This classification Maine's freshwater organic wetlands (peatlands) uses nutrient source, geomorphic-hydrologic setting, gross topography of the peat mass, microtopographic pattern, and presence of pools for distinguishing peatland types. We apply these primarily hydro-geomorphological criteria to landscape units called peatland complexes. Vegetation, while important at lower levels of classification and for the description of individual peatlands, is not used in this classification because, typically, all but the smallest peatland complexes in Maine have major areas of differing vegetational physiognomies and types. This classification resembles the peatland division of Canada's current wetland classification more than the classification in most common use in the United States because a large percentage of Maine peatlands are northern in character. Our classification contains eight peatland types. Two of these are divided into two subtypes. Only one of the types has unique floristic and vegetational elements: plateau (coastal) bog. Large Maine peatlands typically contain multiple complexes, commonly of more than one type. We use the term multiple-unit peatland for these multiple complexes.
Our classification and survey of the distribution of peatland types in Maine is based on a large representative sample of peatlands. We conducted an air photo survey of ∼1100 peatlands throughout the state, observed a representative subset of 171 of these from aircraft at low altitude, and studied a representative subset of 100 of the 171 on the ground. However, to classify a peatland using our system, only air photo study or aerial observation is needed.
The distribution of peatland types in Maine is controlled by gradients of topography, geological substrate, climate, and hydrology. Peatlands are least abundant in the well-drained western uplands. Unpatterned fens occur throughout Maine; hundreds of the smaller ones (∼<10 ha) occur in ice-block depressions (kettles). Five types, all of which also occur in Canada, reach their eastern North American southern limits in Maine: ribbed (string) fens at 45°∼30′N, eccentric bogs at 45°∼10′N, domed bogs with concentric pattern at 44°∼45′N, plateau bogs at 44°∼15′N, and gently convex bogs at 43°∼20′N. These latitudinal limits exclude individual southern outliers). Distributions of peatland types in adjacent areas of New Hampshire and Canada are consistent with those in Maine. We propose that the geographic position of Maine along a steep south (coastal) to north (inland) climatic gradient and a less-steep west-southwest to east-northeast climatic gradient paralleling the coast are the major factors accounting for the diverse representation of peatland types in so limited an area.