Table of Contents: What is a bog? What makes a bog a bog? Geographic distribution of peatlands A bog by any other name.... Characteristics of non-forested peatlands Processes of peatland formation Bog succession Peatlands as dynamically stable ecosystems Bibliography Keywords:

What is a bog?

Sphagnum peatlands -- bogs and acidic fens -- are ecosystems of austerity. Environmental conditions are harsh, with paradoxical contrasts. Water is abundant,but nutrients are scarce. Slow decomposition occurs at the peat surface; within the peat depths, trunks of cedars remain preserved for hundreds or thousands of years. The simple mosaic of bog vegetation belies the complex adaptations that plants have evolved for survival in bog and acidic fen communities: from feathery,water-loggedSphagnum mosses, to leathery and evergreen ericad shrubs, to strange and delicate insectivorous plants.

In temperate southern New England, bogs are relicts of the region's glaciated past. Peatlands are isolated pockets of northern flora. For plants that were originally adapted to cold climates, Sphagnum peatlands provide a favorable habitat and a refuge from invasion by more temperate species. Compared to fields or forests or other New England ecosystems, species diversity is low in peatlands. A stark, somber beauty predominates for much of the year, occasionally interrupted by ostentatious display: the riotous, magenta outburst of flowering sheep-laurel; the heady, honeyed-pungent perfume of sweet pepperbush in bloom; an ethereal field of cotton-grass gone to seed; the dazzling sapphire of the pond's watery eye shining in the sun.

Nutrient-poor and acidic, a bog is a peat-accumulating wetland comprised of acidophilic vegetation, particularly Sphagnum mosses species and ericad shrubs. Although bogs are water-saturated, they have virtually no inflow or outflow of mineral-bearing water. Isolated from the groundwater table, their only source of nutrients is precipitation. Most bogs in Massachusetts, Connecticut, and Rhode Island are level bogs, also called flat or basin bogs. They have their origins at stream headwaters, in isolated valley bottoms or depressions, around pond or lake margins, or in kettlehole ponds formed by receding glaciers more than 10,000 years ago. Most of the bogs in southern New England have some degree of surface water or groundwater input, so they are technically classified as poor or acidic fens; in Maine, these peatlands are called unpatterned fen ecosystems.

True bogs include raised bogs, with peat domes high above the basin surface, and blanket bogs, where peat covers hundreds of acres of terrain, and are found in Maine, the Great Lakes Region, and Canada where cool and moist conditions generate deposition and accumulation of thicker layers of peat. In the peatlands of southern New England, peat accumulates but does not expand beyond the physical limits of the basin or depression. Also, the bog surface usually is level or only slightly elevated. Level bogs are not restricted to southern New England; they are common throughout the glaciated regions of the United States. 

Although most of the level bogs and acidic fens of southern New England are not true bogs hydrologically, their plant communities are characteristic of peatland vegetation. An individual peatland usually contains several natural communities. Black Pond bog, a "typical" kettlehole bog of southern New England, consists of a floating mat of Sphagnum peat spreading centripetally over a pond, and several distinct plant communities, in a pattern of concentric rings or zones, between the leading edge of the mat and the upland beyond the bog.

In kettlehole bogs, the plant communities often form concentric rings around the pond; in pond border bogs, the ringed zonation is less distinct or is absent. Vegetation patterns change from pond edge to upland edge based on environmental gradients of hydrology, topography, water chemistry, abundance of nutrients, and depth of the peat mat. Due to fluctuating water levels and nutrient regime, size and species composition of the plant communities can vary from peatland to peatland. The caveat here is: not only does vegetation vary floristically from one peatland to another, it also varies within an individual peatland, as a bog is usually home to more than one type of plant community. As often as not, the plant assemblages grade into transition zones that may be barely distinguishable out in the bog. In some bogs, one or more community types may be absent. Bogs and acidic fens are fascinating not only because they are wet and soggy, because each one is truly unique. 

What Makes a Bog a Bog?

Because they receive little or no mineral input from stream flow or groundwater, bogs are oligotrophic -- nutrient and solute poor. Bog waters not only lack nutrients, they are deficient in free oxygen (anoxic), and are highly acidic. The acidity of bog water ranges from pH 3.0 to pH 4.0, comparable to wine or tomato juice respectively. When the environment is sufficiently oligotrophic and anoxic for Sphagnum species, these mosses not only colonize a mat or shoreline, they perpetuate acidic, anaerobic, nutrient-depleted conditions, and form the bulk of the peat.

Once Sphagnum mosses have invaded a wetland, two conditions are essential for bog formation and maintenance: (1) Plant productivity -- or growth -- must surpass decomposition in order for a surplus of peat to build up. (2) On an annual basis, precipitation must exceed evapotranspiration -- providing a positive water balance -- to sustain the water-saturated, acidic environment necessary for preservation and accumulation of peat. Thus, bog formation initially depends on allogenic or external abiotic factors, such as climate and precipitation on a regional scale; and hydrology, topography, nutrient-regime, and water chemistry on a local scale. Over time, environmental factors play a lesser role in shaping ecosystem processes; internal, biotic factors, or autogenic processes, become increasingly influential to bog growth and maintenance. The dominant internal control is imposed by the Sphagnum mosses which, once established, create and/or change the physical environment to their competitive advantage. Under favorable environmental conditions, Sphagnum becomes an "ecosystem" engineer by transforming several physical and chemical aspects of its surroundings. Sphagnum produces thick layers of peat, directs vegetative succession, modifies water chemistry, changes hydrology, and acidifies bog waters.

