manipulated during subsequent drawdowns. For example, in cattail Typhaspp.
and Whitetop Rivergrass Scolochloa festucaceaswamps in North America, den-
sities of breeding wildfowl are highest where there are equal proportions of
swamp and open water (Kaminski and Prince 1981; Linz et al. 1996). If the
swamp has died back due to herbivory by wildfowl and/or Muskrats Ondatra
zibethicus, disease, erosion, or other environmental stresses, it can be re-estab-
lished by lowering water levels in spring and summer to allow germination of
emergent plants again. In the Oostvaardersplassen in the Netherlands, a draw-
down period of 4 years was needed to re-establish reedbed which had largely dis-
appeared due to erosion and grazing by Greylag Geese Anser anser(Ter Heerdt
and Drost 1994). If the swamp has expanded too much, then the area of open
water can be increased by lowering water levels, cutting, or burning patches of
swamp, and then re-flooding. Burning creates more open ground than cutting,
and so tends to result in more growth of ruderal vegetation during subsequent
drawdowns (de Szalay and Resh 1997). Densities of breeding waterfowl tend to
be highest where there is a high level of interspersion of swamp and open water
(e.g. de Szalay and Resh 1997; Kaminski and Prince 1981).
Using drawdowns to manage the relative proportions of open water and emer-
gent vegetation is impractical or unacceptable in certain wetlands. In particular,
it risks temporary or permanent extinction of less mobile invertebrates in isolated
wetlands where it is only possible to dry out all, or most, of the habitat. In these
situations it is necessary to control succession by removing vegetation and low-
ering the ground level. However, this has several disadvantages compared to
using periodic drawdowns. It does not create a temporary increase in inverte-
brate productivity in the shallow water following periodic drying out (see
Section 14.9.3), nor does it provide suitable conditions for ruderal vegetation.
In addition, permanent flooding maintains anoxic conditions in the sediment,
which are thought to exacerbate the effects of eutrophication in causing die-back
of reeds in Europe (Van der Putten 1997), unlike periodic drawdowns which
allow oxidation of reed litter. Management of reedbeds without periodically
drying them out is discussed in Section 14.10.6.
14.9.2Increasing food abundance for birds in shallow freshwater
The abundance of seeds for wintering wildfowl can be increased by providing
suitable bare, saturated mud for prolific seed-producing ruderal vegetation to
germinate and grow in spring and summer. This can be done by lowering water
levels or irrigating dried out wetlands to keep the soil moist. These seeds can be
made available to wildfowl by re-flooding in autumn. This technique is known as
“moist-soil management” (Smith and Kadlec 1983; Haukos and Smith 1993).
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