with Examples from Estonia
|Introduction||Phases of development|
|Significance of peat||Bogs and mires of Estonia|
|Endla nature reserve||Related sites & references|
|Rank weeds and lush, slimy water plants send an odour of decay and a heavy miasmatic vapor into our faces, while a false step plunged us more than once thigh-deep into the dark, quivering mire, which shook for yards in soft undulations around our feet. The Great Grimpen Mire, as described in "The hound of the Baskervilles" by Sir Arthur Conan Doyle.|
|Linnusaare (bird island) bog, part of the Endla mire complex in east-central Estonia. A sparse pine forest grows on peat hummocks. Photo date 9/00, digital photograph © Jeremy Aber.|
|Linnusaare bog. Pine is the only tree that can tolerate the acid water and lack of nutrients that are typical in mature bogs. Photo date 9/00, digital photograph © Jeremy Aber.|
Wetlands in which peat has accumulated are widespread in formerly glaciated regions of northern Eurasia, the northern United States, and Canada. In regions with abundant precipitation, little evaporation, and slight runoff, various kinds of swamps, marshes, fens, and bogs may form. All these English words are united under the general term mire nowadays. Equivalent words in other European languages include mose (Danish), Moor (German), suo (Finnish), soo and raba (Estonian), and boloto (Russian). At least 19 words refer to different kinds of mires in dialects of northern Finland (Aapala and Aapala 1997). Some of these words were adopted from Sami language, including the terms tundra, aapa, and palsa, which are used internationally in ecological research nowadays. Mires can form in two general ways.
|Phases in mire development|
In the early stage of development, the mire occupies a topographic depression and is well supplied with nutrient-rich surface or ground water. This phase may take the form of a treeless fen or forested swamp. Fens and forested swamps often have a hummocky surface, due to variable growth of ground vegetation. Plant growth is greater and decomposition less on hummocks, which gradually gain in height above the saturated zone. At this stage, peat moss (Sphagnum) can colonize the hummock. As peat accumulates above water, the hummocks continue to increase in height (phase I below).
|Observation tower under construction on the margin of Valgesoo (White mire), a small peat bog southeast of Tartu, Estonia. Photo date 10/2000; © J.S. Aber|
|View from observation tower over Valgesoo. The bog is surrounded by birch and spurce forest. The central portion of the bog (middle distance) has a dwarf pine forest. Valgesoo is in an early stage of development and is just starting to become convex (raised in the center). Photo date 10/2000; © J.S. Aber|
|Central portion of Valgesoo showing small pine trees, moss, and grass cover. Groundwater is only 20-50 cm deep (Aaviksoo et al. 1997). Photo date 10/2000; © J.S. Aber|
Continued accumulation of peat causes the hummocks to merge together until an open bog results. This bog takes the form of a plateau or low mound. The raised bog has grown to a level above the surrounding terrain (phase II below). The bog is sufficiently high to prevent surface runoff or ground water from reaching the peat surface. Mineral soils beneath the thick peat are so deep that plant roots cannot reach this source of nutrition. In this manner, the bog becomes deprived of nutrients necessary for most plants to survive. Furthermore, metabolism of peat moss releases acid. Bog water is yellow-brown in color and has a range of pH 3-4, which creates a situation quite unfavorable for most other plants. Only peat moss, dwarf shrubs and stunted pine can grow under these conditions; the bog surface resembles a tundra landscape. Rapid peat accumulation begins to take place.
|Five main stages in the development of a mire.|
I. Centrally thinned wooded bog or open fen, surrounded by a bog pine forest.
II. Centrally bare (treeless) raised bog with beginnings of bog hollows.
III. Concentric rings consisting of hollows and hummock ridges, standing water forms pools in hollows, marginal pine forest.
IV. Narrow bog pools surrounding irregular hollows and small lakes in the center.
V. Outlet stream divides bog in two (or more) lobes that continue to develop into a complex mire.
Schematic illustrations, not to scale. Adapted from Masing (1997, p. 45).
