MANGROVE HABITATS

Florida Mangrove Species:
The term mangrove is loosely used to describe a wide variety of often unrelated tropical and subtropical trees and shrubs which share common characteristics.  Globally, more than 50 species in 16 different families are considered mangroves (Tomlinson 1986).  In Florida, the mangrove community consists of 3 main species of true mangroves:  the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans) and the white mangrove (Laguncularia racemosa).  The buttonwood (Conocarpus erectus) is often considered a fourth mangrove species, however, it is classified as a mangrove associate because it lacks any morphological specialization common in true mangrove species, and because it generally inhabits the upland fringe of many mangrove communities.   

Red mangroves, Rhizophora mangle, dominante the shoreline from the upper subtidal to the lower intertidal zones (Davis 1940, Odum and McIvor 990), and are distinguished from other mangroves by networks of prop roots that originate in the trunk of the tree and grow downward towards the substratum.  Red mangroves may attain heights of 25 m, with leaves a glossy, bright green at the upper surface, with somewhat more pale undersides. Trees flower throughout the year, peaking in spring and summer. Propagules of the red mangrove are pencil-shaped and may reach 30 cm in length as they mature on the parent tree (Savage 1972, Carlton 1975).  

Black mangroves typically are found growing immediately inland of red mangroves and may reach 20 m in height.  They are characterized by their conspicuous pneumatophores, vertical branches that may extend upward in excess of 20 cm from cable roots lying below the soil.  Pneumatophores develop into extensive networks of fingerlike projections that surround the bases of black mangroves to provide them with proper aeration.  The leaves of black mangroves tend to be somewhat narrower than those of red mangroves and are often found encrusted with salt.  Black mangroves flower throughout spring and early summer, producing bean-shaped propagules (Savage 1972, Carlton 1975, Odum and McIvor 1990). 

White mangroves are more prominent in high marsh areas, typically growing upland of both red and black mangroves.  White mangroves are significantly shorter than red or black mangroves, generally reaching 15 m in height.  Their leaves are oval in shape, and somewhat flattened.  Trees flower in spring and early summer, and produce small propagules which measure only 1 cm.

Mangroves occur in dense, brackish swamps along coastal and tidally influenced, low energy shorelines.  In Florida, mangrove forests extend from the Florida Keys to St. Augustine on the Atlantic coast, and Cedar Key on the Gulf coast.  Factors such as climate, salt tolerance, water level fluctuation, nutrient runoff, and wave energy influence the composition, distribution, and extent of mangrove communities.  Temperature also plays a major role in mangrove distribution.  Typically, mangroves occur in areas where mean annual temperatures do not drop below 19°C (66°F) (Waisel 1972).  Mangroves are damaged under conditions where temperatures fluctuate more than 10°C within short periods of time, or when they are subject to freezing conditions for even a few hours.  Further, Lugo and Patterson-Zucca (1977) showed that stress induced by low temperatures leads to decreasing structural complexity in black mangroves, with tree height, leaf area, leaf size and tree density within a forest all negatively impacted. 

Mangrove Adaptations:
In general, mangrove species share 4 important traits that allow them to live successfully under environmental conditions that often exclude other species.  Some of these adaptations include:  morphological specialization, i.e., aerial prop roots, cable roots, vivipary, and other features that enable mangroves to adapt and thrive in their environments;  the ability to excrete or exclude salts;  habitat specificity within estuaries, with no extension into upland terrestrial communities;  and taxonomic isolation from other generically related species inhabiting upland communities (Tomlinson 1986).

          Root aeration:
Another adaptation exhibited by mangroves is observed in root aeration.  Soils in mangrove areas tend to be fairly axoxic, preventing many types of plants from taking root.  Mangroves have adapted to this condition by evolving shallow root systems rather than deep taproots.  Red mangroves aerate their roots by way of drop roots and prop roots which develop from lower stems and branches, and penetrate the soil only a few centimeters. Prop roots act to both stabilize the tree, and provide critical aeration to the roots. The above-ground areas of these roots are perforated by many small pores called lenticels that allow oxygen to diffuse first into cortical air spaces called aerenchyma, and then into underground roots (Scholander et al 1955, Odum and McIvor 1990).  Water is prevented from entering the tree via lenticels due to their highly hydrophobic nature which allows the red mangrove to exclude water from prop roots and drop roots even during high tides (Waisel 1972).

