Tropical Hardwood Hammock

Tropical hardwood hammock is a closed canopy forest, dominated by a diverse assemblage of evergreen and semi-deciduous tree and shrub species, mostly of West Indian origin. It is also important habitat for ferns and orchids of West Indian origin. Tropical hardwood hammock is not a fire maintained community, although fire may burn into tropical hardwood hammocks under certain conditions. Soils in tropical hardwood hammocks are primarily composed of organic material which has accumulated directly on top of mineral substrate, and are moist, but rarely inundated. Tropical hardwood hammocks have been described and/or classified by a number of authors ( e.g. Harshberger 1914, Small 1929, Davis 1943, Craighead 1971, Craighead 1974, Duever et al. 1979, Snyder et al. 1990, Ross et al. 1992). At least five major types of hammocks can be described here: (1) rockland hammock "islands" on limestone substrate in or on the edges of pine rockland or marl prairie communities on the Miami Rock Ridge and in Big Cypress National Preserve; (2) Keys rockland hammock on limestone substrate making up the dominant forest type in the Florida Keys; (3) coastal berm hammock on storm-deposited berms in the Sand Keys (west of Key West), the Florida Keys, and along the northern shores of Florida Bay; (4) tree island hammock in the Everglades marsh and surrounding marl prairie and rocky glades; and, (5) shell mound hammock on aboriginal sites. Tropical hardwood hammocks here also include the more open coastal rock barren and sinkhole communities as classified by the Florida Natural Areas Inventory (FNAI and Florida Department of Natural Resources 1990). Coastal rock barren is a rare community occurring in tiny patches in the Florida Keys (FNAI and Florida Department of Natural Resources 1990). Sinkholes are found in areas of karst limestone, primarily in hammocks on the Miami Rock Ridge.

Tropical hardwood hammock is characterized by a diverse, closed canopy of hardwood species, primarily of West Indian origin, and by a fairly open shrub layer and a sparse, species-poor herb layer. Hammock composition changes over space, and hammocks in different regions (e.g. , the Miami Rock Ridge versus the Florida Keys) are composed of different species. Snyder et al. (1990) recorded over 150 species of tree and shrub species that were native to tropical hardwood hammocks in South Florida. Kruer (1992) noted differences in plant community composition and dominant species between lower Keys and upper Keys hammocks.

Tropical hardwood hammocks are habitat for a few endemic vascular plants. South Florida endemics limited in their distribution to tropical hardwood hammocks are Biscayne spleenwort (Asplenium x biscayneanum) and Ames’ halbard fern (Tectaria x amesiana)--both epipetric hybrid ferns endemic to the Miami Rock Ridge, and Keys indigo (Indigofera mucronata var. keyensis)--an endemic herb which is found along hammock edges and in coastal rock barrens. South Florida endemics found in tropical hardwood hammocks in addition to other communities include Blodgett’s wild-mercury (Argythamnia bodgettii)--found in hammock gaps, false leadplant (Dalea carthagenense var. floridana)--found along hammock edges, and Chromolaena frustrata--which is found along hammock edges and in coastal rock barrens.

While few plant species are endemic to tropical hardwood hammocks, hammocks are critical habitat for West Indian species where the northernmost portions of their ranges extend into South Florida. Plants with their entire United States distribution in South Florida, and which are limited to tropical hardwood hammock habitats include Bahama strongback (Bourreria succulenta), buccaneer palm (Pseudophoenix sargentii), crabwood (Gymnanthes lucida), Florida boxwood (Schaefferia frutescens), Florida oncidium (Oncidium floridanum), ghostplant (Leiphiamos parasitica), green thatch palm (Thrinaxradiata), Key’s nutrush (Sclerialithosperma), Key’s tree cactus (Pilosocereus robinii), Krug’s holly (Ilex krugiana), least halberd fern (Tectaria fimbriata), lignum vitae (Guajacum sanctum), mahogany mistletoe (Phoradendron rubrum), manchineel (Hippomane mancinella), milkbark (Drypetesdiversifolia), pearlberry (Vallesiaantillana), princewood (Exostema caribaea), red stopper (Eugenia rhombea), slender spleenwort (Asplenium dentatum), spicewood (Calyptranthes pallens), West Indian cherry (Prunus myrtifolia), West Indian mahagony (Swietenia mahagoni), wild cinnamon (Canella winterana), wild dilly (Manilkara jaimiqui ssp. emarginata), and wild-tamarind (Lysilomalatilisiliquum).

