood hammocks and pine rocklands if they agree not to build on these areas and to manage them for 10 years. This program is still active and protects many hammock sites. There isn't a similar program in Monroe County, where many hammocks are still privately owned and divided into small plots. The Forest Resources Program is also working with the Boy Scouts of America to connect private sites with Eagle Scout projects. They are also looking for ways to provide money to help manage private lands.

Scientists are still debating whether to let hammocks grow naturally, with a mix of old and new growth, or to help restore them with human effort.

The biggest effort to restore tropical hardwood hammocks is being done by the Miami-Dade County Parks and Recreation Department, Natural Areas Management Division. This program has been very active since Hurricane Andrew in 1992. They have done a lot of restoration work in hammocks in four parks on the Miami Rock Ridge: The Charles Deering Estate, R. Hardy Matheson Preserve, Castellow Hammock Park, and Matheson Hammock Park. The University of Miami has been checking on this program at three hammocks, and the Institute for Regional Conservation has provided ongoing technical help. The Natural Areas Management Division has also helped and trained the City of Miami to restore tropical hardwood hammocks there. This included two of the three remaining parts of Brickell Hammock: parts of Alice Wainwright Park and Simpson Park Hammock.

In the Florida Keys, a lot of work has started to control non-native plants. Most of these non-native plants are found at the edges of hammocks and in disturbed areas. Fairchild Tropical Garden and The Nature Conservancy have also been working to reintroduce and increase populations of rare plants in tropical hardwood hammocks in the Florida Keys.

Tropical hardwood hammocks can also be restored even after they have been destroyed. For example, almost all the tropical hardwood hammocks in the Florida Keys are "secondary" forests. This means they grew back after the land was cleared for farming or settlements. Hammocks on Elliott and Rhodes Keys recovered well after 35 years of natural regrowth. Hammocks can also start growing in areas of pine rockland that were cleared and then left alone. These secondary forests can be good for wildlife, even when they are relatively young. The Key Largo woodrat and the Schaus swallowtail butterfly both use relatively young secondary forests in the Florida Keys.

People who love native plants have been encouraging the use of native plants and the restoration of native plant communities in South Florida since the early 1970s. Tropical hardwood hammocks were one of the first natural communities people tried to create from scratch. Efforts to create them began as early as 1965.

One problem with using native plants in landscaping is that some species are being planted outside their natural historical range. There, they can become established and potentially become invasive. For example, the pitch-apple (Clusia rosea), which might be native to the lower Florida Keys, has been widely planted in gardens throughout southeastern Florida. It has now started to grow wild in southeastern Florida and threatens several natural communities, including tropical hardwood hammocks. In South Florida, native species have very specific natural ranges. These ranges must be respected when planning restoration projects.

Keeping an eye on tropical hardwood hammocks and how they are managed is also very important.

Images for kids

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  Mahogany Hammock Trail, an accessible tropical hardwood hammock in Everglades National Park.