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Originally published in "The Palmetto", the magazine of the Florida Native Plant Society,
Volume 18, Number 2, Summer 1998
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| This article is concerned primarily with freshwater wetlands dominated by trees, and while most often referred to as "swamps," they are now identified by the more precise phrase "forested wetlands." In any case, the type of plant community in question is what most people would recognize as a "swamp" if they walked into it, as their feet would likely get wet or sink into soft soil. Such water-saturated soils are dominated by plants which tolerate such extreme, although usually seasonal, conditions. These ecosystems are a significant part of Florida's natural landscape, and hold plants which have developed fascinating strategies for coping with adverse conditions. Wetlands have also been central to the human history of Florida, and humans, with conflicting short-term goals, are now determining the future of what remains of these productive, beautiful systems. | ||
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The humorous title chosen is meant to remind us of earlier years when Florida wetlands were peddled to uninformed buyers, who then found themselves with valueless property; the true "value" of such land may best be measured in a realm which is outside of commerce and short-term return on dollars invested. The curious term "reclamation" was often used when wetlands were drained for human occupation; this implies that they once did belong to us, and were somehow taken (by whom?), thereby justifying our reclaiming them as rightful property. We will someday deeply regret the arrogance, lack of foresight, and simple greed which has dominated wetland policy and actions over the last few generations, and is apparently continuing with current landholders and decision makers. | |
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Although arbitrary surveyors' lines signal political boundaries between most states, much of the current Florida landmass is defined by a natural boundary, where the land meets the Atlantic Ocean or Gulf of Mexico. In calmer tidal areas, these boundaries are diffuse, and mangroves or salt marsh plants dominate, forming forests in the south and fields to the north; in high energy areas, the shoreline is clearly defined by loose sand mounded into dunes, either bare and shifting or colonized by plants adapted to fluctuating degrees of salt, sandblasting, and drought. Within the boundaries delimited by maritime ecosystems (a topic for another article), there are large areas of land where rain or river water collects, or where freshwater seeps to the surface. When water is standing on the surface for more than 10% of the growing season, the site then meets the technical definition of "palustrine" (from the Latin for "swamp.") Of all the popular terms for wetlands, the word "swamp" probably has the largest number of connotations, most of them negative. In fact, the various common names used for such areas have caused great confusion, and eventually had to be addressed when wetland protection legislation was passed and promptly challenged. Today, wetland delineation is based on defensible observations of soil types, plant communities, and hydroperiod, all carefully outlined in technical manuals and established in the field by trained professionals. Palustrine systems include a wide range of wetland plant communities, most of which we would call swamps, but better denoted by technical terms, such as hydric hammock, wet prairie, the Everglades "swale" or marsh, and many others. Note that "marsh" is used for sites dominated by herbaceous plants, and with few or no trees. The Florida Natural Areas Inventory lists at least twelve distinct plant communities of forested wetlands in Florida, each with synonyms under other authors' classifications. These ecological definitions are generally descriptive, reflecting substrate, water source (basin depressions, floodplains, or seepage), dominant plant species, and natural maintenance cycles, such as fire. As might be expected, baldcypress (Taxodium distichum), pondcypress (Taxodium ascendens), and tupelo (Nyssa spp.) are frequent components of our forested wetlands. These particular species exhibit many special adaptations, but are also magnificent trees with fascinating histories. |
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| Human history has always been linked with wetlands, partly because of our need for freshwater, but also because wetlands can be very productive ecosystems. From our point of view, there is a down side to this biological productiveness, since it includes organisms that can be harmful to humans, from microscopic disease organisms to larger predators, all lurking in the shallows. The popular picture of a primeval swamp, rife with enormous snakes, alligators, leeches, mosquitoes, and malaria is unfortunately the dominant idea for most, and has been reinforced by elaborate artwork from the 19th century, usually depicting intrepid British explorers fending off imaginary creatures rising out of a great steaming swamp jungle. |