Black mangroves utilize a different strategy for aeration of root tissues.  Black mangroves have cable roots which lie only a few centimeters below the soil surface, and raditate outward from the stem of the tree (Odum and McIvor 1990).  A network of erect aerial roots extends upward from the cable roots to penetrate the soil surface.  These erect roots, called pneumatophores, contain lenticels and aerenchyma for gas exchange, and may form dense mats around the base of black mangrove trees, with pneumatophores attaining as much as 20 cm or more in height depending on the depth of flood tides (Odum and McIvor 1990). 

          Salt balance:
Mangroves are facultative halophytes, meaning they have the ability to grow in either fresh or salt water depending on which is available.  However, despite the fact that mangroves are able to grow in fresh water, they are largely confined to estuaries and upland fringe areas that are at least periodically flooded by brackish or salt water (Gilmore and Snedaker 1993).  Mangroves are rarely found growing in upland communities.  Simberloff (1983) and Tomlinsion (1986) suggested that one reason mangroves do not develop in strictly freshwater communities is due to space competition from freshwater vascular plants.  By growing in saline water, mangroves reduce competitive threat, and thus are able to dominate the areas they grow in. 

As facultative halophytes, mangroves not only tolerate, but thrive under saline conditions.  They accomplish this either by preventing salts from entering their tissues, or by being able to excrete excess salts that are taken in.  Red mangroves (Rhizophora mangle), for example, exclude salts at their root surfaces.  This is accomplished nonmetabolically via a reverse osmosis process driven by transpiration at leaf surfaces in which water loss from leaves produces high negative pressure in xylem tissue.  This, in turn, allows water to freely diffuse into plant tissues.  In addition to excluding salts, red mangroves also have the ability to exclude sulfides from their tissues.  This sometimes results in elevated pore water concentrations of sulfides in localities where poor flushing of the mangrove area is common (Carlson and Yarbro 1987). 

In contrast to salt exclusion observed in red mangroves, other species such as black mangroves, white mangroves and buttonwoods each utilize salt excretion as a salt-balancing mechanism.  Salt concentrations in the sap of these species may be up to ten times higher than in species that exclude salts (Odum and McIvor 1990). Salt-excreting species are able to take in high salinity pore water, and then excrete excess salts using specialized salt glands located in the leaves.  Atkinson et al. (1967) suggested this process involved active transport, and thus required energy input from mangroves to drive the process.

          Reproduction and dispersal:
Reproductive adaptions in mangroves include vivipary and hydrochory (DEF=dispersal of propagules via water).  Red and black mangroves are considered to be viviparous because once seeds are produced, they undergo continuous development rather than entering a resting stage to await germination in appropriate soil.  White mangroves are not considered to be viviparous;  however, germination in this species often occurs during the dispersal period (Feller 1996).  Mangrove reproductive structures, called propagules rather than seeds, germinate and develop embryonic tissue while still attached to the parent.  Propagules eventually detach from the parent and float in water for a certain period of time before completing embryonic development (Rabinowitz 1978a, Odum and McIvor 1990) and taking root in new areas.  For germination to be completed, propagules must remain in water for extended periods of time.   The obligate dispersal period in red mangroves is approximated to be 40 days;  in black mangroves, it is estimated at 14 days;  and in white mangroves it is estimated at 8 days (Rabinowitz 1978a). This combined strategy of vivipary and long-lived, floating propagules allows not only wide dispersal of mangroves, but also allows for seedlings to establish themselves quickly once appropriate substrata are encountered (Odum and McIvor 1990).

Productivity and Nutrient Flux in Mangroves:
Mangrove forests are among the world’s most highly productive ecosystems, with gross primary production estimated at 3 – 24 g C/m^-2 day ^-1, and net production estimated at 1 – 12 g C/m^-2 day ^-1 (Lugo and Snedaker 1974, Lugo et al. 1976).  Red mangroves have the highest production rates, followed by black mangroves and white mangroves (Lugo et al. 1976).  Black mangroves have been shown to have higher respiration rates, and thus lower primary production,  in comparison to the red mangroves, due perhaps, to the higher salinity stress red mangrove trees come under (Miller 1972, Lugo and Snedaker 1974).

Mangrove communities, like many tidal wetlands, accumulate nutrients such as nitrogen and phosphorus, as well as heavy metals and trace elements that are deposited into estuarine waters from terrestrial sources, and thus act as nutrient “sinks” for these materials.  Mangrove roots, epiphytic algae, bacteria and other microorganisms, as well a wide variety of invertebrates take up and sequester nutrients in their tissues, often for long periods of time.  Mangroves also continually act as sources for carbon, nitrogen, and other elements as living material dies and is decomposed into dissolved, particulate and gaseous forms.  Tidal flushing then assists in distributing this material to areas where other organisms may utilize it.   

