Swamps Defined by Their Trees

“…..though you may think me perverse, if it were proposed to me to dwell in the neighbourhood of the most beautiful garden that ever human art contrived, or else of a Dismal Swamp, I should certainly decide for the swamp.” — Henry David Thoreau, whom scholars describe as a ‘patron saint of swamps’

Swamps are forested wetlands that form for two main reasons: poor drainage or the presence of a high water table. Swamp water ranges from standing or slow-moving, to seasonal flooding. Swamps in the southeastern United States tend to have deeper waters that, in general, are only accessible by boats. Northern swamps, by contrast, are shallower and can largely be traversed with high boots. The periodic floods that occur in swamps help deposit alluvial soil. In turn, the waterlogged, spongy soil leads to further accumulation of organic material, such as dead leaves or plants. This organic soil supports many types of water-tolerant trees that have evolved to thrive in wet conditions.

Mangrove Swamp

A mangrove swamp is defined by the U. S. Environmental Protection Agency as a “coastal wetland found in tropical and subtropical regions.” It is an area where both fresh and salt water converge. Mangrove trees have adapted well to their brackish water environment. The three most common varieties in the United States are the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans) and the white mangrove (Laguncularia racemosa). Mangrove leaves contain glands that excrete excess salt. Mangroves also have specialized root systems to stabilize them in the ground. Prop roots can descend from a mangrove tree’s branches or extend from the tree trunk for additional support. Mangrove swamps play an important role in preserving the coastline ecosystem. Their roots trap and filter out sediments that would otherwise blanket coral reefs further downstream. The presence of a mangrove swamp in a coastal region prevents excess erosion from waves and storms.

Deciduous Swamp Hardwoods: Red Maple

A deciduous swamp — sometimes called a hardwood swamp — is located near floodplains, alongside lakes and rivers, and in poorly drained basins. Its canopy of trees is dominated by deciduous trees that can tolerate the cycle of saturated soil, inundation, erosion and periodic sediment deposit. For example, the red maple (Acer rubrum) is highly adaptable to local conditions to help it survive in the swamp. Even the National Wildlife Federation has stated, “It’s unlikely that any other tree in North America can match the red maple’s wide range of growing conditions.” Its root system makes red maple moderately flood-tolerant, and it produces a large number of seeds in the springtime. These seeds germinate and sprout rapidly to help populate an area with red maples. New Hampshire, for example, has vast tracts of red maple swamps.

Deciduous Swamp Hardwood: Water Tupelo and Swamp Blackgum

Another type of deciduous swamp, the aptly named tupelo-blackgum swamp, features two related hardwoods — the water tupelo (Nyssa aquatica) and the swamp blackgum tree (Nyssa sylvatica var. biflora). The water tupelo’s swollen base provides two advantages: it aerates the trunk tissue and gives a stable structural base that anchors the tree solidly on the soggy ground. Additionally, the water tupelo discharges oxygen into its roots to make the surrounding soil chemistry more conducive to the tree’s uptake of nutrients from the waterlogged substrate. Water tupelo grows well in standing water, but its seeds germinate on dry ground, to coincide with the seasonal subsiding of the swamp’s water levels.

In like manner, the swamp blackgum tree grows well on continuously saturated soil, while also having seeds that germinate during dry spells. Swamp blackgum seeds can be dormant underwater. Growth of the sapling from a germinated seed is rapid so that the apex and leaves are high enough above the next onslaught of moving water. This explains why swamp blackgums do not grow in the deep parts of the swamp. Curiously enough, young saplings can sprout from the stumps of downed or harvested swamp blackgums. In other words, these trees have evolved the strategy to suppress buds on their trunks that, in turn, spring back to life when the original tree is felled by man, lightning, or fire. Then, too, the swamp blackgum’s roots deploy a similar mechanism to that of the water tupelo in aerating the soil to improve nutrient uptake.

Coniferous Swamp Evergreen: White Cedar

Coniferous swamps, also termed evergreen forested swamps, exist on poorly drained areas subject to year-round saturation, especially from springtime flooding. Abundant in this swamp are the conifers, or cone-bearing softwoods commonly known as evergreen trees — like the Atlantic white cedar (Chamaecyparis thyoides) and the Northern white cedar (Thuja occidentalis). Swamps dominated by the Atlantic white cedar have acidic soil that is low on nutrients, which slows the decomposition process. Consequently, this swamp’s leaf litter accumulation makes the ground highly organic. Atlantic white cedar is well-adapted to the highly organic swamp soil and can handle a wide range of water depths. The tree’s tissue even has an exclusion mechanism that selects which nutrients are taken up from the soil, and which are excluded. Another adaptation is a root system that can be shallow and widespread when the ground is saturated, yet also extend deeper when the ground is dry. Seeds of the Atlantic white cedar thrive in the swamp and germinate best in organic-rich soil, like the conditions of swamp peat. Saplings that grow in the swamp substrate are up to three times bigger than saplings found in other types of soils far from the swamp’s conditions.

As for swamps dominated by Northern white cedar, they are often characterized as mineral-rich sites located in limestone regions. Thus, the surface might be covered with organic layers that are acidic, but underneath are sublayers of limestone-derived soils that are more neutral or slightly alkaline. This means a different soil chemistry exists in the Northern white cedar swamp. Nonetheless, one swamp adaptation of the Northern white cedar is its ability to grow adventitious roots. These are roots that emerge from a location other than the traditional underground root system. Adventitious roots can extend from the trunk, branches, or leaves so that the tree is more structurally supported. Similarly, Northern white cedar can propagate from uprooted cedar trees via roots that form from vertical branches.

