DWARF MANGROVE FORESTS AND
SAVANNAS OF SOUTH FLORIDA
Dwarfing or stunting is one response of vegetation to stress or environmental gradients. Dwarf vegetation has been described from a variety of locations including elfin or cloud forests on tropical mountains, pygmy pines on the east coast and west coast of the United States, krummholz in the Rocky Mountains, dwarf cypress and dwarf mangroves in south Florida. In all of these vegetation types trees that otherwise grow tall are dwarfed or stunted by natural stressors. Because dwarfing occurs in such a diversity of situations, it may present a unique kind of evolutionary bioassay for stress that can be used to help understand certain human impacts on vegetation. The purpose of this study is to begin to synthesize the literature on dwarf mangroves in south Florida and to provide some measurements to gain preliminary empirical perspective on this vegetation type.
Although there are many references in passing to the existence of dwarf mangroves, very little detailed information on them is available in the literature. In fact, there appears to be only one field study ever conducted on dwarf mangroves and the data from this study only occurs in tables of review papers (Lugo and Snedaker 1974; Pool et al. 1977; Snedaker 1989). All of the principal mangrove species found in the Caribbean region are known to occur as dwarfs including: red mangroves (Rhizophora mangle), black mangroves (Avicennia germinans), white mangroves (Laguncularia racemosa) and buttonwood (Conocarpus erectus). However, the majority of dwarf mangrove stands in south Florida seem to be dominated by red and black mangroves. Dwarf mangroves are usually less than 1.5 m tall and relatively old, on the order of 40 years based on leaf scar counts (Lugo and Snedaker 1974). These plant communities cover relatively large areas in the Florida Keys and in the Everglades on topographic flats that receive at least storm-driven tidal inputs. They dominate the "lower Saline Everglades" belt, between the sawgrass marsh and the tall mangrove zone, of Egler (1952), and form transitions with a number of vegetation types in Lugo's (1980) model of mangrove successional pathways. Craighead (1971) notes different morpho-logical forms of dwarf mangrove trees including "spiders" or low red mangrove clumps with outstretching prop roots and "natural bonsai" type trees with stunted or gnarled canopy growth.
Dwarf mangroves have been recognized as a separate type of mangrove forest in the classification system of Lugo and Snedaker (1974). This classification was further developed by Cintron et al. (1985) and Snedaker (1989) and has become the standard for use in mangrove work. For example, it recently was used by Bacon (1994) to organize his analysis of over 200 Caribbean coastal wetlands for response to sea level rise. A number of stressors have been proposed to cause dwarfing in mangroves including fire, desiccation, limited soil volume, low nutrients, lack of tidal flushing and high salinities (Craighead 1971; Egler 1952; Gilmore and Snedaker 1993), but there is no consensus. Most of the stressors are interrelated, so multiple causal factors are probably involved.
Exploratory field measurements of tree height and density were made at 5 dwarf mangrove stands in south Florida. Three stands were located in the Keys: Site 1 on the Gulf side of Saddle Bunch Key, site 2 on the Atlantic side of Park Key and site 3 on the Gulf side of Middle Torch Key. Two other stands were located in Everglades National park: Site 4 at the lower park boundary on U. S. Route 1 near Card Sound and site 5 near Paurotis Pond on the road to Flamingo.
Sampling was done with transects of 10 m x 10 m quadrats extending into the stands. Field time was limited so only a few plots could be measured at each site. All trees within the plots at the Keys sites (1 - 3) were counted and measured for height. In the Everglades dwarfs had the "spider" form which made it impractical to separate individuals, so clumps were counted and measured for height at these sites (4 and 5). Thus, data from sites 1 to 3 are not directly comparable to sites 4 and 5.
