Sphagnum
Sphagnum is a genus of approximately 380 accepted species of mosses, commonly known as sphagnum moss, also bog moss and quacker moss. Accumulations of Sphagnum can store water, since both living and dead plants can hold large quantities of water inside their cells; plants may hold 16 to 26 times as much water as their dry weight, depending on the species. The empty cells help retain water in drier conditions.
As Sphagnum moss grows, it can slowly spread into drier conditions, forming larger mires, both raised bogs and blanket bogs. Thus, Sphagnum can influence the composition of such habitats, with some describing Sphagnum as 'habitat manipulators' or 'autogenic ecosystem engineers'. These peat accumulations then provide habitat for a wide array of peatland plants, including sedges and ericaceous shrubs, as well as orchids and carnivorous plants.
Sphagnum and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. In addition, bogs, like all wetlands, develop anaerobic soil conditions, which produces 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.
Under the right conditions, peat can accumulate to a depth of many meters. Different species of Sphagnum have different tolerance limits for flooding and pH, and any one peatland may have a number of different Sphagnum species.
Description
An individual Sphagnum plant consists 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 has compact clusters of young branches that give the plant its characteristic tuft-like appearance. Along the stem are scattered leaves of various shapes, named stem leaves; the shape varies according to species.Sphagnum has a distinctive cellular structure. The stem portion consists of two important sections. The pith which is the site of food production and storage, and the cortical layer' which serves to absorb water and protect the pith. Mosses have no vascular system to move water and nutrients around the plant. Thus tissues are thin and usually one cell thick to allow them to diffuse easily. Sphagnum mosses have two distinct cell types. There are small, green, living cells with chlorophyll that produce food for the plant. Additionally, there are larger hyaline or retort cells that are barrel shaped and have a pore at one end to allow for water absorption and improved water-holding capacity. These unique cells help Sphagnum'' to retain water during prolonged UV exposure.
Lifecycle
Sphagnum, like all other land plants, has an alternation of generations; like other bryophytes, the haploid gametophyte generation is dominant and persistent. Unlike other mosses, the long-lived gametophytes do not rely upon rhizoids to assist in water uptake.Sphagnum species can be unisexual or bisexual ; 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 almost entirely of a shiny green, spherical spore capsule that becomes black with spores. 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 explosively discharges its cap, called an operculum, and shoots the spores some distance. The spores germinate to produce minute protonemae, which start as filaments, can become thalloid, and can produce a few rhizoids. Soon afterwards, the protonema develops buds and these differentiate into its characteristic, erect, leafy, branched gametophyte with chlorophyllose cells and hyaline cells.
Carpets of living Sphagnum may be attacked by various fungi, and one fungus that is also a mushroom, Sphagnurus paluster, produces conspicuous dead patches. When this fungus and other agarics attack the protonema, Sphagnum is induced to produce nonphotosynthetic gemmae that can survive the fungal attack and months later germinate to produce new protonema and leafy gametophytes.
Spore dispersal
As with many other mosses, Sphagnum species disperse spores through the wind. The tops of spore capsules are only about 1 cm above ground, and where wind is weak. 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 per second, but alternative mechanisms have been recently proposed. High-speed photography has shown vortex rings are created during the discharge, which enable the spores to reach a height of, 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.Human activities like slash-and-burn and cattle grazing are believed to promote the growth and expansion of Sphagnum moss. Oceanic islands such as the Faroe Islands, the Galápagos or the Azores have recorded a significant increase in their Sphagnum populations after human settlement.
Taxonomy
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 :- Sphagnum sect. Acutifolia plants generally form hummocks above the water line, usually colored orange or red. Examples: Sphagnum fuscum and S. warnstorfii.
- Sphagnum sect. Cuspidata plants are usually found in hollows, lawns, or are aquatic, and are green. Examples: Sphagnum cuspidatum and S. flexuosum.
- Sphagnum sect. Sphagnum plants have the largest gametophytes among the sections, forming large hummocks, their leaves form cuculate apices, and are green, except for S. magellanicum Example: Sphagnum austinii.
- Sphagnum sect. Subsecunda plants vary in color from green to yellow and orange, and are found in hollows, lawns, or are aquatic. Species always with unisexual gametophytes. Examples: Sphagnum lescurii and Sphagnum pylaesii.
Distribution
Sphagnum mosses occur mainly in the Northern Hemisphere in peat bogs, conifer forests, and moist tundra areas. Their northernmost populations lie in the archipelago of Svalbard, Arctic Norway, at 81° N.In the Southern Hemisphere, the largest peat areas are in southern Chile and Argentina, part of the vast Magellanic moorland. Peat areas are also found in New Zealand and Tasmania. In the Southern Hemisphere, however, peat landscapes may contain many moss species other than Sphagnum. Sphagnum species are also reported from "dripping rocks" in mountainous, subtropical Brazil.
Conservation
Several of the world's largest wetlands are sphagnum-dominated bogs, including the West Siberian Lowland, the Hudson Bay Lowland and the Mackenzie River Valley. These areas provide habitat for common and rare species. They also store large amounts of carbon, which helps reduce global warming.According to an article written in 2013, the U.S. got up to 80% of sphagnum peat moss it uses from Canada. At that time, in Canada, the peat bog mass harvested each year was roughly 1/60th of the peat mass that annually accumulated. About 0.02% of the of Canadian peat bog are used for peat moss mining. Some efforts are being made to restore peat bogs after peat mining, and some debate exists as to whether the peat bogs can be restored to their premining condition and how long the process takes. "The North American Wetlands Conservation Council estimates that harvested peatlands can be restored to 'ecologically balanced systems' within five to 20 years after peat harvesting." Some wetlands scientists assert that "a managed bog bears little resemblance to a natural one. Like tree farms, these peatlands tend toward monoculture, lacking the biodiversity of an unharvested bog."
PittMoss, a peat moss alternative made from recycled newspaper, has emerged as a sustainable substitute in growing media. Coir has also 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. Semi-open cell polyurethane materials available in flaked and sheet stock are also finding application as sphagnum replacements with typical usage in green wall and roof garden substrates.