Understanding Wetland Communities: From Bogs to Vernal Pools

by Rose Paul, Director of Science and Stewardship

LaPlatte River Marsh Natural Area © Sarah Wakefield

Some people call any wet, mucky place a bog or swamp. But give it a closer look, and you can begin to identify a delightful variety of wetland natural communities.

The first factor to observe is water, is it flowing or stagnant, are there inlets or outlets, is water present year round? Second look at the plant life, are there any trees or shrubs in the wetland, do sedges or mosses dominate? Finally pick up a handful of the soil and squeeze it, does the water run clear leaving recognizable fragments of plants or are you left with a handful of fine black muck?

Take care not to be fooled by a name. For example, Eshqua Bog in Hartland, a place well-loved for its showy lady’s slipper orchids, is really a rich fen.

Bogs and Fens

Bogs are the most nutrient-poor of the wetlands. With little to no inflow or outflow most of their water and minerals come from rainfall and airborne dust. Stagnant water has a reduced oxygen content, acids build up from decomposition, and bog plants are forced to find creative ways to absorb nutrients. Acid-loving dwarf shrubs, called heaths, have partnerships with fungi in their roots that make soil nutrients more available. Carnivorous pitcher plants, sundews and bladderworts go after their food more directly, trapping and dissolving little insects.

The most abundant plants are sphagnum mosses and heaths, including cranberries, leatherleaf, Labrador tea and bog laurel. Tamaracks and black spruce fringe the bog where it is drier.

Life in the fen is much easier than in a bog. Water flows slowly through them flushing out the acids of decomposition, and they’re fed by an upwelling of groundwater that is laden with calcium and other minerals.

A rich fen looks like a lush wet grassland and is dominated by sedges. Rich fens have many more species of plants and different mosses than bogs. These so-called brown mosses are abundant but less obvious than in bogs, tucked under the luxuriant sedge growth. High spots called hummocks are home to some of our native wetland shrubs like alternate-leaved buckthorn, shrubby cinquefoil and the white-leaved hoary willow.

Some bogs and fens started out as ponds at the end of the glacial era and, if conditions were just right, gradually filled in. The marshy edges of ponds growing with species such as cattails and rushes are one step in a centuries-long evolution from open water pond to marsh to wooded swamp.

Marshes and Swamps

The marshes we see today usually occur in ponds, lakes, slow-moving rivers and at river mouths. The constant movement of waves or currents increases the oxygen content in the water, brings a constant inflow of nutrients, and promotes decomposition. A fine, black fine muck soil develops that is high in well decomposed organic matter. A variety of grasses and sedges thrive in marshes, including the familiar cattails. Woody plants are absent from waterlogged marshes with one exception, buttonbush shrubs, which can tolerate growing in standing water.

Swamps are forested wetlands dominated by woody shrubs and trees. They occur in low spots that have wet soils, but not so wet that woody plants are excluded. They are only intermittently wet, during times like spring flooding and heavy summer storms. Swamps can be small dips in the landscape, such as an acre or so, or they can occupy many hundreds of acres. Some trees like black ash, green ash, red maple and swamp white oak are well adapted to the seasonally wet conditions of swamps.

One type of wooded swamp occurs in the floodplain of rivers. In a broad river valley floodwaters deposit sediment on the banks of the river, elevating the banks slightly above the surrounding land. These elevated ridges are called levees. There is a progression from floodplain forest trees along the levee, to swamp trees in the lower land beyond the levee. Typical floodplain forest species include silver maple, green ash, sycamore and cottonwood and the now declining American elms and butternuts. Nowadays intact floodplain forests are uncommon as the fertile soils have been cleared for agriculture or development.

Seeps and Vernal Pools

Seeps and vernal pool are small and occur in depressions in the forest, often at the base of slopes.

Water in a seep comes from groundwater discharge, where cracks in the bedrock direct flow toward the surface. A seep can persist throughout the year and often forms the headwaters of perennial streams. One feature of a seep is the constant temperature of groundwater, which is typically at 47 degrees and varies only a little during the year. This constant temperature allows vegetation such as grasses and sedges to get an early start in the spring, providing food for wildlife like bears and deer

Vernal pools are small, often less than ½ acre depressions in forests and they can occur quite high in the landscape, in little low spots on ridgelines. They typically lack inlets and outlets, gathering water from snowmelt and rainfall. The pools are lined with bedrock or dense gravel called hardpan that can hold water well into the summer months.

Vernal pools are most easily recognized, even in the dry times, by their lack of vegetation. Visit them in the spring when they teem with amphibious wildlife and listen for the deafening chorus of wood frogs and peepers. Many species of frogs and salamanders depend on vernal pools for breeding, egg-laying, and larval development. Because they dry out each year, vernal pools lack the predators, like small fish, that would normally prey on such a bounty of food.

Beaver Ponds

Beaver ponds are another ephemeral wetland but their cycle is measured in years, not months. Beaver dams create temporary ponds that make great habitat for species such as brook trout, otters, turtles, great blue herons and dragonflies. Eventually the beavers will abandon the pond once they’ve consumed all the available food and the untended dam will begin to fail, draining the pond slowly and allowing a sequence of natural communities to take over.

When there is still standing water, a shallow emergent marsh dominated by grasses and sedges will occur for a few years. As the marsh dries out woody species can take hold, often starting with willows and alders. This shrub swamp phase will eventually transition to a wooded swamp dominated by trees, setting the stage for a new population of beavers to move in, as their preferred food source returns.

Each wetland community type has a number of variations depending on many physical features and the ensuing plant life. The book Wetland, Woodland, Wildland, available at most bookstores, is a wonderful reference to the rich variety of Vermont’s natural communities.

This article is based on information found in Wetland, Woodland, Wildland, written by Elizabeth Thompson and Eric Sorenson.