Geographic distribution of peatlands

Peatlands comprise an estimated 2.3 million square miles, or approximately 1% of the earth's surface. Worldwide, true bogs are restricted to cool and moist temperate regions of the Northern Hemisphere, generally at latitudes above 40º N. Geographical distribution of bogs occurs primarily across the boreal and subalpine regions of Europe, the British Isles, Siberia, Alaska, Northern Canada, and the north central and northeast United States.

In North America, Sphagnum peatlands occur throughout the range of Pleistocene glaciation, which includes the Northeast and the Great Lakes region of Michigan, Wisconsin, and Minnesota; and also in unglaciated parts of New Jersey and northern Pennsylvania, Illinois and Indiana. Less common farther south, bogs usually are limited to higher altitudes, for example, in the Appalachian mountains, where cooler and wetter conditions prevail. Controlling mechanisms for southern distribution of bog communities appear to be the intensity of solar radiation and the length of the growing season during the summer.

At lower latitudes -- both temperate and tropical -- Sphagnum peatland ecosystems can exist where precipitation exceeds evapotranspiration. Either cooler temperatures or higher precipitation/relatively constant humidity may slow evapotranspiration losses sufficiently to create favorable conditions for bogs to form. In the south, warm and humid -- rather than cool and humid -- climates may sustain the development of peatlands. Although Sphagnum peatlands can be found in depressions within poorly-drained sediments of the southeast coastal plain, other kinds of peat-based wetland communities dominate. Occurring along the Atlantic coastal plain are the pocosins of North Carolina, which are evergreen shrub bogs, and "peaty swamps" (Hofstetter 1983), or cedar swamps, dominated by Atlantic white cedar (Chamaecyparis thyoides) and Sphagnum mosses. The Pine Barrens of New Jersey and the Great Dismal Swamp (Virginia/North Carolina) contain extensive cedar swamps.

Entirely different ecosystems from Sphagnum-dominated Northern peatlands, the cypress swamps and grassy marshes of the Southern Atlantic coastal plain are also peat-based wetlands. Among the largest -- and most famous -- are the Okefenokee Swamp in Georgia, Big Cypress Swamp in Florida, and the Everglades. The organic peat soils of cypress swamps are comprised of a variety of shrubby and herbaceous plants species; the peat of the Everglades is predominantly saw-grass (Cladium spp.) peat. 

Ombrogenous Bogs in the Northeast

Figure 1: Southern boundary of ombrotrophic, or true bogs, in the Northeast. (Damman and French 1987)

New England contains approximately 20,000 peat bogs and acidic, peaty fens, but few are true ombrotrophic, or exclusively rainfed, bogs. Canada and the northern United States support several types of genuine, ombrotrophic or ombrogenous peatlands, also called raised bogs.Northern and coastal Maine and the Adirondack region in New York State are the southernmost limits of raised bogs in the Northeast. (See figure 1.).

Stretching across hundreds of acres throughout the state of Maine, raisedbogs range in shape from gently convex to distinctively domed, with the peat surface extending 10-30+ feet above the bog's border. Another type of raised bog with distinctive topography is the plateau bog, which is found along the coast of Maine. (See Figure 2.)

Because they depend entirely on precipitation for nutrients, ombrotrophic peatlands requires a cool and moist climate year round, including summers. Sphagnum peat development appears to be hampered by prolonged dessication as well as by negative water balance during dry summer months. In most of New England, the months of July and August are too hot and dry to sustain growth of raised bogs.

Southern New England is dotted with several types of smaller forested and non-forested peatlands. The scope of this article focuses on the non-forested peatlands -- referred to as level bogs or fens-- that are found throughout the entire Northeast. (See Figure 3.) Level bogs that originate in kettlehole ponds or along pond borders are also called basin bogs, and they have relatively level surface elevations within the confines of the basin. Characteristic of many basin bogs is a mat of Sphagnum peat, which may be up to several meters thick, that floats on the surface of the water. Because the bog mat is not grounded to the bottom of the basin, it quivers or quakes when stepped upon -- earning the name "quaking bog." The level peatlands of southern New England are actually classified as poor fens, transitional fens, or acidic fens rather than as bogs, because they receive some import of mineral-bearing water; in Maine they are classified as unpatterned fen ecosystems.

RAISED BOGS

	DOMED OR  CONVEX RAISED BOG
	PLATEAU BOG
Figure 2. Ombrotrophic raised bogs of Northern New England (From Damman and French 1987.)

	KETTLEHOLE BOG POND BORDER BOG
	MOAT BOG
	LAKE-FILL BOG
Figure 3. Flat or basin bogs of Southern New England and the Northeast. (From Damman and French 1987)

A bog by any other name....