The canopy height of tropical hardwood hammocks varies according to substrate and climate. On the Miami Rock Ridge, a mature hammock will have a closed canopy at 18 m (59 ft) or less, while those on the Florida Keys have a canopy 9 to 12 m (30 to 39 ft) tall (Snyder et al. 1990). Typical canopy species of tropical hardwood hammocks include gumbo-limbo (Bursera simaruba), paradise tree (Simarouba glauca), pigeon-plum (Coccoloba diversifolia), strangler fig (Ficus aurea), wild mastic (Sideroxylon foetidissimum), and willow-bustic (Sideroxylon salicifolium). Although a temperate species, live oak (Quercus virginiana) can be found in or on the margins of many tropical hardwood hammocks outside of the Florida Keys. Other canopy trees include short-leaf fig (Ficuscitrifolia) and wild-tamarind--both mostly associated with rockland hammocks, West Indian mahogany--which naturally occurs in the northern Florida Keys and in hammocks along the northern shores of Florida Bay, and Gulf licaria (Licaria triandra)--a tropical species historically known only from a small area near downtown Miami. Some epiphytes also occur in the hammock canopy, including Spanish-moss (Tillandsiausneoides) and ballmoss (T . recurvata).

Common subcanopy and understory trees and shrubs include black ironwood (Krugiodendron ferreum), inkwood (Exothea paniculata), lancewood (Ocoteacoriacea), marlberry (Ardisiaescallonoides), poisonwood (Metopiumtoxiferum), satinleaf (Chrysophyllumoliviforme), and white stopper (Eugenia axillaris). Additional rockland hammock species include crabwood and spicewood. Coastal hammocks typically include Jamaica-dogwood ( Piscidia piscipula ), saffron-plum (Sideroxylon celastrinum), Spanish stopper (Eugenia foetida), and sea-grape (Coccoloba diversifolia). Buttonwood (Conocarpuserecta) can often be found in hammocks along the interface with mangrove swamps and salt marshes. In the Florida Keys and along the northern shores of Florida Bay, additional subcanopy and understory species include Bahama strongbark (Bourreria succulenta), beeftree (Guapira discolor), darling-plum (Reynosiaseptentrionalis), Florida boxwood, green thatch palm, Jamaica caper (Capparis cynophallophora), Key’s tree cactus, lignum-vitae, limber caper (Capparis flexuosa), manchineel, mayten (Maytenus phyllanthoides), milkbark, pearlberry, princewood, red stopper, torchwood (Amyris elemifera), and wild dilly. Species associated with aboriginal activity include red mulberry (Morusrubra) and soapberry (Sapindussaponaria), and those associated with wet areas in hammocks (such as sinkholes) include cocoplum (Chrysobalanus icaco), hackberry (Celtis laevigata), and pond-apple (Annona glabra). Several species, including Krug’s holly, and West Indian cherry are limited in distribution to tropical hardwood hammocks on the Miami Rock Ridge. Subcanopy and understory species with extremely limited distributions include: bitterbush (Picramnia pentandra)--occuring only in coastal hammocks on the Miami Rock Ridge; Bahama tree cactus (Pilosocereus bahamensis), cinnecord (Acacia choriophylla) and soldierwood (Colubrina elliptica)--found only in the upper Florida Keys; and cupania (Cupania glabra), maidenbush (Savia bahamensis), and rough strongback (Bourreria radula)--found only in the lower Florida Keys. Vines often associated with the hammock subcanopy include pull-and-hold-back (Pisonia aculeata), Tournefortia hirsutissima, and T . volubilis. Epiphytes found in the sub-canopy and understory include Florida peperomia (Peperomiaobtusifolia).

Taken from: U.S. Fish and Wildlife Service. 1999. South Florida Multi-Species Recovery Plan. Atlanta, Georgia. Pp. 3: 122, 124-127.

Listed Species Occurring in Tropical Hardwood Hammock Habitat

  • Florida Panther
  • Kirtland's Warbler
  • Eastern Indigo Snake
  • Key Deer
  • Key Largo Cotton Mouse
  • Key Largo Woodrat
  • Schaus Swallowtail Butterfly
  • Stock Island Tree Snail
  • Key Tree Cactus