Predictions of common reed expansion into new areas

The common reed can form dense reed beds in wetland areas which often push out other plants.
The common reed can form dense reed beds in wetland areas which often push out other plant species. A new study maps the spread of the potentially damaging common reed along the southern coast of Finland. 

The common reed (Phragmites australis) is a contrary species: a wetland plant found throughout the world, it nonetheless can cause serious problems. The expansion of the aggressive European variety into North American sites has led to the suppression of the less vigorous native subspecies. At the same, in the Baltic Sea it plays an essential role in local ecosystems, acting as a buffer between land and sea, and providing nesting sites for shorebirds and spawning areas for fish species.

But in the last few decades, it has begun to rapidly expand into new areas along the Gulf of Finland, disrupting the balance among species in local wetlands. Most significantly, the reed is a hostile competitor, forming extensive reed beds which shade out other plants so that they eventually disappear completely.

With this in mind, a study set out to map the past and future expansion of the reed in four sites along the south coast of Finland. The results confirmed that the reed has continued to spread into clear waters and shorelines in recent years, progressively extending into deeper waters over time. The researchers found that the most suitable sites for the establishment of new reed colonies are shallow shores near river outlets. Bearing in mind the features of the local landscape, they anticipate that the reed will continue to expand for the foreseeable future at a rate of between 1 and 8 %.

The common reed is able to reproduce vegetatively by putting down horizontal runners; this enables it to rapidly overtake areas adjacent to existing colonies, if conditions are suitable. This also means that it is unlikely that only small patches of reed will be found in open waters – once it gets into a new place it won’t be long until it is the dominant plant species there. Often the best method of suppressing growth is with traditional grazing by livestock.

The main factors that appeared to influence the occurrence of the common reed are easily measured – the depth of water, the openness of the shoreline, and the distance to river outlets. This makes it relatively simple to identify other areas that might be susceptible to reed colonisation in the future. If these factors are taken into consideration, then hopefully management plans can be implemented before the reed becomes too much of a problem elsewhere.

Reference: Anas Altartouri, Leena Nurminen, & Ari Jolma (2014). Modeling the role of the close-range effect and environmental variables in the occurrence and spread of Phragmites australis in four sites on the Finnish coast of the Gulf of Finland and the Archipelago Sea Ecology and Evolution : 10.1002/ece3.986

Image: Jarmo Holopainen


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