Northern peatland ecosystems include bogs and fens, as well as conifer, hardwood, and shrub swamps. Complex interactions of biological factors and environmental conditions create myriad variations among and within peatlands; therefore, they are difficult to categorize. There are neither universal definitions of peatland types, nor standardized names. A "bog" in the United States is a "muskeg" in Canada, a "hochmoor" in Germany. To add to the confusion, wetland terminology differs between European and North American scientists, as do the methods of peatlands classification. Such differences hinder comparisons between North American and European peatlands. Most peatlands research, however, has focused on the bogs of Europe due to their extensive size, range, and economic value.

Mitsch and Gosselink (2000) illustrate some of the terminology differences in their book Wetlands (Third edition). In the United States, "fen" and "marsh" are casually interchangeable; Europeans differentiate fens from marshes and marshes from swamps. Another major difference in wetlands classification is that, according to the European definition, a swamp is dominated by reeds and is a non-forested wetland rather than a peatland. In the United States, a swamp is known as a peat-based, forested wetland.

When is a bog truly a bog? When is it really a fen? It depends on whom you ask. In its strictest usage, "bog" defines only ombrotrophic (rainfed) peatlands, but the word also has been used loosely to include all peat-forming wetlands. Today, many wetland scientists prefer "peatland" (U.S.) or "mire" (Europe) as the all-encompassing term for peat-based wetland ecosystems. Bogs and fens are "the fundamental subdivision" of peatlands, according to A.J.P. Gore (1983), who noted that "even at the widest scale of enquiry it appears that these two words can adequately subdivide all" peatlands. The boreal regions of Europe and Canada contain many more peatland types than does the glaciated Northeast. In Canada, for example, 18 types of bogs and 17 types of fens have been classified. 

Peatlands research in the United States has concentrated on the Great Lakes region, which contains the most extensive peatlands, and, more recently, in Maine.  Much work has been done in Maine to study and classify its numerous and varied peatlands, especially its raised bogs that are not found anywhere else in New England. The small peatlands of New England received little scientific attention in terms of ecology and development before the 1990's. The first, comprehensive, regional attempt at classifying peatlands of New England is Damman and French's The Ecology of Peat Bogs of the Glaciated Northeastern United States: a Community Profile (1987). They categorize bogs -- in relation to source of water and vegetation type -- as landforms.

Damman and French's landform system can be used as a general guideline to bog classification with the caveat that visually identifying a peatland by landform may not be possible in the field because vegetation is only one parameter for classification. Determining whether a dwarf shrub bog is a lake-fill or a perched bog requires additional knowledge of hydrology and topography. Development and succession patterns of an individual bog cannot be determined accurately without stratigraphical studies -- coring the vertical peat layers for the sequential history of deposition and formation. Damman and French noted that their study underscored the need for more research to garner a thorough understanding of bog processes in the Northeast.

Over the past decade, interest in peatlands has grown, resulting in new studies by the individual New England states to inventory and classify their peatlands and the plant communities that comprise bogs and fens. (The New England states have been classifying all of their terrestrial and wetland natural communities; the Nature Conservancy is developing a regional classification that fits the United States National Vegetation Classification (USNVC) system.)   Currently, peatlands in New England are subdivided into raised or flat bogs; poor, intermediate or rich fens; and swamps. 

Scientists use several criteria to separate peatlands into bogs and fens: hydrology, water chemistry, nutrient regime, topography, and/or vegetation. One of the most widely accepted methods of classifying peatlands is according to hydrology, the distribution and movement of water through the ecosystem. An ombrogenous peatland has no source of water other than precipitation. Raised bogs -- both convex and plateau -- are ombrogenous because the surface peat layers are isolated from surface and groundwater flow. Geogenous peatlands receive water from streams, surface runoff, groundwater and/or seepage as well as from precipitation; fens -- even acidic fens -- are geogenous.

HYDROLOGICAL CLASSIFICATION OF PEATLANDS

OMBROGENOUS			Domed or Convex Raised Bogs Plateau Bog
GEOGENOUS	Limnogenous		Marsh Swamp Bog
	Topogenous		Swamp Marsh Kettlehole Bog
	Soligenous		Aapa Mire and Slope Fen

Table 1. Classification of peatlands based on hydrology. Two major categories are geogenous peatlands, which are open to surface and groundwater flow, and ombrogenous peatlands, which only receive precipitation. (After Damman 1986)

Geogenous peatlands are further categorized by topographical as well as hydrological development. Limnogenous peatlands are influenced by external flows, such as flooding, associated with lakes or streams. Topogeneous peatlands are influenced by upland runoff and/or groundwater flow and originate in topographical depressions; kettlehole and pond border bogs are topogenous, as are perched water peatlands in isolated valleys. Soligenous peatlands are influenced by seepage and water flowing down a gradient; they occur in northern New England.

