This class includes bog vegetation frequent in the boreal and subarctic zones of Eurasia and North America, but it can also occur as azonal vegetation on suitable sites elsewhere in the world. It occupies sites saturated with water in areas where precipitation exceeds evaporation. Bryophytes (mostly Sphagnum species) adapted to permanent saturation and extremely low nutrient and mineral content dominate, and their remains are the main component of peat. Some of them form tall hummocks, creating a typical hollow-hummock microtopography on the bog surface. Bog water has a very low pH because of humic acids releasing from organic matter and active acidification by Sphagnum species. Only low amounts of essential nutrients remain for vascular plants, represented by very few species, such as some species of Cyperaceae, dwarf evergreen shrubs and carnivorous Drosera species. Grasses and broad-leaved dicot herbs are nearly absent. Bogs with a well-developed tree layer occur in continental areas, where water table strongly declines in dry summers. In central Europe, several species of Pinus dominate vegetation mostly in bog margins with fluctuating water level. Generally, bogs can develop through autogenic succession from mineral-rich fens due to organic matter accumulation, or paludification due to saturation of mineral soil with water in precipitation-rich areas. The class Oxycocco-Sphagnetea does not include vegetation of bog hollows, which is assigned to the class Scheuchzerio palustris-Caricetea nigrae.