Sphagnum is a genus of between 151 and 350 species of mosses commonly called peat moss, due to its prevalence in peat bogs and mires. A distinction is made between sphagnum moss, the live moss growing on top of a peat bog on one hand, and sphagnum peat moss (North American usage) or sphagnum peat (British usage) on the other, the latter being the decaying matter underneath. Bogs are dependent on precipitation as their main source of nutrients, thus making them a favourable habitat for sphagnum as it can retain water and air quite well. Members of this genus can hold large quantities of water inside their cells; some species can hold up to 20 times their dry weight in water, which is why peat moss is commonly sold as a soil conditioner. The empty cells help retain water in drier conditions. In wetter conditions, the spaces contain air and help the moss float for photosynthetic purposes. Sphagnum and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. An additional reason is that the bogs in which Sphagnum grows are submerged, deoxygenated, and favor slower anaerobic decay rather than aerobic microbial action. Peat moss can also acidify its surroundings by taking up cations such as calcium and magnesium and releasing hydrogen ions.
Individual peat moss plants consist of a main stem, with tightly arranged clusters of branch fascicles usually consisting of two or three spreading branches and two to four hanging branches. The top of the plant, or capitulum, has compact clusters of young branches. Along the stem are scattered leaves of various shape, named stem leaves; the shape varies according to species. The leaves consist of two kinds of cell; small, green, living cells (chlorophyllose cells), and large, clear, structural, dead cells (hyaline cells). The latter have the large water-holding capacity.
Sphagnum, like all other land plants, has an alternation of generations; like other bryophytes, it is the haploid gametophyte generation that is dominant and persistent. Sphagnum species can be unisexual (male or female, dioicous) or bisexual (male and female gametes produced from the same plant; monoicous); In North America, 80% of Sphagnum species are unisexual. Gametophytes have substantial asexual reproduction by fragmentation, producing much of the living material in Sphagnum peatlands. Swimming sperm fertilize eggs contained in archegonia that remain attached to the female gametophyte. The sporophyte is relatively short-lived, and consists entirely of a shiny black, spherical spore capsule. Sporophytes are raised on stalks to facilitate spore dispersal, but unlike other mosses, Sphagnum stalks are produced by the maternal gametophyte. Tetrahedral haploid spores are produced in the sporophyte by meiosis, which are then dispersed when the capsule ruptures.
Taxonomy and Phylogeny
Peat moss can be distinguished from other moss species by its unique branch clusters. The plant and stem color, the shape of the branch and stem leaves, and the shape of the green cells are all characteristics used to identify peat moss to species. Sphagnum taxonomy has been very contentious since the early 1900s; most species require microscopic dissection to be identified. In the field, most Sphagnum species can be identified to one of four major sections of the genus—classification and descriptions follow Andrus 2007 (Flora North America):
- Sphagnum sect. Acutifolia: plants generally form hummocks above the water line, usually colored orange or red. Examples: Sphagnum fuscum Sphagnum warnstorfii
- Sphagnum sect. Cuspidata: plants usually in hollows, lawns, or aquatic; plants green. Examples: Sphagnum cuspidatum, Sphagnum flexuosum
- Sphagnum sect. Sphagnum: largest gametophytes among the sections, forming large hummocks, leaves with cuculate (hood-shaped) apices. Green, except for Sphagnum magellanicum Examples: Sphagnum austinii
- Sphagnum sect. Subsecunda: plants various colors, from green to yellow and orange (but never red), found in hollows, lawns, or aquatic. Species always with unisexual gametophytes. Examples: Sphagnum lescurii, Sphagnum pylaesii.
The reciprocal monophyly of these sections and two other minor ones (Rigida and Squarrosa) has been resolved using molecular phylogenetics. All but two species normally identified as Sphagnum reside in one clade, two other species have recently been separated into new families within the Sphagnaceae reflecting an ancestral relationship with the Tasmanian endemic Ambuchanania and long phylogenetic distance to the rest of Sphagnum. Within main clade of Sphagnum there is relatively short phylogenetic distance, and molecular dating methods suggest nearly all current Sphagnum species are descended from a radiation that occurred just 14 mya. This is controversial[by whom?] due to the poor construction and calibration of the genetic clock. Normal and correctly used genetic clocks employ a series of genes with determined substitution rates, in this work Shaw et al. used nuclear, mitochondrial and chloroplast genes with a mean substitution rate. The problem arises when a chloroplast gene with low substitution rate is analyzed with a nuclear substitution rate (which is extremely high) it would give a more recent date since the analysis would conclude that less mutations means a more recent divergence and evolution.
Peat mosses occur mainly in the Northern Hemisphere where different species dominate the top layer of peat bogs and moist tundra areas. The northernmost populations of peat moss lie in the archipelago of Svalbard, Arctic Norway at 81° N.
In the Southern Hemisphere, the largest peat moss areas are in New Zealand, Tasmania, southernmost Chile and Argentina, but contain comparatively few species. Many species are reported from mountainous, subtropical Brazil, but uncertainty exists regarding the specific status of many of them.
