About Quagmires — and other mires

Historically, we have called wetlands some name that implied low value or even evil and unquestionably available for dumping garbage or other fill. Today we know more about how wetlands function and about their many values to us, to other creatures and to moderating water flows. Learning more about the types of wetlands may help end the ill-considered undervaluing of these fascinating ecological systems and the subconsciously negative names given to them.

 

Bog is a term commonly used to refer to wetlands. However, bog has a very specific meaning to folks who know wetlands. Bogs usually form on mainly organic, as opposed to mineral, soils. Bogs form in depressions, sometimes a lakebed, or, just after the glacier, in depressions left by the melting of a giant ice block. Bogs have very low plant diversity. There is essentially no flow of surface water into a bog from mineral soil. It gets only rainwater falling right on it. Consequently there is an extreme shortage of all the nutrients that would normally come from mineral soils. Shortage of nutrients prevents survival of most types of plants. Thus the very low plant diversity. The living plants, usually mainly sphagnum moss, control the chemistry of the rainwater as it becomes bog water. Sphagnum stays living at the growing tip and dies from the bottom up. As sphagnum dies, the dead plant matter attracts alkaline ions from the bog water and holds them so they cannot react chemically with the acidic ions from the decaying plant matter. Thus the bog becomes very acid, sometimes reaching the acidity level of vinegar. In such a bog, the continued saturation with water combined with the acidity virtually halts decomposition. Remnants of the vegetation history of a bog can be found preserved in the deep peat. Drilling down and taking cores of the peat reveals that history. The lack of decay preserved a man in a bog in Jutland in remarkably complete condition for over 1000 years. Peat, especially sphagnum peat, is so antiseptic that it was used as wound dressings in World War One.

 

So a ‘bog’ is a very special type of wetland, not just any low, wet place. And we would not expect to find any types of plants in bogs that need large amounts of nutrients. Cattails (Typha) would starve in acid bogs.

 

A fen is a different type of peat-forming wetland. In fens, the peat often is formed mainly from sedges, rather than sphagnum. Fens differ from bogs because they obtain some water flowing from mineral soil, not just from rain falling on them as bogs do. The incoming water brings more nutrients than are available in bogs. The chemistry of the water in fens is alkaline, not acidic as in bogs. Nutrient flow into fens is somewhat limited and, in addition, the alkalinity of fen water alters the solubility of some nutrients making them unavailable to plants. So the vegetation in fens is still limited although more diverse than bog vegetation. The common presence of the pitcher plant (Sarracenia) in fens shows that the availability of nitrogen to plants is limited. In bogs, where pitcher plants also occur, the acidity keeps the nitrogen from being available. In fens, it is the alkaline chemistry that makes the nitrogen unavailable to plants. (Nitrogen is available to the plants only at one particular peak along the pH scale.) Pitcher plants obtain their needed nitrogen by catching and digesting insects and small animals such as frogs. The low availability of nitrogen from the fen water is the reason for the success of the pitcher plants where other plants cannot survive.

 

A marsh is a low place in the topography that is rich in mineral nutrients and is dominated by non-woody, nutrient-demanding plants. The permanence and depth of the water can vary as can the amount of organic matter deposited. Marshes may have great diversity of both fully aquatic and semi-aquatic plants or the vegetation diversity can be low and may be dominated by one species such as cattail. The capture of solar energy is very high, making it available as food for herbivores, (‘primary production’). This high ecological production together with the structure of the vegetation in high diversity marshes provides many habitats for animals.

 

Wooded swamps, like marshes, are rich in available nutrients but here the vegetation is dominated by woody species. Depending on the depth and permanence of the water, both evergreen and deciduous trees may survive the seasonal flooding. Prolonged submergence of roots can prevent root access to air and may inhibit or kill some species.

 

Wetlands are special places in any landscape mosaic. They add high rates of food production for herbivores and consequently are rich in food for carnivores. The diversity of their vegetation is a foundation for high biodiversity locally and the inclusion of wetlands in the landscape mosaic raises diversity across the landscape. Wetlands catch the peak of any flood and store it. They are special places as vernal ponds in the spring and as water sources in the dry season. They feed saved water either into the groundwater or at moderate flow rates down the watershed, protecting it from the ravages of the full peak flows of floods. Without wetlands, a landscapes is poorer in many ways.

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