Hydrology influences water chemistry. Flowing water carries nutrients (nitrogen, phosphorus, and potassium) and other minerals from soils into and out of wetland ecosystems. The amount of flow-through, the mineral chemistry of the soil through which the water passes, and the water chemistry are factors in peatland development. When groundwater and/or surface flows are impeded or absent, and dissolved minerals are in short supply, the resulting highly acidic, nutrient-poor, oxygen-deficient conditions are conducive to peatland formation. Ombrogenous peatlands depend exclusively on precipitation for water and are have no supply of flowing, mineral-laden water. Ombrogenous peatlands are more commonly referred to as ombrotrophic, a term that simply means "rainfed" but is used by many wetland scientists use to describe water and soil chemistry as well as hydrology. In contrast to ombrotrophic bogs, minerotrophic peatlands are geogenous and have contact with mineral-bearing water. Minerotrophic wetlands may be highly variable in nutrients, ranging from eutrophic (nutrient rich) marshes and fens to mesotrophic (moderate in nutrients) fens and swamps to oligotrophic (nutrient poor) level bogs and poor fens. Technically, the level and quaking bogs of New England would be classified as weakly minerotrophic fens -- also called transition or poor fens -- because they are influenced by groundwater, however slightly. From a strict hydrological definition, there are no true ombrotrophic bogs - raised and exclusively rainfed - in southern New England.

The level, oligotrophic, acidic peatlands of southern New England can be loosely termed "bogs" when water chemistry, nutrient-regime, and, in particular, vegetation are factored into peatland classification..

RAISED BOG	LEVEL BOG	POOR (ACIDIC) FEN
West Quoddy, ME	Poutwater Pond, Holden, MA	Ponkapoag bog, Canton, MA
INTERMEDIATE FEN		RICH FEN
Hockomock Swamp, Easton, MA		Indian Brook cattail marsh, S. Natick, MA

Bogs and poor acidic fens  are peatlands with low species diversity and similar plant communities. The Sphagnum mat of a level bog -- while not raised above the bog basin -- can accumulate sufficient peat to isolate the bog surface and its vegetation from the original source of groundwater and minerals.  In contrast, level bogs and acidic fens bear little resemblance to most minerotrophic fens which tend to be marshy, nutrient-rich, and support vastly different and more diverse plant communities. (See Table 2. to compare characteristics of bogs and fens.) 

Processes of Peatland Formation

The majority of acidic peatlands and level bogs in southern New England developed in depressions or ponds of glacial origin. Pleicestone glaciers, melting 10,000-13,000 years ago, created the stony soil and hummocky topography that is typical of New England. Many types of wetlands -- including ponds, marshes, and fens -- formed in the poorly-drained depressions and hollows that dot the landscape. Kettleholes and kettle ponds are among the best-known features of the glaciated Northeast. In the wake of the retreating Pleicestone glacier 10,000-13,000 years ago, huge chunks of ice were buried in sand and gravel. When the ice blocks melted, they left steep-sided basins in the glacial till, many of which filled with water to become kettle ponds. The sluggish, deep waters of some kettle ponds provide nutrient-deficient, acidic, anoxic (oxygen deficient) conditions that few species other than Sphagnum mosses and shrubs like leatherleaf tolerate. A floating mat of Sphagnum on a framework of leatherleaf takes hold at the margins of these ponds. The mat expands both vertically and horizontally into the water as peat accumulates to create quaking Sphagnum bogs.

Chain of kettlehole peatlands (Town of Yarmouth GIS)

Water-saturated conditions help retard decomposition so that peat can build up. External, abiotic factors such as cool climate and humidity keep precipitation high and evapotranspiration low to maintain a water-logged environment. In turn, the formation of peat layers restricts the flow of water. With little or no inflow, bog waters rapidly lose oxygen. Oxygen-deficient waters cannot support aerobic microbial decomposers; therefore, organic plant matter decays very slowly. With little or no water outflow, organic matter is not flushed out of the system and, instead, accrues as peat. Because productivity is generally low in peatland ecosystems, decompostion must be lower than primary production and slower than peat accumulation in order for peatlands to develop. Other physical factors that can accelerate or decelerate decompostion of Sphagnum and other plant remains include: the type and amount of nutrients entering and leaving the bog, water chemistry, and hydrology. Internal, biotic factors mediated by Sphagnum mosses create physical and chemical conditions necessary for peat formation and preservation.

Two virtually opposite processes are responsible for peatland development: terrestrialization (lake-fill) or paludification (swamping). Terrestrialization transforms aquatic ecosystems into peat-based wetlands by the filling in of ponds or depressional basins. Paludification converts terrestrial ecosystems, such as meadows or forests, into peat-based wetlands when these uplands become permanently swamped, thus creating water-saturated conditions for peat development. 

Paludification

Paludified peatlands can encroach upon uplands laterally, downslope, or even upslope. Laterally, Sphagnum peat mats in lake-fill bogs can spill over basin boundaries. Similarly from basin bogs at the bottom of a slope or valley or slope, Sphagnum mosses can climb up slopes as steep as 18º to 25º. Peatlands grow upslope because water-saturated Sphagnum raises water table levels as it continues to grow and deposit layers of peat. Downslope paludification occurs from continuous water seepage. Constantly soggy conditions enable peatlands to develop directly on fairly mineral as well as acidic substrates.