As do many other mosses, Sphagnum disperses its spores through the wind, but the tops of spore capsules are only about 1 cm above ground, and the wind is too weak that low. As the spherical spore capsule dries, the operculum is forced off, followed by a cloud of spores. The exact mechanism has traditionally attributed to a "pop gun" method using air compressed in the capsule, reaching a maximum velocity of 3.6 meters per second, but alternative mechanisms have been recently proposed. High speed photography has shown that vortex rings are created during the discharge, which enable the spores to reach a height of 10 to 20 cm, further than would be expected by ballistics alone. The acceleration of the spores is about 36,000G. Spores are extremely important in establishment of new populations in disturbed habitats and on islands.
Decayed, compacted Sphagnum moss has the name of peat or peat moss. This is used as a soil conditioner which increases the soil's capacity to hold water and nutrients by increasing capillary forces and cation exchange capacity (CEC). This is often necessary when dealing with very sandy soil, or plants that need an increased moisture content to flourish. One such group of plants are the carnivorous plants, often found in wetlands like bogs. Dried Sphagnum moss is also used in northern Arctic regions as an insulating material. Peat moss is also a critical element for growing mushrooms; mycelium grows in compost with a layer of peat moss on top, through which the mushrooms come out, a process called casing.
Anaerobic acidic Sphagnum bogs are known to preserve mammalian bodies extremely well for millennia. Examples of these preserved specimens are Tollund Man, Haraldskær Woman, Clonycavan Man and Lindow Man. Such Sphagnum bogs can also preserve human hair and clothing, one of the most noteworthy examples being Egtved Girl, Denmark. Because of the acidity of peat, however, bones are dissolved rather than preserved. These bogs have also been used to preserve food. Bog butters have been found in Scottish and Irish peat bogs. Containing butter or lard, bog butters have been found that are up to 2000 years old.
Sphagnum moss has also been used for centuries as a dressing for wounds, including during both World Wars. It is absorptive and extremely acidic, inhibiting the growth of bacteria and fungi. However, see Health Dangers below.
Sphagnum moss is used as an environmentally-friendly alternative to chlorine in swimming pool sanitation. The moss inhibits the growth of microbes and reduces or eliminates the need for chlorine in swimming pools.
Peat moss is used to dispose of the clarified liquid output (effluent) from septic tanks in areas that lack the proper soil to support an ordinary disposal means or for soils that were ruined by previous improper maintenance of existing systems.
In New Zealand, both the species S. cristatum and S. subnitens are harvested by hand and exported worldwide for use as hanging basket liners, as a growing medium for young orchids, and mixed in with other potting mixes to enhance their moisture retaining value.
It is also used at horse stables as a bedding in horse stalls. It is not a very common bedding, but some farm owners choose peat moss to compost with horse manure.
There is a difference in naming conventions for similar things related to sphagnum moss. The terms that people use when referring to moss peat, peat moss, and bog moss can be taken out of context and be used when reference is actually being made about a plant that is still growing, as opposed to the decayed and compressed plant material. These terms are commonly used for both forms of the same plant material, resulting in confusion as to what the speaker is actually talking about.
Large-scale peat harvesting is not sustainable as it takes thousands of years to form the peat "bricks" that are harvested in just a week. In particular, the extraction of large quantities of moss is a threat to raised bogs. Coir has been touted as a sustainable alternative to peat moss in growing media. Another peat moss alternative is manufactured in California from sustainably harvested redwood fiber and sold under the brand name LignaPeat.
During the 17th century in the Dutch Republic, peat bogs were drained to feed a burgeoning peat mining industry. The system of dikes and waterways existing today in the Netherlands was once a peat bog.
Care is taken during the harvesting of Sphagnum moss in New Zealand to ensure that there is enough moss remaining to allow regrowth. This is commonly done using a 3 year cycle. If a good percentage of moss is not left for regrowth, the time that it takes for the swamp to revert to its original state can be up to a decade or more if serious damage has occurred.
This "farming" as done in New Zealand is based on a sustainable management program approved by New Zealand's Department of Conservation. This plan ensures the regeneration of the moss, while protecting the wildlife and the environment. Most harvesting in New Zealand swamps is done only using pitchforks without the use of heavy machinery. During transportation, helicopters are commonly employed to transfer the newly harvested moss from the swamp to the nearest road. This is an important component of the transportation process, as it prevents damage to other components of the ecosystem during the initial transportation phase. The removal of sphagnum moss in a managed environment does not cause a swamp to dry out. In fact the swamp environment is improved such that the regrown moss is normally better quality than the previously harvested moss that was removed.
The greatest threat to the existence of sphagnum moss swamps is the intentional draining for encroaching farmland.
Sphagnum moss can potentially harbour the chronic fungal disease sporotrichosis. Sporothrix schenckii spores enter the skin via abrasions, scratches, and small puncture wounds as a result of unprotected contact exposure to Sphagnum moss.
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