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1.1 Introduction Phragmites australis (Cav.) Trin.exSteud is a cosmopolitan species, considered to be the most widely distributed reed species in the world (Maroyi & Oyen, 2012). The occurrence of Phragmites on almost all the continents simply means it can tolerate a wide range of climate (Richard & van Wilgen, 2004). Vulnerability of protected areas to invasion is exacerbated by several anthropogenic disturbances.

The large size, high reproductive potential and rapid growth rate of Phragmites has led to differences of opinion held by biologists and resource managers with respect to the plant’s ecological value and potential usefulness for environmental Bart & Hartman, 2000). In this capacity it is capable of invading adjacent areas and crowding out other wetland plant species, reducing the overall plant diversity of the affected system species with a limited stable distribution to an invasive species producing dense monospecific stands (Maroyi & Oyen, 2012).

Phragmites forms dense monotypic stands, once established, rapidly takes over, creating dense patches and consumes available growing space for other plant species (Maroyi & Oyen, 2012). Phragmites has competitive abilities that allow it to quickly displace the wetland plants. Studying the community level impacts in the field, by comparing invaded and un-invaded plots, can identify potential effects of an invading alien species and provide valuable information for landscape management and nature conservation (Hejda, & Pysek, 2006).

The invasions threaten indigenous plant and animal species, and alter ecosystem function and natural disturbance regimes. Initially concerns were raised according to Guthrie (2007) regarding the potential for introduced plants to replace natural vegetation, with detrimental consequences. These warnings were ignored until the 1900s when invasion was realized as a serious problem in South Africa. Biological invasions affect biodiversity worldwide at various scales (MacDougall & Turkington 2004).

Phragmites is a serious weed and once established, it is difficult to eradicate, and it can block canals, streams, and drainage ditches (Maroyi & Oyen, 2012). It is common on irrigated land, where it infest all crops. Phragmites is considered to be the most widely distributed reed species in the world (Bird 1962). It is tolerant to climate, altitude, flooding, frost, high pH, salt, a wide range of soils, and invasion by alien species. (Gordon-Gray & Ward, 1971). Research in the past decade has improved knowledge of the patterns of invasion and substantial progress in understanding the mechanisms of invasion has been achieved.

Species composition and species diversity are functions of each other. Species composition will determine the diversity of an ecosystem, and vice versa. As an attribute of biodiversity, species composition (and species diversity) will affect other attributes, such as ecosystem functioning and ecosystem structure (Bart & Hartman, 2000). Plant species diversity can be used as an indicator of environmental change, such as disturbance to soil conditions (Blossey, 2003).

1.2 LITERATURE REVIEW

1.2.1	Phragmites australis

Phragmites is a cosmopolitan species that occurs on all continents, except Antarctica, and is thought to be the most widely distributed angiosperm. Phragmites occurs in tropical Africa in a belt around the dense forest zone, from Senegal east to Eritrea and from Ethiopia and Eritrea south to Mozambique, Zimbabwe, Namibia, Swaziland, South Africa and Madagascar (Maroyi & Oyen, 2012). In South Africa a number of climatic zones exist: humid, predominantly summer rainfall conditions hence Phragmites is found to occur throughout these zones (Human, 2009).

Phragmites tolerates salinity by extending its tap roots into deeper less saline water, and that it typically grows and establishes best in sites with low salinity, provided it can cope with varying water levels (Muukkonen et al., 2006). Phragmites has become a dominant species in a wide range of wetland habitats, which often leads to the displacement of other macrophyte communities, degradation of habitat for wildlife and an alteration in ecosystem processes (Mucina & Rutherford, 2006).

Not only has Phragmites increased its dominance in sites where it was historically found, but it has also spread into parts of the world where it was not found. This invasion has been attributed to factors such as disturbance, shoreline development, pollution, and eutrophication of waterways. It has been assumed that all Phragmites pose a threat to native biodiversity and will cause negative impacts on the ecosystem as a whole (Zimmermann et al., 2004).

1.2.2 	Habitat Phragmites is especially common in alkaline, tolerates moderate salinity and can also thrive in highly acidic wetlands (Ailstock et al., 2001). However, Phragmites does not require, nor even prefer these habitats to freshwater areas. Its growth is greater in fresh water but it may be outcompeted in these areas by other species that cannot tolerate brackish, alkaline or acidic waters. It is often found in association with other wetland plants including species from the following genera: Nymphaea, Typha, Glyceria, Myrica, Triglochin, Calamagrostis, and Phalaris (Howard et al., 1978). Phragmites occurs in disturbed areas as well as pristine sites. It is especially common along roadside ditches, and piles of dredge spoil (Ricciuti, 1983). Various types of human manipulation and/or disturbance are thought to promote Phragmites invasion (Roman et al. 1984). For example, restriction of the tidal inundation of a marsh may result in a lowering of the water table, which may in turn favor Phragmites. Likewise, sedimentation may promote the spread of Phragmites.

Phragmites tends to grow at or above the mean water level (Ailstock et al., 2001). It is classified as a facultative wetland species and can occasionally be found in upland areas (Reed, 1988). Reduced tidal action, lowered water table and decreased salinity may favors Phragmites within salt marsh settings (Marks et al., 1994). While Phragmites favors a landscape position that is higher relative to the water table than many wetland species, it is relatively tolerant of hydrological variability and can survive conditions that many competitors cannot (Marks et al., 1994).