Leaf litter, including leaves, twigs, propagules, flowers, small braches and insect refuse, is a major nutrient source to consumers in mangrove systems (Odum 1970).  Generally, leaf litter is composed of approximately 68 – 86 % leaves, 3 – 15 % twigs, and 8 – 21 % miscellaneous material (Pool et al. 1975).  Leaf fall in Florida mangroves was estimated to be 2.4 dry g m^-2 day ^-1 on average, with significant variation depending on the site (Heald 1969,  Odum 1970).  Typically, black mangrove leaf fall rates are only ½ those of the red mangrove (Lugo et al. 1980). 

Once fallen, leaves and twigs decompose fairly rapidly, with black mangrove leaves decomposing faster than red mangrove leaves (Heald et al. 1979).  Areas experiencing high tidal flushing rates, or which are flooded frequently, have faster rates of decomposition and export than other areas.  Heald (1969) also showed that decomposition of red mangrove litter proceeds faster under saline conditions than under fresh water conditions, and also reported that as the decay process proceeds, nitrogen, protein, and caloric content within the leaf all increase.

Types of Mangrove Forests:
Gilmore and Snedaker (1993) described 5 distinct types of mangrove forests based on water level, wave energy, and pore water salinity:  1) mangrove fringe forests, 2) overwash mangrove islands, 3) riverine mangrove forests, 4) basin mangrove forests, and 5) dwarf mangrove forests. 

          Mangrove fringe forests:
Mangrove fringe forests occur along protected coastlines and the exposed open waters of bays and lagoons.  These forests typically have a vertical profile, owing to full-sun exposure.  Red mangroves dominate fringe forests, but when local topology rises toward the uplands, other species may be included in zones above the water line.  Tides are the primary physical factor in fringing forests, with daily cycles of tidal inundation and export transporting buoyant materials such as leaves, twigs and propagules from mangrove areas to adjacent shallow water areas.  This export of organic material provides nutrition to a wide variety of organisms and provides for continued growth of the fringing forest.

          Overwash islands:
Like fringe forests, mangrove overwash islands are also subject to tidal inundation, and are dominated by red mangroves.  The major difference between mangrove fringe forests and overwash islands is that, in the latter, the entire island is typically inundated on each tidal cycle.  Because overwash islands are unsuitable for human habitation, and because the water surrounding them may act as a barrier to predatory animals such as raccoons, rats, feral cats, etc., overwash islands are often the site of bird rookeries.

          Riverine mangrove forests:
Riverine mangrove forests occur on seasonal floodplains in areas where natural patterns of freshwater discharge remain intact.  Salinity drops during the wet season, when rains cause extensive freshwater runoff;  however, during the dry season, estuarine waters are able to intrude more deeply into river systems, and salinity increases as a result.  This high seasonal salinity may aid primary production by excluding space competitors from mangrove areas.  Further, nutrient availability in these systems becomes highest during periods when salinity is lowest, thus promoting optimal mangrove growth.  This alternating cycle of high runoff/low salinity followed by low runoff/high salinity led Pool et al. (1977) to suggest that riverine mangrove forests are the most highly productive of the mangrove communities. 

          Basin mangrove forests:
Basin mangrove forests are perhaps the most common community type, and thus are the most commonly altered wetlands.  Basin mangrove forests occur in inland depressions which are irregularly flushed by tides.  Because of irregular tidal action in these forests, hypersaline conditions are likely to occur periodically.  Cintron et al. (1978) observed that the physiological stress induced by extreme hypersalinity may severely limit growth, or induce mortality in mangroves.  Black mangroves tend to dominate in basin communities, but certain exotic trees such as Brazilian pepper (Schinus terebinthifolius) and Australian pine (Casuarina spp.) are also successful invaders.  Basin mangrove forests contribute large amounts of organic debris to adjacent waters, with the majority being exported as whole leaves, particulates, or dissolved organic substances typical of waters containing high tannin concentrations.

          Dwarf mangrove forests:
Dwarf mangrove forests occur in areas where nutrients, freshwater, and inundation by tides are all limited.  Any mangrove species can be dwarfed, with trees generally limited in height to approximately 1 meter or less.  Dwarf forests are most commonly observed in South Florida, around the vicinity of the Everglades, but occur in all portions of the range where physical conditions are suboptimal, especially in drier transitional areas.  Despite their small size and relatively low area to biomass ratios, dwarf mangroves typically have higher leaf litter production rates; thus primary production in dwarf forests is disproportionately high when compared with normal mangrove forests. 

