Reeds (Phragmites)

Phragmites is a genus of four species of large perennial grasses found in wetlands throughout temperate and tropical regions of the world. The World Checklist of Selected Plant Families, maintained by Kew Garden in London, accepts the following four species:

Phragmites australis (Cav.) Trin. ex Steud. – cosmopolitan
Phragmites japonicus Steud. – Japan, Korea, Ryukyu Islands, Russian Far East
Phragmites karka (Retz.) Trin. ex Steud. – tropical Africa, southern Asia, Australia, some Pacific Islands
Phragmites mauritianus Kunth – central + southern Africa, Madagascar, Mauritius

Taxonomy
The cosmopolitan common reed has the generally accepted botanical name Phragmites australis. (Cav.) Trin. ex Steud. About 130 other synonyms have been proposed, and some have been widely used.[citation needed] Examples include Phragmites communis Trin., Arundo phragmites L., and Phragmites vulgaris (Lam.) Crép. (illegitimate name).

Subspecies and varieties
Recent studies have characterised morphological distinctions between the introduced and native stands of Phragmites australis in North America. The Eurasian phenotype can be distinguished from the North American phenotype by its shorter ligules of up to 0.9 millimetres (0.04 in) as opposed to over 1.0 millimetre (0.04 in), shorter glumes of under 3.2 millimetres (0.13 in) against over 3.2 millimetres (0.13 in) (although there is some overlap in this character), and in culm characteristics.

Phragmites australis subsp. americanus – the North American genotype has been described as a distinct subspecies, subsp. americanus, and
Phragmites australis – the Eurasian genotype is sometimes referred to as subsp. australis. but this is a synonym.
Phragmites australis subsp. altissimus (Benth.) Clayton is an accepted subspecies of P. australis.
Phragmites australis var. marsillyanus (Mabille) Kerguélen is an accepted variety of Phragmites australis.

Invasive status
In North America, the status of Phragmites australis was a source of confusion and debate. It was commonly considered an exotic species and often invasive species, introduced from Europe. However, there is evidence of the existence of Phragmites as a native plant in North America long before European colonization of the continent. It is now known that the North American native forms of P. a. subsp. americanus are markedly less vigorous than European forms. The recent marked expansion of Phragmites in North America may be due to the more vigorous, but similar-looking European subsp. australis.

Phragmites outcompetes native vegetation and lowers the local plant biodiversity. Phragmites forms dense thickets of vegetation that is unsuitable habitat for native fauna. Phragmites displaces native plants species such as wild rice, cattails, and native wetland orchids. Phragmites has a high above ground biomass that blocks light to other plants allowing areas to turn into Phragmites monoculture very quickly. Decomposing Phragmites increases the rate of marsh accretion more rapidly than would occur with native marsh vegetation.

Phragmites australis subsp. australis is causing serious problems for many other North American hydrophyte wetland plants, including the native Phragmites australis subsp. americanus. Gallic acid released by Phragmites is degraded by ultraviolet light to produce mesoxalic acid, effectively hitting susceptible plants and seedlings with two harmful toxins. Phragmites is so difficult to control that one of the most effective methods of eradicating the plant is to burn it over 2-3 seasons. The roots grow so deep and strong that one burn is not enough. Ongoing research suggests that goats could be effectively used to control the species.

Natural enemies
Since 2017, over 80% of the beds of Phragmites in the Pass a Loutre Wildlife Management Area have been damaged by the invasive "roseau cane scale", Nipponaclerda biwakoensis, threatening wildlife habitat throughout the affected regions of the WMA. While typically considered a noxious weed, in Louisiana the reed beds are considered critical to the stability of the shorelines of wetland areas and waterways of the Mississippi Delta, and the die-off of reed beds is believed to accelerate coastal erosion.

Growth and habitat
Phragmites australis, common reed, commonly forms extensive stands (known as reed beds), which may be as much as 1 square kilometre (0.39 sq mi) or more in extent. Where conditions are suitable it can also spread at 5 metres (16 ft) or more per year by horizontal runners, which put down roots at regular intervals. It can grow in damp ground, in standing water up to 1 metre (3 ft 3 in) or so deep, or even as a floating mat. The erect stems grow to 2–6 metres (6 ft 7 in–19 ft 8 in) tall, with the tallest plants growing in areas with hot summers and fertile growing conditions.

The leaves are long for a grass, 20–50 centimetres (7.9–19.7 in) and 2–3 centimetres (0.79–1.18 in) broad. The flowers are produced in late summer in a dense, dark purple panicle, about 20–50 cm long. Later the numerous long, narrow, sharp pointed spikelets appear greyer due to the growth of long, silky hairs. These eventually help disperse the minute seeds.

It is a helophyte (aquatic plant), especially common in alkaline habitats, and it also tolerates brackish water, and so is often found at the upper edges of estuaries and on other wetlands (such as grazing marsh) which are occasionally inundated by the sea. A study demonstrated that Phragmites australis has similar greenhouse gas emissions to native Spartina alterniflora. However, other studies have demonstrated that it is associated with larger methane emissions and greater carbon dioxide uptake than native New England salt marsh vegetation that occurs at higher marsh elevations.

Common reed is suppressed where it is grazed regularly by livestock. Under these conditions it either grows as small shoots within the grassland sward, or it disappears altogether.

In Europe, common reed is rarely invasive, except in damp grasslands where traditional grazing has been abandoned.

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