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Once again, part of this image has a basis in fact, because swamp trees show unusual adaptations to flooded soils. Stilt roots, prop roots, and aerial roots, draping down from overhead branches, all appear as sinister tentacles attempting to capture and consume the innocent wanderer. On flooded soils, roots grow upward like stalagmites, form loops, or send many small breathing roots up from the mud, perhaps looking like fingers of chthonic beings and adding to the lugubrious mood of the forest. Large flaring buttresses at the base of tree trunks add to the strange architecture, and unstable soils (including the cliché quicksand) all contribute to giving these forests a reputation for danger and evil. Even the name "malaria" is a contraction of the Italian "mala aria" meaning "bad air," since the disease was blamed on "vapors" rising from the swamps. Yet these same sites attract philosophers and scientists, and may reveal to us a glimpse of the world as it was before domination by human presence and its overwhelming artifacts. There are several underlying reasons for the unusual architecture of plants in swamps, including physiology and mechanical or physical limitations. Green plants require a balance of basic resources for successful growth: sunlight, liquid water, gaseous oxygen and carbon dioxide, and minerals. In the real world, the perfect combination of these factors is rare, except perhaps in tropical rainforests. Deserts present obvious shortcomings in the limited amount of available water; polar regions resemble deserts in limited water, because if present it is almost always frozen, but light is also severely reduced in the winter. Wetlands would seem to be the ideal environment, with abundant freshwater, sunlight, minerals, and atmospheric gasses, but vascular plant roots require considerable gas exchange. Simple physics limits the movement of gas into deeper water (think of a snorkel several feet long, and how difficult it would be to breathe, fighting against water pressure.) Microorganisms and chemical reactions further consume oxygen, and toxic elements or compounds accumulate in the substrate. As a result, most woody plants are not tolerant of flooded soils, and those which grow in flooded sites have evolved beautiful strategies for survival. In the most extreme cases, swamp plants must germinate and grow underwater. In fact, very few wetland trees do so, some because their fruits are buoyant (for dispersal) or growth may be inhibited until gas exchange improves. When conditions are right, they must germinate quickly on exposed muddy soil (an event which might occur only a few times per decade, indicating the benefits of occasional drought), establish a firm root system to prevent being washed away, grow in height as much as possible, then prepare to live like an aquatic plant, spending half the year under water. In Florida, most wetland tree and shrub seedlings might have to endure these conditions for just a few years; in the Amazon Basin, where water levels rise 40 feet every season, mature trees and palms that offer a shady canopy in the dry season become aquatic plants in the wet season, with their entire crowns submerged like coral reefs. This is an extreme example of adaptation to flooding, and the methods by which these plants survive is still under investigation. Certainly, water quality and other factors in the habitat play a role, but the overall picture of the flooded forest is very intriguing. Plants are the most efficient devices on earth which collect and store solar energy. The chemical process which is used to release this energy depends on the availability of oxygen. In the absence of oxygen, the chemical cycle leads to the production of ethanol (pure alcohol), which at higher concentrations is lethal to living organisms. Some research has suggested that wetland plants use a modified metabolic pathway when oxygen is lacking, leading to different, non-lethal, end products (such as malate, a salt or ester of malic acid), but these results are not universally accepted. Improved aeration, as allowed by structural changes described below, may help in limiting the amount of anaerobic respiration and its toxic by-products. In temperate zones, flooding during the winter season causes less harm because plants are dormant, and the biochemical cycles are slowed dramatically. This kind of suspended animation may also occur in tropical species, especially when light levels are reduced, as happens in deep water. |
| A third strategy occurs in some wetland species, where flooding will cause the death of secondary roots, but the plants are apparently capable of regenerating new roots when conditions allow, resulting only in slower overall growth of the individual. In this vein, it should be noted that many wetland trees will grow well on higher ground, but are less able to compete with other upland species. By evolving tolerance to conditions which would kill most other plants, wetland trees thrive with reduced or absent competition, sometimes forming large stands of only one or two species. As a result, some wetlands may actually be less diverse in species than adjacent uplands, but the ecotone (or transition between the two) is always rich in species and is especially valuable for foraging and hunting animals. |
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Structural adaptations to wet soils are easier to observe and interpret. Roots, stems, leaves, and fruits all show some adaptation to flooding. Again, oxygen is the driving factor, so while most roots are positively geotropic (moving toward the earth), many wetland roots grow in the opposite direction, and are often called "breathing roots," but can also be assigned technical terms such as pneumorhizae, pneumatophores, or pneumathodes. The most dramatic and common example is that of cypress "knees," a part of the root system which forms conical of rounded projections, from a few inches to twelve feet in height. The production of knees seems to be most abundant on sites which are alternately wet and dry, and the height of the knees is commonly thought to reflect the highest water level. Many experiments have been done to establish the function of these knees, and although it seems logical that they should help in obtaining gasses, the proof is not absolute. One convenient but sad misconception was expressed by a harvester of cypress trees for shredding into mulch; he stated that cypress knees are actually young cypress trees, ready to grow up as replacements after the adults are cut and dragged out of natural swamps! |
