These regularly inundated wetlands form a forested border along large rivers, creeks, and lakes, or occur in depressions as circular domes or linear strands. Cypress swamp communities are strongly dominated by either bald cypress or pond cypress, with very low numbers of scattered black gum, red maple, and sweetbay. Understory and ground cover are usually sparse due to frequent flooding but may include such species as buttonbush, lizard's-tail, and various ferns. The canopy of a cypress swamp is dense, and is produced by cypress and other trees as well as epiphytic plants.
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These systems are strongly hydric and highly organic. They typically stay wet year round, having higher water levels than marsh systems, which may have annual dry periods due to the drought-like summer weather of southwest Florida. This means that the luxuriant flora cycles through growth and decomposition creating a low oxygen aquatic organic mucky substrate suitable for submerged and emergent plant life. Often, the fires frequent in southwest Florida will burn quickly around the edges of the swampy areas further reducing competitive, invasive, species. Typically a fire burns through a cypress swamp every 20-100 years.
The cypress swamps are the least ephemeral of the freshwater wetlands. They may be submerged as much as nine months out of the year. However, it should be noted that in order to reproduce, bald and pond cypress must have at least a few small periods when the soils are exposed to the sun and air.
The soils of cypress swamps are generally very acidic. While there is a great deal of organic matter present, it is not readily available. In fact the rates of decomposition due to the lack of oxygenated water means that dead materials are slow to decompose. During the few times yearly that the water levels fall enough that the peat and muck bottoms may be exposed, the rates of decomposition and re-growth actually increase even though the system is more stressed. While gross production may be at a high level, the net production remains low.
Often the nutrient cycles of a specific area can vary greatly from year to year. It is imperative to understanding these cycles that long term sedimentary observations are implemented. "Differences in organic matter accumulation result from differences in hydroperiod, fire frequency, and water source. (Larson, 1982).
The effects of evapotranspiration are less drastic in the deeper waters of the cypress swamps due to the dense overstory. Often the moisture and water are trapped between the aqueous substrate and the dense vegetation cover deep into the dry seasons when the rest of the wetlands are already drying out, and being exposed to the sunlight, which increases the nutrient cycle by oxidizing the organics in their substrates.
The long hydroperiod in a cypress swamp determines the types of plants that can thrive in such a system. Plants need not only to be flood tolerant, but also to be able to survive in soil with a low oxygen content. Because the soil is inundated for much of the year, oxygen in the soil is gradually depleted. Plants that grow in this soil have specially adapted roots or other mechanisms to allow them to survive. For example, one hypothesis about cypress knees is that they are an adaptation which allows cypress trees to survive in low oxygen conditions when the rest of their root system is underwater.
Cypress trees, both bald cypress or pond cypress, are unusual in that they are deciduous conifers—they lose their needles in late fall and grow new needles in the spring. Once mature, cypress trees are extremely flood tolerant, which allows them to become the dominant species in this system.
Although fire is infrequent in this system, it serves to prevent hardwood species from becoming dominant. Lack of fire, in combination with a change in water fluctuations, may result in the introduction of hardwood species and eventual transformation of the cypress swamp into a different ecosystem.
A popular misconception is that water in a swamp is stagnant, when in fact it moves slowly through the swamp. Therefore, changes to the water in systems that are "upstream" from a cypress swamp will eventually affect the water and living organisms in the swamp as well. Conversely, swamps affect "downstream" ecosystems by the water that flows out of the swamp. Swamps act as filters for sediments and pollutants that enter the system, which benefits the systems into which those swamps drain. In Lee County, for example, water that enters the Six Mile Cypress Slough eventually drains into Estero Bay.
Changes to the hydrology of a cypress swamp can have strong impacts on the system. Drainage of a swamp will allow less flood-tolerant species to become established. Wastewater discharge may increase both the volume of water and the amount of pollutants entering a swamp. Pollutants tend to settle out of water traveling through a swamp, and excessive amounts of these pollutants can damage or kill the plants and animals living there.
Historically, cypress trees have been used as a source of lumber in Florida, with the result that most cypress swamps contain second-growth cypress. Removal of cypress trees changes the overall species composition of the ecosystem, and removes habitat for species such as epiphytes and cavity nesting birds. Another use of swamps by humans is as a source of peat. Removal of peat from a swamp is extremely detrimental, as it accumulates very slowly and therefore is not easily replaced.