Raised bogs are unique in their isolation from the surrounding environment. The bog surface is separated from underlying ground water and mineral soil, and is inaccessible to runoff from the adjacent land. The only source of moisture is from direct precipitation, and the nutrients come from dust and other wind-blown sources--bird droppings, spiders drifting on invisible threads, leaves from nearby forests, etc. In this self-contained mode, raised bogs are relatively autonomous, stable and long-lasting elements of the landscape (Aaviksoo et al. 1997).
|View across Meenikunno, a large bog southeast of Tartu, Estonia. Meenikunno is a mature, raised bog that encompasses several lakes and mineral islands. The center of the bog is a treeless plateau (light yellow zone in middle distance). Photo date 10/2000; © J.S. Aber|
|Dwarf pine trees, grass, and moss cover in the southern portion of Meenikunno bog. Notice the wooden board path to right. Photo date 10/2000; © J.S. Aber|
Peat moss is a paradox of sorts. Moss is well known for holding a great deal of water in the body of the plant as well as in the openings between plant stems. Each moss species has a preferred growth position in relation to the saturated zone. Sphagnum cuspidatum grows as floating mats on pools or just at water level around pools. Most other mosses, such as S. fuscum, rubellum, and magellanicum, grow in slightly higher and drier positions on peat hummocks above the saturated zone.
|Vertical kite aerial photograph over central portion of Männikjärve Bog, east-central Estonia. This mature bog exhibits striking color zonation of moss and other vegetation in this autumn view. A = Sphagnum cuspidatum floating in water, B = S. cuspidatum around pool shore, C = S. rubellum above water, D = pine trees on hummocks along with dwarf shrubs. The board walk is 2 feet (60 cm) wide. Digital image date 9/01 (Aber et al. 2002, fig. 2).|
A mature raised bog begins to show its age when the surface develops a wrinkled texture of hollows and hummock ridges, which form a characteristic concentric pattern (phase III above). The hollows may contain pools of standing water that are "perched" at levels above ground water and surface drainage in the surrounding terrain. Presence of water in the pools inhibits growth of peat, and the pools gradually expand, deepen and merge into small lakes through time (phase IV above). Eventually water from the lakes may erode outlet channels that drain to the surrounding, lower terrain. The original bog is split by the outlet streams; this process leads to mire complexes with multiple bogs, internal lakes, and intervening streams (phase V above). The pattern of a mature bog can be quite complicated, depending on its raised surface, presence of mineral "islands," and distribution of pools, lakes, and streams (map below).
|Sketch map of the Nigula mire complex in southwestern Estonia. Notice the pattern of pools and hollows. Mineral islands support forests surrounded by various bog types. The wooden path (red line) is 6.8 km (4¼ miles) long. Taken from Nigula Nature Reserve, Estonia.|
|Vertical kite aerial photograph over eastern portion of Nigula Bog. Sphagnum cuspidatum forms a silvery green "mat" floating in parts of some pools. The reddish-brown zones include S. magellanicum and S. rubellum. Heather covers much of the mottled green surface along with a few small pines (note shadows). A portion of the boardwalk (~40 cm wide) is visible in lower left corner. Digital image date 9/01 (Aber et al. 2002, fig. 3).|
|Oblique kite aerial photograph over Salupeaksi, a tree-covered "mineral" island in the middle of Nigula Bog. Notice the marked forest zones of the island. A = pine, B = birch (partly bare), C = ash, elm, maple and other deciduous hardwoods, some of which display fall colors. Photo date 9/01 (Aber et al. 2002, fig. 4).|
In the late 19th century, Axel Blytt (Norway) and Rutger Sernander (Sweden) developed a Holocene sequence of vegetation zones based on macrofossil remains in peat--wood, leaves, seeds, bark, etc. This sequence forms the basis for Holocene stratigraphy in northern Europe. Early in the 20th century, this scheme was expanded by Lennart von Post (Sweden) with the discovery that pollen is preserved in peat. Wind-blown pollen represents most of the tree and grass species in the adjacent region, and so gives a picture of vegetation conditions surrounding the peat bog. Hundreds of plant genera and species can be identified from pollen, which allows for accurate reconstructions of vegetation and environmental conditions throughout the late Pleistocene and Holocene (last 12,000 years). Peat bogs can, thus, be regarded as archives that contain huge amounts of coded information about past environments and climates.
|Bogs and mires of Estonia|
Estonia is a country rich in wetlands. Located at the far eastern end of the Baltic Sea, Estonia is bounded by Russia to the east, Finland to the north, and Latvia to the south. More than one-fifth of the territory is covered by swamps, marshes, fens and bogs (Orru et al. 1993). More than forty of these mires and bogs are protected as national parks, nature preserves, or mire conservation areas. The mires of Estonia can be divided in two general types (Ilomets 1997).
