Figure 814

Phragmites australis declines have been noted at over 45 European lakes, most in central and eastern Europe. The dots denote lakes where reed declines have been documented. (From Ostendorp et al. 1995. Ecological Engineering 5: 51-75. Reprinted with permission from Elsevier Science.)

• The disappearance of bird species like great reed warbler (Acrocephalus arundi-naceus), little bittern (Ixobrychus minutus), little grebe (Podiceps ruficollis) and purple heron (Ardea purpurea)

• Reduction in size of some fish species such as pike (Exos lucius), tench (Tinca tinca), carp (Cyprinus carpio), and rudd (Scardinius erythrophthalmus)

b. Causes of the Decline

The alteration of hydrology is one of the major causes of the decline of reed stands throughout Europe. Many of the documented cases of European reed decline have occurred where water levels are controlled by locks, weirs, and dams (Rea 1996; van der Putten 1997). Alterations in hydrology that restrict extreme fluctuations and thereby avoid both flooding and exposed sediments can result in the decline of reed beds. In Sweden, Switzerland, and other European countries, Phragmites australis has declined because lake water levels are regulated and the natural summertime dry periods no longer occur. P. australis is under stress when constantly submerged because less oxygen and carbon dioxide are available. Growth and germination are inhibited and the plants deplete carbohydrate reserves.

Other factors may further influence the decline of P australis stands, such as mechanical damage, organic matter accumulation, development of intensely reducing soil conditions, eutrophication, phytotoxin accumulation, raised temperatures, and insect or fungal damage. These factors interfere with the reed's internal aeration as well as its water and nutrient uptake, leading to stunted growth and the death of underground plant parts. The accumulation of decaying organic matter exacerbates the problem because levels of phy-totoxins such as hydrogen sulfide and volatile organic acids (formic, acetic, butyric, pro-pionic, caproic) increase, further contributing to root, rhizome, bud, and shoot death, premature shoot senescence, and ultimately leading to a full dieback (Figure 8.15; Ostendorp et al. 1995; Armstrong et al. 1996; Clevering 1998). Allochthonous sources of organic matter such as wastewater effluent can also bring about the accumulation of phytotoxins and a high oxygen demand, and lead to reed decline (Cizkova et al. 1996; Kubin and Melzer 1997).

c. Solutions to the Phragmites australis Decline

Phragmites australis does exist in healthy and expanding stands in Europe: in oligotrophic waters, sand pits, temporarily wet roadside ditches, and abandoned meadows. In all of these sites, dry summertime conditions and low nutrient inputs seem to be factors in the plant's success. The solution to the decline may be to restore low water levels, at least during part of the growing season, and to reduce nutrient inputs (Rea 1996; van der Putten 1997).

Lake managers and ecologists are attempting to restore P. australis stands in a number of Swiss lakes. Restoration measures include reed protection against mechanical damage (fences), wave-dissipating structures (brushwood piles and refilling of substrate), nutrient export from the reed stands (winter mowing), and other measures such as reed plantings and the prohibition of public access (Ostendorp et al. 1995).

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