Paludification occurs on both regional and local scales. Regional paludification occurs in northern latitudes with hyperhumid, climatic conditions, where it is the more significant process for the establishment and preservation of thousands of acres of blanket peatlands. Development of these paludified peatlands is associated with post-glacial rises in sea level and climatic changes that favor cool, moist regimes. In the centuries following the last glaciation, peat has enveloped vast, topographical landscapes, forming the spectacular blanket bogs of Ireland, Northern Europe, Canada, and the smaller patterned peatlands of the Great Lakes region of the United States.

Small-scale paludification occurs when a local water table rises due to drainage changes in a watershed, clearing of forests, or damming of streams by people or beavers. In southern New England, where the climate is neither cool nor moist enough to sustain large-scale paludification, swamping occurs only on a local scale Ñ usually due to the work of beavers. The process of paludification in southern New England has not been well studied; peatland research has focused primarily on kettle-hole and pond margin bogs created through terrestrialization.

Terrestrialization

Just as variation in floristics exists among lake-fill peatlands, formation and successional patterns through terrestrialization also vary due to environmental (external) and biotic (internal) factors.

Figure 4: Peatland formation through hydrarch sucession. Red arrows indicate a "bottom-up" or marsh sequence, from the bottom of the basin upwards. Blue arrows indicate the bog sequence in nutrient-rich lakes and ponds, where a "top down" process of mat formation occurs after an initial deposition of sediments on the basin bottom. Green arrows indicate the "top Ñdown" bog sequence for mat formation in kettlehole ponds and other acidic, nutrient-poor ponds. (From In Search of Swampland, Ralph Tiner.Copyright 1998. This material is used by permission of Ralph Tiner.)

In general, peatland formation through terrestrialization can be initiated by:

• a top-down process through the formation of a floating mat of peat that encroaches horizontally upon the pond's surface and vertically toward the basin:
• in deep, nutrient-poor, acidic kettleholes and ponds
• in nutrient-rich or neutral-to-alkaline lakes and ponds (with a bottom-up beginning)
• a bottom-up process when organic sediments sink to the bottom of the pond and gradually accumulate, filling the basin.

Peat mat formation in nutrient-rich or neutral-to-alkaline lakes and ponds

Progression from open water to Sphagnum peat appears to follow different patterns of hydrarch succession in shallow, eutrophic ponds and lakes than in deep, oligotrophic kettlehole ponds. Terrestrialization of shallow lakes and ponds represents the classical model of lake-fill in which several stages of sedimentation and peat deposition by pioneer species occur before Sphagnum gains a stronghold on the mat.

When conditions for bog formation are suitable, terrestrialization of shallow ponds and lakes follows a fen-to-bog succession which results in several stratified layers of peat. Early in the lake's existence, particulate minerals and organic debris settle out of the water column and form layers of lake mud and sedimentary peat, respectively, at the bottom of the pond. Accumulation of organic sediments creates a false bottom and shallower waters at lake edges.

Peat mat formation in deep, nutrient-poor, acidic kettlehole ponds

Development of a quaking mat follows a different pattern in deep kettle ponds due to their mineral-deficient, acidic waters and steeply-sloped sides. Supporting fewer aquatic microorganisms and macrophytes than nutrient-rich lakes, kettle ponds do not build up false bottoms of mud and sedimentary peat. Consequently, sedges are unable to invade deeper shoreline margins and form a bog mat.

In kettlehole ponds, leatherleaf is both the principal pioneer of the bog mat and the floating matrix on which sedges, Sphagnum mosses, and shrubs become established. Leatherleaf spreads by means of an adventitious root system that produces the stable framework for the floating mat on which sedimentbuildup.jpg Sphagnum accumulates.

As the leatherleaf framework of roots and branches grows increasingly dense, Sphagnum colonizes the mat. Sphagnum peat and partially or undecomposed stems and roots of leatherleaf continuously enlarge the mat. Over time, the floating bog mat deepens vertically and spreads horizontally over the pond's surface.

In Southern New England, swamp loosestrife (Decodon verticillatus) helps to create and extend the floating mat along with leatherleaf. Swamp loosestrife is a woody herb that grows at the pond's edge, its branches arching over into the water.

When the stems touch the water, their tips develop roots.

Swamp loosestrife produces a floating network of branches that traps organic sediments and supports leatherleaf at the leading edge horizontal spreading of of the bog.

In both kettlehole ponds and shallow lakes, the floating mat stabilizes and encroaches farther into the pond as it accumulates peat beneath the vegetative surface. Debris falling from the mat creates peat layers in the pond basin below.

In older peat deposits, the mat becomes grounded and compacted where the bottom and floating peat layers join together. The grounded mat can support larger shrubs and even trees. Concentric rings or distinct vegetation zones are typical of many kettlehole bogs.

Peatlands that develop from a marsh

Peatlands can develop when a lake basin fills with sediments and peat from the bottom up, forming a marsh that will eventually impede water flow and accumulate peat. Tiner (1998) terms lake-to-marsh-to-peatland succession "marsh sequence"; Moore and Bellamy (1974) refer to it as "flow-through" succession, a term many wetland scientists have adopted.