1.2.3	Morphology Phragmites commonly forms extensive stands, in suitable conditions it can spread five metres or more per year by horizontal runners and can grow in variety of habitats (Marks et al, 1991). The upright, aerial stems serve mainly for photosynthesis and seed formation. According to Haslam (1970), rhizomes in soil are commonly long, thick and with no branches and capable of rapid growth above and below ground. In water they are shorter, more slender and have multiple branches. Below ground level Phragmites exists as a matrix of rhizomes and stolon's that produce the dense reed stands (Human, 2009). These aerial stems are derived from rhizome buds which are probably formed during the previous year’s growth. At the end of each growing season all the aerial stems die and are replaced in the following year by the growth of pre-existing rhizome buds. Phragmites australis has a firm, erect, hollow aerial cane-like stem, leaves are long for grass, aligned on one side of the stem and at maturity the leaves appear to be flat (Lambert & Casagronde, 2006).

1.2.4	 Uses

Phragmites australis is a multi-purpose species in various cultures with the stems being used to make walls, fences, plaiting baskets and mats. Young leaves and stems are forage for cattle and horses, while the stems are harvested for fuel. Flowering stalks yield a fiber suitable for rope making. Stems can also be used medicinally as a remedy for arthritis, bronchitis and cancer. Phragmites is also useful as a water clearing plant because of its dense growth, spreading root system and its ability to absorb plant nutrients and heavy metals. (Maroyi & Oyen, 2012).

Numerous parts of Phragmites can be prepared for consumption. For example, the young stems while still green and fleshy can be dried and pounded into a fine powder. The rhizomes and roots also serve as emergency food Pens for writing on parchment were cut and fashioned from this reed, and the stems were used as a linear measuring device (Marks, 1994). It is also useful in the production of homogeneous boards. It can be processed into a fine fibrous material suitable as filler in upholstery. It is also used for thatching and for making partitions, fences, coarse mats, baskets, and sandals (Ehrenfeld, 2003).

1.2.5	Reproduction

The plants generally flower and set seed between July and September and may produce great quantities of seed. Phragmites seeds are shed from November through January and so may be among the first propagules to reach these sites. The seeds are normally dispersed by wind but may be transported by birds such as red-winged blackbirds that nest among the reeds (Haslam 1972). If the seeds germinate and become established the young plants will usually persist for at least two years in a small, rather inconspicuous stage, resembling many other grasses (Ehrenfeld, 2003).

Newly opened sites may be colonized by seed or by rhizome fragments carried to the area by humans in soils and on machinery during construction or naturally in floodwaters (Maroyi & Oyen, 2012). Later after the input of nutrients, they may take off and assume the tall growth form that makes the species easily identifiable. Increases in soil nutrient concentrations, may come from runoff from farms and urban areas. It has also been suggested increases in nutrient concentrations, especially nitrates, are primarily responsible for increases in Phragmites populations (Lambert & Casagronde, 2006).

Temperature, salinity and water levels affect seed germination. Water depths of more than 5 cm and salinities above 20 ppt (2%) prevent germination (Kim et al. 1985; Tucker 1990). Germination is not affected by salinities below 10 ppt (1%) but declines at higher salinities. Percentage germination increases with increasing temperature from 16 to 25 °C while the time required for germinating decreases from 25 to 10 days over the same temperature range. It is not clear whether it establishes from rhizome pieces washed in with the wrack or from seed that blows in later (Kim et al. 1985).

Phragmites australis reproduces sexually and asexually. Marks et al (1994), found that the established stands of Phragmites propagate through vegetative reproduction via dispersal of rhizome fragments by water currents, animals and humans. It tolerates fire if water is above the soil surface (Maroyi & Oyen, 2012). Phragmites is highly successful colonizer in that it propagates in several ways by producing large quantities of seed, dispersion, rhizomes and stolon fragments. Cross pollination generally occurs, while self-pollination and agamospermy (seed production without fertilizations) are also possible (Burdick & Konisky, 2002).

1.2.6	 Ecology

Phragmites has a low tolerance for wave and current action which can break its vertical stems and impede bud formation in the rhizomes (Haslam 1970). It can survive, and in fact thrive where the sediments are poorly aerated). Air spaces in the above-ground stems and in the rhizomes themselves assure the underground parts of the plant with a relatively fresh supply of air. In addition the build-up of litter from the aerial shoots within stands prevents or discourages other species from germinating and becoming established (Haslam 1970).

The rhizomes and adventitious roots themselves form dense mats that further discourage competitors. These characteristics are what enable Phragmites to spread, push other species out and form monotypic stands. In this capacity it is capable of invading adjacent areas and crowding out other wetland plant species, reducing the overall plant diversity of the affected system. By forming monocultures, Phragmites may supplant other species considered to be more important as food or cover for wildlife (Chambers, 1997).

1.2.7	Effects of invasion

Invasion refers to the series of changes observed in a plant community following a disturbance event whereby new species are introduced in a plant community (Guthrie, 2007). Invasion has the potential to disrupt regeneration process of other species, by decreasing germination, reducing early growth rates and selectively increasing mortality of other plants species (Graneli, 1989).Biological invasions constitute the second largest threat to biodiversity globally, indigenous plant and animal species, and alter ecosystem function and natural disturbance regimes (Guthrie, 2007).

1.2.8	Invasion as a threat

Invasive species are able to survive, reproduce, spread at rapid rates across different landscapes, they constituted a threat to nature conservation through substitution of species and they are a sign of habitat degradation (Burdick & Konisky, 2002).The negativity of invasion is noted mainly in the agricultural sector, crop production, range management and forestry. Characteristics of invaded environments often include: geographical and historical isolation; low vegetative diversity and production of native species. Areas where the vegetation and soil have been disturbed are more susceptible to invasion (Moyo & Fatunbi, 2010).

2.1 References

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