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In coastal Florida, the most commonly seen pneumatophores are the peg roots of black mangrove (Avicennia germinans), about the size of a pencil, often carpeting the ground as very dense mats. However, similar structures are produced by many palms, and by certain hardwood trees under specific conditions; tulip poplar (Liriodendron tulipifera) occurs throughout the eastern deciduous forest in upland situations. In the coastal plain, as far south as | |
| Orlando, it is found growing on hummocks in swamps, where although rarely inundated, the soil saturation is enough to cause the formation of small vertical peg roots. Swamp tupelo (Nyssa sylvatica var. biflora) and water tupelo (Nyssa aquatica) produce roots which form loops above the soil surface, sometimes wrapping around cypress knees. It has been argued that these structures help trap debris, and thus build up a mound of soil at the base of the mature tree, helping it to enlarge its own hummock of aerated soil, free from competition. Stilt roots, prop roots, and other aerial roots grow out from the trunks or branches of trees, eventually connecting with the soil below, creating a strong physical support as well as a pipeline for water and nutrients. Red mangrove (Rhizophora mangle) in coastal areas shows the best developed stilt roots, and strangler fig (Ficus aurea) usually shows some aerial root development, and prop roots when growing in flooded soils, almost as if to keep the trunk out of the water. Actually, strangler fig seems to prefer better drainage, and often occurs as an epiphyte, but if a seed germinates and grows on a log or tree stump, the roots will persist and enlarge, long after the stump has decomposed. | ||
| Cypress and tupelo often show basal swelling in the trunk, a feature that recurs in many species of wetland trees. It has been argued that this gives trees more stability in unstable soils, especially when large plank buttresses radiate from the trunk. While most evident in tropical trees, such as our native strangler fig, both cypress and Florida elm (Ulmus floridana) develop respectable buttresses on wet soils. Bell or bottle-type swelling of the base of the trunk is found in cypress, swamp tupelo, water tupelo, pond apple (Annona glabra), pop ash (Fraxinus caroliniana), and even red maple (Acer rubrum), but attains truly exaggerated proportions in cypress trees in north Florida. | ||
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Another explanation for basal swelling (also called stem hypertrophy) is suggested by the internal wood anatomy: large, low density cells dominate and may allow better gas exchange. The low density wood is easy to identify when dry, because it is almost as light as balsa; the wood of many swamp trees has in fact been used to make floats for fishing nets. A more pronounced internal change, often visible in cross-sections of plant roots and stems, is the occurrence of large gas-filled cavities in cortical tissue arranged around the central xylem. These canals, known as aerenchyma tissue, act as pipes to carry greater volumes of gasses more efficiently. In this way, wetland plants have evolved an internal architecture which allows normal biochemical processes to continue despite the limiting factors and stresses imposed by the environment. | |
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While the search for increased aeration is easy to understand, some anatomical features of swamp plants seem contradictory. This is seen in the leaves of many wetland plants, which show a strong similarity to those of very dry areas. Sclerophylly, the production of small, hard textured, glossy leaves is common, contrary to what might be expected where water would not seem limiting. In fact, seasonal dry periods, high levels of sunlight, wind, high rainfall, and inundation all have a damaging effect on leaf tissue. Some swamps are limited in available nutrients, and plants can conserve energy by producing small, durable leaves, especially when abundant light is available. These leaves have thick cuticles and are often coated with wax, as much to keep water out (which would leach nutrients) during floods and rains, as to keep water in during droughts. Other anatomical changes can be more ephemeral and are produced only during periods of inundation. Lenticels are small pores on the surface of plant stems and roots, and often appear as raised dots or lines on the bark of woody plants. Normal lenticels are essential for the entry of oxygen, but are often hypertrophied (abnormally large) when plants are flooded, indicating a more active function, and again helping to provide the gas exchange needed for growth. Lenticels can often be seen on pneumatophores. Adventitious roots are those roots produced directly from a stem above ground, and their presence is a common response to seasonal flooding. These roots again help with gas exchange, but also absorb any available dissolved nutrients. In cases where flooding deposits silt, these roots then grow into the new level of soil, and