Bog Succession

The classical definition of hydrarch succession is the transformation of an ecosystem from an open lake or pond to vegetated wetlands to a forested upland. According to the classical theory, succession occurs gradually through a linear, sequential change in plant communities directed by the vegetation itself. Succession begins with an aquatic ecosystem, such as a lake or pond, and culminates in a terrestrial forest, the stable climax community. Traditionally, peatlands and other wetlands were deemed only transitional stages of hydrarch succession; sooner or later, peat bogs would be obliterated by the invasion of upland shrubs and trees, culminating in black spruce, northern white cedar, or other hardwood forests.

Today, the idea that peatlands are replaced by upland climax communities is little more than myth. Aquatic systems that develop into wetland ecosystems remain wetlands; hydrarch succession is the replacement of one type of wetland ecosystem with another. For example, a fen may be replaced with a dwarf shrub bog that may subsequently be succeeded by a black spruce swamp. Wetland ecosystems do not become terrestrial ecosystems without major disturbances or alterations in hydrological regime (natural or caused by people) that lower the water table. While some examples of classical lake-to-wetland-to-forest succession exist, it is not clear that succession is due to internal, vegetative controls rather than to external, environmental factors.

Bogs and conifer swamps are usually their own climax communities. Particularly in northern and subarctic regions, many peatland ecosystems have remained intact for up to 5000-8000 years. Stratigraphic data of several peatlands in Maine indicate that organic deposition began between 9400 and 10,000 years ago after the last glaciers retreated. Cores of Caribou Bog reveal that its Sphagnum -dominated peatland has existed for the past 5500 years and is underlain by more than 20 feet of peat. 

Peatlands as Dynamically Stable Ecosystems

Figure 5: Hydrarch sucession,a sequence of wetlands developing from the gradual filling-in of lakes. (From In Search of Swampland, Ralph Tiner.Copyright 1998. This material is used by permission of Ralph Tiner.)

One cannot simply look at a bog, even one of the level peatlands of southern New England, and know exactly how it formed. Surface vegetation is a poor indicator of bog formation and succession because it reflects current conditions only. The actual history of a bog lies well below the living, surface vegetation in its layers of accumulated peat. In order to know a bog's pattern of development, it is necessary to core the peat strata. To analyze the past, wetland scientists sample core sequences of the peat layers underlying the bog surface. They study the physical properties of the peat, its vegetative make-up and degree of decomposition, and look for fossils preserved in the peat. Studying stratigraphy can reveal (1) the historical development of a bog, (2) shifts in the distribution of plant species due to climatic or chemical changes and disturbances, and (3) previous cycles of terrestrialization and paludification. Stratigraphy affords a paleoecological record of the peatland itself and the surrounding environs, as it is possible to identify pollen grains, fungal spores and other fossils that were blown into the bog and preserved by the peat.

Most stratigraphical studies of peatlands in the United States have focused on the development of large, northern peatlands; little research has been carried out on quaking bogs and acidic fens of Southern New England. Nevertheless, stratigraphical sequencing has produced evidence that the process of peatland formation follows several pathways of terrestrialization and paludification and varies from bog to bog. Tallis (1983) cored peat in 36 forested and open Sphagnum bogs in New England, the Great Lakes region and the Pacific coast to try to find a pattern of peatland development and concluded that "in North America, as in the British Isles, there is no single preferred course of [peatland] development. Development is again almost exclusively progressive and involves both terrestrialization and paludification." The most common stratigraphical sequence that Tallis found was lake mud to sedimentary peat to sedge peat to forest peat to Sphagnum peat, which roughly corresponds to the developmental sequence : lake > marsh/fen > forested swamp/bog > open bog. 

Davis and Anderson (1991) (figure 6) cored 107 peat samples at 15 eccentric raised bog complexes in Maine with similar results. They found several pathways to peatland formation that had originated either in open lake water or on mineral substrates, leading them to conclude that both terrestrialization and paludification played a role in forming the eccentric bogs of Maine. One particular terrestrialization sequence was more prevalent than others. Comparing their work with that of Tallis, they reported: "we find that the main hydrosere in the eccentric bogs, namely lake to open fen and semi-wooded fen to open bog and semi-wooded bog, is typical for North America."  Davis and Anderson's (1991) study of eccentric raised bogs, clearly shows dynamic patterns of peatland succession - that succession is cyclical as well as linear. For example, although the thickness of the arrows indicates that the most common developmental sequence to be open fen > open-wooded fen > wooded fen, sometimes development is reversed. Neither is succession from open bog to open-wooded bog a one-way process. Many types of disturbance such as fire, disease, or major hydrological change can create cyclical patterns, reversals, or interruptions in peat development. A bog that appears to follow a linear pattern of succession spatially actually may have passed through several cycles of succession temporally since postglacial times.For example, if wildfire or disease destroy the vegetation of the tall shrub and conifer zones on a grounded bog mat, the floral composition of the mat may revert to sedge or Sphagnum peat, beginning the succession sequence anew. Likewise, paludification of uplands reverses hydrarch succession, creating wetlands from a "climax" terrestrial ecosystem. Upland trees and plants die when their roots are submerged by the rising water table, which produces ideal conditions for peat development and accumulation.