Coniferous Swamp Evergreen: Black Spruce and Balsam Fir

Black spruce (Picea mariana) and balsam fir (Abies balsamea) are further examples of evergreen trees that live in coniferous swamps. In fact, these two trees are the arboreal denizens that comprise the specialized wetland known as the spruce-fir swamp. Black spruce have the ability to sprout adventitious roots. They can also layer, which is when roots sprout from the branches that come into contact with the ground. Accordingly, black spruce’s co-dominant, the balsam fir tree, is noted for its seeds, which can remain dormant. The U.S. Department of Agriculture’s Forest Service has documented that the viability of a balsam fir seed diminishes once it reaches the ground — unless it lands in a cold swamp. Swamp conditions allow for the seed’s viability to persist for up to 3 more years. Balsam fir are also known for having strongly developed lateral root systems that continue to extend horizontally. Interestingly, the balsam fir’s needles are green on top, yet whitened below. This indicates that the green surface seeks to maximize photosynthetic ability, particularly since the excess moisture in the environment leads to fog development which can limit sunlight infiltration.

Palustrine Scrub-Shrub Swamp: Swamp Cottonwood

Shrub swamps are dominated by shrubby vegetation — like buttonbush, meadowsweet, swamp rose, arrowwood, ferns and goldenrod. But a special type of shrub swamp, called the palustrine tidal scrub-shrub swamp, is interspersed with some trees. A palustrine tidal scrub-shrub swamp consists of the mini isles and mudflats that form in salt marshes exposed to coastal tides or to periodically changing river water levels. Over time, scrub-shrub swamps evolve into the delta landforms at river mouths and even the barrier islands off the coast. But during their beginning stages of development, these scrub-shrub swamps have what is described as a broken or sparse canopy of trees. Swamp cottonwood (Populus heterophylla) is well-adapted to the scrub-shrub swamp because it can grow in sites that are too wet for other trees, like very saturated river bottoms and low-lying areas near tidewater. Water is relied upon as a transport agent for the swamp cottonwood’s seeds. The U.S. Department of Agriculture’s Forest Service has even stated that the best seedling establishment for the swamp cottonwood is along often-flooded creeks or rivers. They are intolerant of shade, which explains why they are typically found as a single tree in an area, as is characteristic of the palustrine tidal scrub-shrub swamp habitat. Another unique adaptation of the swamp cottonwood is that branches and twigs that break off and are then carried away downstream by high water can, in turn, establish themselves as new cottonwood trees once the high water recedes.

Deciduous Conifers: Bald Cypress Swamp

The bald cypress (Taxodium distichum) is both a deciduous tree and a conifer, thus enabling it to persist in both deciduous and conifer swamps. The existence of deciduous conifers is attributed to an evolutionary strategy that allows this versatile group to thrive in a wider range of environmental conditions. Swamp soil is less porous because water has filled up the spaces that normally would be taken up by air. As a result, swamp sediments and substrates are lacking in oxygen. The bald cypress has thereby developed the aerenchyma, which is porous tissue full of air-filled cavities within the roots and lower trunk. The aerenchyma’s air pockets function like an internal atmosphere in the root system to help promote better nutrient uptake from swamp soil. Bald cypress also have modified roots known as pneumatophores — sometimes called ‘knees’ — to help provide better gas exchange at the root level. Pneumatophores emerge from the horizontal roots found below the water’s surface. In so doing, these pneumatophores become conduits of air for submerged roots. Bald cypress likewise have buttressing of their lower trunks, wherein the base of their tree trunks swell to reinforce stability in the swamp. Bald cypress swamps are mainly found in the southeastern United States.

Deciduous Conifers: Larch Swamp

Another type of swamp populated by deciduous conifers is the larch swamp. It is dominated by the larch tree (Larix laricina), which also goes by other names like American larch, eastern larch, and tamarack. The larch is tolerant of a wide variety of environments, and favors swamp conditions. Like the bald cypress tree, the larch also has root tissue that can be oxygenated despite stagnant swamp water surrounding it. The larch, as a deciduous conifer, sheds its needles, which is unlike evergreen conifers. Biologists contend that evergeen conifers are adapted to regions where nutrients are low so it makes sense for these trees to hold onto their needles rather than replace them since there are so few nutrients to take up from the ground to help in the regeneration of new needles. But, in the case of the larch, it tends to live in places with better nutrients, so it has the means to rebuild a crop of new needles each growing season. The larch’s root system is well-adapted to swamp conditions because it spreads widely, often spreading over areas greater in radius than the tree’s height. Larch have also been known to exhibit layering behavior, especially when lower branches are covered with organic litter and consequently pushed into the ground to begin sprouting new roots. Larch even have the unusual characteristic of producing root sprouts as far as 30 feet away from the mother tree. Also interesting is that the larch is known to be a pioneer tree. In other words, it is generally the first tree to invade the wetlands of the more northerly latitudes — like the bogs in the Great Lakes, New England or Canada before they evolve into the swamps that become populated by other trees.