Taken from Estonia in the Baltics.
|Baltic Sea coast on the Island of Vormsi, northwestern Estonia. This region was submerged beneath early phases of the Baltic Sea in the early Holocene. As the land rebounded following deglaciation, the island emerged a few meters above sea level. A spruce forest covers most of the land area in this view. Kite aerial photograph, Aug. 2000.|
|Teosaare Bog (right foreground) in the Pandivere Upland of east-central Estonia. Part of the Endla mire complex with numerous raised bogs, swamps, and small lakes. Kite aerial photograph, Sept. 2001.|
The development of Estonian mires began soon after the land was deglaciated and emerged from the sea; however, the rate of mire formation was slow in the early Holocene. At least 5000 years were required for infilling of lakes and uplift of the land after retreat of the last ice sheet some 12,000 years ago (Ilomets 1997). The major period of mire initialization was in the middle Holocene, from about 7000 to 4000 years ago. As peat accumulated, raised bogs began to appear with a peak in their formation during the interval 4000 to 2000 years ago. Peat accumulates at rates of 1 to 1½ mm per year. Given their mid-Holocene age, most bogs in Estonia have peat deposits 5 to 7 meters deep, and maximum known peat thickness is 17 m (Masing 1997).
Human activities have influenced bog development during the past few centuries. Many bogs were drained to promote forest growth or for mining of their peat resources. Following the forcible annexation of Estonia by the Soviet Union in World War II, bog management came under centralized control from Leningrad (now St. Petersburg, Russia). Major emphasis was placed on draining bogs and extracting peat for economical purposes. Furthermore peat bogs in northeastern Estonia were heavily impacted by deposition of fly ash from nearby power plants (fueled by oil shale).
|Clearcut for experimental agriculture in forest at Teosaare Bog; artificial drainage for improved forestry in left background; Endla Lake in right background. Part of the Endla mire complex in east-central Estonia. Kite aerial photograph, Sept. 2001.|
|Peat mining at Rannu Soo, west of Tartu, Estonia. Drainage ditches remove water from the peat, which is then "milled" into ridges for drying. The peat is used for fuel in a nearby power plant--see smokestack in right background. Photo date 8/2000; © J.S. Aber|
|Rannu Soo peat mine. A pile of milled and dried peat awaits loading for transport to a nearby power plant. Photo date 8/2000; © J.S. Aber|
In spite of this bureaucratic situation, some progress was made on bog research and conservation. The Ramsar Treaty, signed in 1971, led to protected status for several essential wetlands. In 1981, several mire protection areas were designated, which were considered most important for hydrologic aspects and for richness in berries. Additional mire reserves were established in the 1980s and '90s. On this basis, Estonia has a substantial number and total area of mires and bogs in protected status today. Peat moss is beginning to grow again in bogs of northeastern Estonia following reduction in fly ash emissions from power plants (Karofeld 1996). The mires of Estonia now are attracting considerable interest from scientists and nature lovers in western Europe, where only fragments of undisturbed bogs remain (Aaviksoo et al. 1997; Masing, 1997).
|Landscape of western Denmark exhibits evidence of extensive human modification. Ancient burial mound to right (<) and remains of an Iron Age wall and ditch to left (>). Agriculture blankets the land with wind turbines in the background. Most original forest and wetlands of Denmark were removed long ago. Kite aerial photograph, Sept. 2005.|
Endla Nature Reserve, Estonia
The present Endla Nature Reserve was created in 1985 as an expansion of the previous smaller Endla-Oostriku mire reserve. It is located immediately south of the Pandivere Upland in east-central Estonia. The Endla mire complex grew up in the depression of former Great Endla Lake (Aaviksoo et al. 1997). Several remnants of this lake still survive, notably Endla Lake and Sinijärv (Blue Lake). These lakes were subjected to several episodes of draining (1872, 1949, 1950) and were reflooded in 1968. The Endla mire complex contains seven bogs separated by narrow rivers, and several significant springs rise in the western part of the complex.