Figure 6. Summary developmental sequences for Maine's eccentric bog complexes.  (Davis and Anderson. 1991. The Eccentric Bogs of Maine)

Stratigraphy has helped scientists to debunk the myth of linear hydrarch succession in which peatlands were considered to be merely transitional communities. Coring peatlands has revealed that: 

Peatland development is multidirectional. Successional patterns include: the open- water-to-peatland progression of terrestrialization, the reverse upland-to-wetland process of paludification, and sequences that appear random or skip one or more developmental stages altogether.  Many of the acidic, poor fens, level bogs, and true raised bogs in the northeastern United Sates appear to be stable, climax communities.  Other peatlands are dynamic; they culminate in neither an open nor a forested bog, but have undergone several successive cycles of terrestrialization and paludification.

 

No two peatlands are exactly alike despite the similarity of bog flora throughout the range of North American peatlands. From one bog to another, vegetation may differ due to latitude, climate, and water or nutrient regime. Locally, composition of plant communities is influenced primarily by water chemistry and water table fluctuations within the bog. Neither vegetation pattern nor bog succession is restricted to a single model; variation from bog to bog is the norm due to complex interactions of external environmental factors coupled with internal changes brought about by the vegetation itself.

Bibliography

Aerts, R., J. T. A. Verhoeven, and D. F. Whigham. 1999. Plant-mediated controls on nutrient cycling in temperate fens and bogs.  Ecology 80:2170-81.

Anderson, D. S. and R. B. Davis. 1997. The vegetation and its environments in Maine peatlands. Can. J. Bot. 75: 1785-1805.

Anderson, D. S. and R. B. Davis. 1998. The flora and plant communities of Maine peatlands. Technical Bulletin 170.  Maine Agricultural and Forest Experiment Station, University of Maine, Orono, ME.

Andrus, R. E. 1986. Some aspects of Sphagnum ecology. Can. J. Bot. 64:416-426.

Bedford, B., M. R. Walbridge and A. Aldous. 1999. Patterns in nutrient availability and plant diversity of temperate North American wetlands. Ecology 80:2151-2169.

Bridgham, S. D., J. Pastor, J. A. Janssens, C. Chapin, and T. J. Malterer. 1996. Multiple limiting gradients in peatlands: a call for a new paradigm. Wetlands 16:45-65.

Crum, H. 1988. A Focus on Peatlands and Peat Mosses. The University of Michigan Press, Ann Arbor, MI.

Damman , A. W. H. and T. W. French. 1987. The ecology of peat bogs of the glaciated Northeastern United States: a community profile. U.S. Fish Wildl. Serv. Biol. Rep. 85(7.16).  Washington, D.C.  100 pp.

Damman, A. W. H. 1979. Geographic patterns in peatland development in Eastern North America, in Proceedings of the International Symposium of Classification of Peat and Peatlands, Finland, September 17-21, 1979. International Peat Society.

Dansereau, P. and F. Segadas-Vianna. 1952. Ecological study of the peat bogs of Eastern North America. Can. J. Bot. 30: 490-520.

Davis, R. B. and D. S. Anderson. 1991. The eccentric bogs of Maine: a rare wetland type in the United States. Technical Bulletin 146.  Maine Agricultural and Forest Experiment Station, University of Maine, Orono, ME.

Davis, R. B. and D. S. Anderson. 2001. Classification and distribution of freshwater peatlands in Maine. Northeastern Naturalist 8:1-50.

Davis, R. B. and D. S. Anderson. 2001. Seminar: environments, vegetation, and development of peatlands. Humboldt Field Research Station, Steuben, ME. Personal communication.

Dunlop, D. A. 1987. Community classification of the vascular vegetation of a New Hampshire peatland. Rhodora 89: 415- 441.

Gates, F. C. 1942. The bogs of northern lower Michigan. Ecol. Monographs 12:215-254.
 

Glaser, P.H. and J.A. Janssens. 1986. Raised bogs in eastern North America: transitions in landforms and stratigraphy. Can. J. Bot. 64:395-415.

Glaser, P.H. 1992. Raised bogs in eastern North America -- regional controls for species richness and floristic assemblages. Journal of Ecology 80:535-554.

Gore, A. J. P. Introduction, in Ecosystems of the World: Mires: Swamp, Bog, Fen, and Moor, Vol. 4A (A.J.P. Gore, ed.). Elsevier, Amsterdam, Netherlands, pp. 1-30.

Heinselman, M.L. 1963. Forest sites, bog processes, and peatland types in the Glacial Lake Agassiz reion, Minnesota. Ecol. Monographs 33: 327-375.

Heinselman, M.L. 1970. Landscape evolution, peatland types, and the environment in the Lake Agassiz Peatlands Natural Area, Minnesota. Ecol. Monographs 40: 235-262.

Hemond, H. F. 1980. Biogeochemistry of Thoreau's Bog, Concord, Massachusetts. Ecol. Monographs 50:507-526.

Hoffstetter, R.H. 1983. Wetlands in the United States, in  Ecosystems of the World: Mires: Swamp, Bog, Fen, and Moor, Vol. 4B (A.J.P. Gore, ed.). Elsevier, Amsterdam, Netherlands, pp. 201-244.

Johnson, C. W. 1985.  Bogs of the Northeast.  University Press of New England,     Hanover, NH.

