How does a bog form?
Bogs (mires) form through two different methods:
- Paludification, or
- Terrestrialization
Terrestrialization accounts for approximately 1/3 of all bog formations. (Alumäe, H, pers. com., 19/9/00) It is simply the closing up of a water body, which is when a small pond or lake begins to be filled with sediments. Plant life and animals die in the water and settle on the bottom of the pond. Also, humans can impact the sediment levels of the water (by physically dumping sediment into the water or through secondary influence by encouraging the plant life). Eventually, after the sediments have built up to sufficient levels, the former pond becomes a plot of swampy soil. This cannot happen where there is a current to remove the soil, so ponds or lakes that are fed by rivers have little chance of becoming swamps/mires/bogs. An Estonian book on the subject has an excellent diagram which shows how terrestrialization works in greater detail.
Paludification, which accounts for the remaining 2/3 of mires, is the second process by which bogs are formed. This is basically when the ground becomes over saturated and the conditions for bog formation become present (Masing 16). This can come from several different sources, man-made and natural. Beaver dams can cause surrounding areas to become over saturated, allowing sphagnum moss to grow and begin to form peat. Man-made dams (actual dams, roads, improperly drained construction) can also cause the same effect. The process can also be entirely natural, without any animal or human influence. However, the natural process is mainly related to physical geography and occurs around springs usually.
Regardless of which process they go through, mires must pass several states before becoming a bog (Alumäe, H, pers. com., 19/9/00). The first state is that of a swamp. A swamp has less water than a bog, but it usually has more water on the surface. Next is the transitional bog. As one might expect, this falls somewhere between swamps and bogs in terms of physical characteristics. Then comes the final stage of a mire, the bog. Bogs have a much higher water content than swamps, but the water can hidden underground like a giant sponge.
One of the major features of a bog is peat. Peat is the built up sediment (mainly dead sphagnum moss) that composes the base of the bog. Most bogs in Estonia have a peat layer of around 6 m, but the highest bog in Estonia has 17 m of peat (Masing 34). Because of the way that peat builds up, bogs are usually taller than the surrounding landscapes. This can be seen in the above mentioned diagram. The peat is so thick that the plant life cannot reach ground water. As bogs build up higher and higher (about 1mm of peat per year), the small ponds begin to stagnate due to their lack of contact with ground water. The only fresh water that reaches the ponds and plants comes from rainfall. All of these factors lead to the limited amount of plant diversity.