Ecological Role of Mangroves:
Mangroves perform a vital ecological role providing habitat for a wide variety of species.  Odum et al. (1982) reported 220 fish species, 24 reptile species, 18 mammal species, and 181 bird species that all utilize mangroves as habitat during some period of life.  Additionally many species, though not permanent mangrove inhabitants, make use of mangrove areas for foraging, roosting, breeding, and other activities. 

Mangrove canopies and aerial roots offer a wealth of habitat opportunities to many species of estuarine invertebrates.  Barnacles, sponges, mollusks, segmented worms, shrimp, insects, crabs, and spiny lobsters all utilize mangrove prop roots as habitat for at least part of their life cycles (Gillet1996 In: Feller 1996).  Additionally, mangrove roots are particularly suitable for juvenile fishes.  A study by Thayer et al. (1987) in the Florida Everglades showed that comparitively more fishes were sampled from mangrove areas than from adjacent seagrass beds.  In this study, 75% of the number of fishes sampled were taken from mangrove areas, while only 25% were sampled from nearby seagrass beds.  Further, when fish densities in each habitat were examined, fish density in mangroves was 35 times higher than in adjacent seagrass beds. 

In addition to providing vital nursery and feeding habitat to fishes, mangroves also assist in shoreline protection and stabilization.  Prop roots of red mangroves trap sediments in low-energy estuarine waters, and thus assist in preventing coastal erosion.  Mangroves also assist in buffering the coastal zone when tropical storms and hurricanes strike.  Because mangroves encounter damaging winds and waves before inland areas do, the branches in their canopies, and their many prop roots create friction that opposes and reduces the force of winds and waves.  Thus, coastlines are protected from severe wave damage, shoreline erosion and high winds (Gillet1996 In: Feller 1996).

A number of spatial guilds for mangrove-associated species were identified by Gilmore and Snedaker (1993).  The sublittoral/littoral guild utilizes the prop root zone of red mangroves associated with fringe forests, riverine forests, and overwash forests.  The prop root zone provides sessile filter feeding organisms such as bryozoans, tunicates, barnacles, and mussels with an ideal environment.  Mobile organisms such as crabs, shrimp, snails, boring crustaceans, polychaete worms, many species of juvenile fishes, and other transient species also utilize the prop root zone of mangroves as both a refuge and feeding area.

The arboreal canopy guild consists of species able to migrate from the water’s surface to the mangrove canopy.  Lagoonal snails such as the coffee bean snail (Melamphus coffeus), angulate periwinkle (Littorina anguilifera), and ladderhorn snail (Cerithidea scalariformis) are among the most common of the invertebrate species in this guild.  Also common are many species of crustaceans such as the common mangrove crabs Aratus pisoni, Goniopsis cruentata, Pachygrapsus transverses, and Sesarma spp., the isopod Ligea exotica, and many species of insects.  Birds also constitute a major component of this spatial guild.

When compared with species that inhabit adjacent seagrass areas, the benthic infaunal guild is generally considered to exist under somewhat impoverished conditions, primarily due to the reducing conditions which often exist in mangrove sediments.  Despite this, the benthic infaunal community in mangrove areas is highly productive, especially when microbial activity is taken into consideration. 

The upland arboreal guild includes those species associated with tropical hardwoods such as mahogany (Swietenia spp.), cabbage palms (Sabal palmetto), dogwoods (Piscidia spp.), oaks (Quercus spp.), red bay (Persea sp.), gumbo limbo (Bersera simaruba), mastic (Mastichodendron sp.), figs (Ficus spp.) and stoppers (Eugenia spp.).  Also included are the various species of bromeliads, orchids, ferns, and other epiphytes that utilize upland trees for support and shelter.  Animals of this spatial guild, primarily birds and winged insects, often reside in the upland community, but migrate to feeding areas located in mangroves.  Common upland arboreal animals include jays, wrens, woodpeckers, warblers, gnatcatchers, skinks, anoles, snakes, and tree snails.

Finally, the upland terrestrial community is associated with the understory of tropical hardwood forests.  The most common members of this guild include various snakes, hispid cotton rats (Sigmodon sp.), raccoons (Procyon lotor), white-tailed deer (Odocoileusus virginianus), bobcats (Felis rufus), gray fox (Urocyon cinereoargenteus), and many insect species.  Many of the animals in this spatial guild enter mangrove forests daily for feeding, but return to the upland community at other times. 

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