Jones, C. G., J. H. Lawton and M. Shachak. 1994. Organisms as ecosystems engineers.          Oikos 69:373-386.

Kearsley, J. 1999. Non-forested acidic peatlands of Massachusetts. Natural Heritage and Endangered Species Program, Masachusetts Division of Fisheries and Wildlife, Westborough, MA.

Kratz, T. K. and C. B. DeWitt. 1986. Internal factors controlling peatland-lake ecosystem development. Ecology 67(1): 100-107.

Kuhry, P., B.J. Nicholson, L. D. Gignac, D. H. Vitt, and S. E. Bayley. 1993. Development of Sphagnum-dominated peatlands in boreal continental Canada. Can. J. Bot. 71: 10-22.

Larsen, J. A.  1982.  Ecology of the Northern Lowland Bogs and Conifer Forests.   Academic Press, New York.

Metzler, K. J. and R. W. Tiner. 1992. Wetlands of Connecticut. State Geological and Natural History Survey of Connecticut, Department of Environmental Protection, Hartford CT.  In cooperation with the U.S. Fish and Wildlife Service, National Wetlands Inventory. Report of Investigations No. 13.

Mitsch, W. J. and J. G. Gosselink. 2000. Wetlands (third edition). John Wiley & Sons, Inc., New York.

Moore, P. D. 1991.  Ups and downs in peatland. Nature 353:299-300.

Moore, P. D. and D. J. Bellamy. 1974. Peatlands. Springer-Verlag, New York.

Motzkin, G. H. and W. A. Patterson III. 1991. Vegetation patterns and basin morphometry of a New England moat bog. Rhodora 93:307-321.

Nichols, G. E. 1915. The vegetation of Connecticut. Bull. of Torr. Bot. Club. 42:169-217.

Niering, W. A. 1991. Wetlands of North America. Thomasson-Grant, Charlottesville, VA.

Pielou, E. C. 1991.  After the Ice Age: the Return of Life to Glaciated North America. University of Chicago Press, Chicago, IL.

Rigg, G. B. 1940. The development of sphagnum bogs in North America. The Botanical Review 6:666-693.

Schwintzer, C.R. and G. Williams. 1974. Vegetation changes in a small Michigan bog from 1917 to 1972. American Midland Naturalist 92:447-459.

Schwintzer, C. R. 1981. Vegetation and nutrient status of northern Michigan bogs and conifer swamps with a comparison to fens. Can. J. Bot. 59: 842-853.

Searcy K. B. and M. G. Hickler. 1999. The plant communities and vascular flora of the peatland within Poutwater Pond nature preserve. Rhodora 101:341-359.

Sperduto, D. D., W. F. Nichols, and N. Cleavitt. 2000. Bogs and Fens of New Hampshire. New Hampshire Natural Heritage Inventory ­ DRED Division of Forests and Lands, Concord , New Hampshire and The Nature Conservancy.

Swain, P. C. and J. B. Kearsley. 2000. Classification of the Natural Communities of Massachusetts. Natural Heritage and Endangered Species Program, Massachusetts Division of Fisheries and Wildlife, Westborough, MA.  Draft version.

Swan, J.M.A. and A.M. Gill. 1970. The origins, spread and consolidation of a floating bog in Harvard Pond, Petersham, Massachusetts. Ecology 51: 829-39.

Tallis, J. H. 1983. Changes in wetland communities, in Ecosystems of the World: Mires: Swamp, Bog, Fen, and Moor, Vol. 4A (A.J.P. Gore, ed.)  Elsevier, Amsterdam, Netherlands, pp. 311-344.

Tiner, R. W. 1994. Maine Wetlands and Their Boundaries.  State of Maine, Department of Economic and Community Development, Office of Community Development, Augusta, ME.

Tiner, R. W. 1998. In Search of Swampland: a Wetland Sourcebook and Field Guide.   Rutgers University Press, New Brunswick, NJ.

Thompson, E. 1996. Natural Communities of Vermont: Uplands and Wetlands. Vermont Nongame and Natural Heritage Program, Department of Fish and Wildlife, Agency of Natural Resources. Waterbury, VT and The Nature Conservancy, VT Chapter.

Thompson, E. H. and E. R. Sorenson. 2000. Wetland, Woodland, Wildland: a Guide to the Natural Communities of Vermont. The Nature Conservancy and the Vermont Dept. of Fish and Wildlife. University of New England Press, Hanover, NH.

Transeau, E. N. 1903. On the geographical distribution and ecological relations of the bog plant societies of northern North America. Botanical Gazette 36:401-420.

van Breemen, N. 1995. How Sphagnum bogs down other plants. TREE 10:270-275.

Vitt, D. H. and P. Kuhry. 1992. Changes in moss-dominated wetland ecosystems. In Bryophytes and Lichens in a Changing Environment, J.W. Bates and A.M. Farmer, eds.). Clarendon Press, Oxford, UK.

Vitt, D. H. and N.G. Slack. 1975. An analysis of the vegetation of Sphagnum-dominated kettle-hole bogs in relation to environmental gradients. Can. J. Bot. 53: 332-359.