Results from the exploratory survey indicate that dwarf mangrove communities can have very different community structures (Table 1). One stand that was growing on an exposed limestone surface (Site 2) was almost completely dominated by many short black mangrove trees. Stands at the other two Keys sites were growing under seemingly comparable conditions in the upper intertidal zone on thick lime mud substrates, but the communities were different. One stand (Site 1) was a savanna with scattered trees almost equally mixed between red and black mangroves, while the other stand (Site 3) was a forest with tree canopies touching that had only red mangroves. At the Keys sites both tree height and density varied over a two fold range of values.
In the Everglades the two stands were generally similar with scattered clumps of red mangroves forming a savanna within a freshwater marsh background. Tree height and density seemed to compensate for each other with the stand at Site 4 having relatively shorter height but higher density than the stand at Site 5.
Dwarf mangrove communities are diverse and take on different forms depending on the mix of stressors that impact them. This diversity is reflected in the different terms used to describe the system from the literature: dwarf, stunted and scrub. These terms are used almost interchangeably but they seem to have very different meanings. Dwarf implies stressed conditions with a genetic basis, stunted implies stressed conditions without a genetic basis and scrub is a physiognomic term with no implication to stress. Since there have been no experimental studies on the system, scrub may be the most appropriate term at this time for the community. However, the uniform morphology of these tiny mangroves, especially in the Everglades, seems to indicate dwarfism, which therefore is used in this paper.
In terms of stress theory, dwarf mangroves may indicate a critical threshold of ecological response of the overall mangrove vegetation type. Different levels of stress can be inferred by considering characteristics of the system. The first level of stress (perhaps low oxygen in flooded soils) eliminates tree species diversity and essentially creates the mangrove community. The next level of stress (perhaps normal salinity and tidal action) causes several special physiological and morphological adaptations and perhaps eliminates the understory from the forest (Lugo 1986). A final level of stress, as listed earlier, causes reduced growth and results in dwarfism or stunting. Additional levels of stress, either natural or human-induced, may completely eliminate higher vegetation. Dwarf mangroves may be a bioassay for the evolutionary potential of coastal wetland ecosystems to respond to stress. Other dwarf systems may represent similar bioassays for different hierarchies of stress. Thus, further study of dwarf mangroves and other dwarf forms of vegetation may lead to new ways of thinking about response of ecosystems to stress.
Melissa Champion assisted with field work in south Florida and with preparation of the manuscript.
Bacon, P. R. 1994. Template for evaluation of impacts of sealevel rise on Caribbean coastal wetlands. Ecological Engineering 3:171-186.
Cintron, G., A. E. Lugo and R. Martinez. 1985. Structural and unctional properties of mangrove forests. In: W. G. D'Arcy and M. D. Correa A., eds. The Botany and Natural History of Panama. Missouri Botanical Garden. Saint Louis, MO. p. 53-66.
Craighead, F. C. Sr. 1971. The Trees of South Florida. Volume 1. The Natural Environments and Their Succession. University of Miami Press. Coral Gables, FL.
Egler, F. E. 1952. Southeast saline Everglades vegetation, Florida, and its management. Vegetatio 3:213-265.
Gilmore, R. G., Jr. and S. C. Snedaker. 1993. Mangrove forests. In: W. H. Martin, S. G. Boyce and A. C. Echternacht, eds. Biodiversity of the Southeastern United States. Vol. 1. Lowland Terrestrial Communities. John Wiley & Sons, New York, NY. p. 165-198.
Lugo, A. E. 1980. Mangrove ecosystems: successional or steady state? Biotropica Supplement on Tropical Succession12:65-72.
Pool, D. J., S. C. Snedaker and A. E. Lugo. 1977. Structure of mangrove forests in Florida, Puerto Rico, Mexico and Costa Rica. Biotropica 9:195-212.
Snedaker, S. C. 1989. Overview of ecology of mangroves and information needs for Florida Bay. Bulletin of Marine Science 44:341-347
Table. 1. Survey data on dwarf mangrove stands in the Florida Keys and Everglades National Park. Quadrat size was 10 m x 10 m.
Average Tree Height (cm)
Average Density (Stems/100m2)