may continue to do so annually, leaving the original grade and base of the tree far below the surface. Actually, flood-borne silt deposits along rivers have created some of the most fertile soils in the world, and allowed the greatest advances in early agriculture. Ironically, the same floods today are reported as disasters, and every effort is made to prevent any recurrence, especially when houses are built at grade in well-known floodplains. Among wetland trees, baldcypress and pondcypress are especially interesting and reveal a fascinating history. Although true conifers, which are generally thought of as evergreen cone-bearing trees of dry, cold latitudes, swamp cypresses (which do bear cones), grow in flooded southeastern sites and are deciduous. Among wetland trees, they are among the most tolerant of flooding, surviving even when swamps are impounded and water levels remain constantly high. It has often seemed odd that this conifer should be deciduous in the south, where cold winters are not a limiting factor as they are closer to the poles, where many trees endure the winter in leafless dormancy. A possible explanation comes from plant fossils found in Alaska, and the picture they indicate is that of a plant community resembling modern-day cypress swamps, complete with a diverse fern understory. It is generally accepted that cycles of warmer polar temperatures did allow abundant plant growth at higher latitudes, but daylength would still resemble the cycles we see today, so for six months of the year, solar radiation would be severely limited. In light of this observation, the loss of leaves, even in warm temperature regimes, suddenly makes sense since leaves without sunlight are more of a liability, exposed to rain, wind, and hungry dinosaur herbivores. Today, the retained deciduous habit may be as beneficial in southern swamps with winter droughts, even though winter darkness is no longer a limiting factor. |
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| Since forested wetlands offer a third, aerial dimension, to the ecosystem, there are organisms which thrive in swamp forests even though they cannot tolerate flooding. This added dimension is the surface area available on trunks, branches, and leaves of woody plants. As in the tropical rainforest, there is competition for sunlit sites with reasonable access to nutrients and water; these same sites are out of reach of many herbivores. | ||
| Orchids, bromeliads, and ferns are the vascular plants which dominate our available swamp scaffolding, but closer inspection will reveal lichens, mosses, and liverworts on bark, usually showing a very distinct zonation based on the high water mark. This permanent characteristic chronicles the history of seasonal flooding and is reliable enough to be used as a part of wetland delineation. |
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Algae and fungi will occur throughout wetland forests, and along with liverworts, can be found as epiphylls, growing on the surface of leaves, much as they do in the humid tropics. Vines often thrive in swamp systems, adding their own complexity to the forest; they create aerial walkways above the water, allowing animals to move throughout the canopy. While making foot travel more difficult, it is unlikely that vines in swamps are any more damaging to trees than they are in any other natural system. Whether seen as obstacles to overland travel, as reservoirs of pestilence, or as oases of life, swamps provide habitat to a vast array of animals we prize, particularly bird life. The last panthers in Florida exist in part because of our own difficulties in navigating or eliminating swamps. Cypress trees hundreds or thousands of years old, akin to the redwoods of the west, have mostly been felled for timber, although a few living examples remain to inspire awe in those who are susceptible. The Fakahatchee Strand, perhaps the richest forested wetland in North America, is mostly in public ownership, but has many difficult management issues to address, as do most natural areas finally acquired and set aside. A long-term view of responsible land management may only begin when we develop an appreciation for the inherent values of biological diversity. Despite our preconceived ideas and learned fears, the swamp is probably a very good place to start, or at least a good place to get wet. Bibliography Brown, Clair A., and Glen N. Montz. 1986. Baldcypress: The Tree Unique, The Wood Eternal. Claitor's Publishing Division, Baton Rouge, Louisiana. Dennis, John V. 1988. The Great Cypress Swamps. Louisiana State University Press, Baton Rouge. Ewel, Katherine C., and Howard T. Odum, eds. 1984. Cypress Swamps. University of Florida Press, Gainesville. Eyre, F.H. editor. 1980. Forest Cover Types of the United States and Canada. Society of American Foresters, Washington, D.C. Florida Natural Areas Inventory (FNAI). 1990. Guide to the Natural Communities of Florida. FNAI and FDNR, Tallahassee, FL. Gilbert, Katherine M., et al. 1995. The Florida Wetlands Delineation Manual. Florida Department of Environmental Protection. Goulding, Michael. 1990. Amazon: The Flooded Forest. Sterling Publishing Co., New York. Kozlowski, T.T., ed. 1984. Flooding and Plant Growth. Academic Press, New York. Lugo, Ariel, Mark Brinson, and Sandra Brown. 1990. Forested Wetlands. Elsevier, New York. Myers, Ronald L., and John J. Ewel. Ecosystems of Florida. University of Central Florida Press, Orlando. Vileisis, Ann. 1997. Discovering the Unknown Landscape: A History of America's Wetlands. Island Press, Washington, D.C. |
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