|View toward the northwest across Männikjärve bog, Endla Nature Reserve. The bog center is seen toward the left background. It's marked by elongated peat hummocks with intervening narrow water pools. The path is an elevated wooden walkway for visitors. Kite flyers are working from a small meteorologic station in lower part of view. Kite aerial photo, date 9/00 (Aber and Aber 2001).|
|Closeup, low-oblique view of the bog-forest margin. Sun glint shows small water pools in the bog, and a trail is faintly visible running toward the upper left corner of view. Kite aerial photo, date 9/00 (Aber and Aber 2001). More KAP of Männikjärve bog.|
|Observation tower on the wooden walkway in the center of Männikjärve bog, Endla Nature Reserve. The next three pictures are taken from the top of this tower. Photo date 9/00, © J.S. Aber.|
|View eastward over Männikjärve bog from top of the observation tower. Dwarf pines occupy the hummocks. The boardwalk is approx. 60 cm (2 feet) wide. Photo date 9/00, © J.S. Aber.|
|View northward over Männikjärve bog from top of the observation tower. Notice the near-circular tiny islands in the pools. Photo date 9/00, © J.S. Aber.|
|View southeastward over Männikjärve bog from top of the observation tower. Pools and islands have elliptical shapes. Photo date 9/00, © J.S. Aber.|
|Endla Lake seen through bare birch trees. This view was taken from another observation tower on the eastern edge of the lake. Endla Lake is surrounded by thick growth of reeds. Photo date 9/00, © J.S. Aber.|
|Bog panorama QuickTime movie (1.14 mb). A 360° panorama from the top of the observation tower in Männikjärve bog, Endla Nature Reserve. Starting view toward south, then progressing to east, north, and west. Movie date 9/00, © Jeremy Aber.|
Among the bogs, Männikjärve bog has been investigated intensively since the early 1900s. A small meteorological station is located in the bog. An elevated, wooden walkway allows visitors to travel across the bog without disturbing the surface and without sinking into the peat and mud. This bog has been utilized for numerous scientific observations and measurements.
|Flux chambers with tubes and battery-driven pumps to collect samples of gas emitted from the bog, Männikjärve Bog. Image courtesy of Peter Frenzel © Germany.|
|Microprofiler with syringe and needle to collect water samples, Männikjärve Bog. Image courtesy of Peter Frenzel © Germany.|
|Peat coring tool (Russian type), Männikjärve Bog. Image courtesy of Peter Frenzel © Germany.|
|Peat core containing piece of ancient wood. Männikjärve Bog. Image courtsey of Peter Frenzel © Germany.|
During the past five millennia, Männikjärve Bog has experienced several episodes of wetter conditions: 3170-2850 years ago, 2450-2000 years ago, 1770-1530 years ago, and 880 years ago to present (Sillasoo et al. 2007). The latter interval includes the Little Ice Age (A.D. 1200-1900). These wet periods correspond to colder temperatures and more continental climatic conditions of the late Holocene.
Recent findings at Männikjärve Bog suggest that bogs play a crucial role in regulating atmospheric greenhouse gases and hence have a significant influence on world climate. Bogs are net sinks for carbon--removal of CO2 from the atmosphere, and bogs are potential sources for methane (CH4), which is a strong greenhouse gas. Bog pools expand during cool/wet climatic intervals; whereas, hummocks and ridges are stable or increasing during warm/dry periods (Karofeld 1998).
Frenzel and Karofeld (2000) demonstrated at Männikjärve Bog that hummocks and ridges are sinks for methane; in contrast, hollows and pools are sites of methane emission. Gradual changes in hummock-hollow distribution, thus, affect methane flux within the bog system. A negative feedback relationship exists, such that warm/dry weather causes an increase in hummocks and ridges and thus reduction in methane emission. Reduced atmospheric methane would, in turn, lead to a decreased greenhouse effect and cooler climate. On this basis, it appears that bogs of the Baltic region have the potential to dampen changes in climate brought about by other factors.
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EB/ES/GE 341 © by J.S. Aber (2016).
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