Hydrology of wet grasslands

Shallow (<50 cm deep) flooding on grassland can be used to attract wildfowl by providing both safe roost sites and suitable feeding conditions. Regular winter flooding followed by grazing or cutting encourages perennial grasses and some other plants important as seed sources for wintering wildfowl, but tends to produce less ruderal vegetation than moist soil management (Section 14.9.2). Regular inundation also encourages some grass species favored by herbivorous wildfowl, notably Creeping Bent Agrostis stolonifera. Retention of shallow floods during the breeding season will provide feeding areas for breeding wildfowl, although more densely vegetated ditches are probably more valuable for brood rearing.

Waders on grassland feed on invertebrates in the soil (primarily earthworms Lumbricidae and leatherjackets Tipulidae), amongst vegetation, and in shallow pools. Flooding previously unflooded grasslands creates a short-term "flush" of displaced soil invertebrates, which can attract waders and other species. However, soil invertebrates are slow to re-colonize areas vacated during flooding. This means that flooding large areas of wet grassland, either at the same time or on rotation, is likely to greatly decrease the total abundance of prey for waders (Ausden et al. 2001). One effective way of maintaining suitable conditions for breeding waders in the long-term is to maintain a mosaic of unflooded grassland with a high water table (if soils are suitable—see below) on which waders can nest and feed on soil invertebrates, and shallow pools that sequentially dry out and concentrate aquatic prey during the breeding season (Ausden 2001). This is easiest to achieve on sites with varied topography, such as unleveled coastal grazing marsh. Excavation can be used to create shallow creeks and pools at otherwise uniform sites, but it can be difficult to create a natural-looking variation in height. Disposal of unwanted spoil can also be a problem. Retention of surface water is easiest on soils with low rates of water transmission, such as clays.

Maintaining a high water table within fields can benefit Snipe Gallinago gallinago and Black-tailed Godwits by keeping the upper soil moist and therefore soft enough for them to probe for soil invertebrates (Green et al. 1990b). A high water table also concentrates soil invertebrates close to the soil surface, particularly on the margins of shallow floods. It may also retard vegetation growth and thereby help maintain suitable conditions for breeding waders that prefer more open conditions, such as Lapwings (Figure 14.5).

It is usually only possible to maintain a high field water table during the breeding season on soils that have a high rate of water transmission, such as undamaged peat. This can be done by maintaining high water levels in surrounding ditches, particularly if these ditches are closely spaced, so that water can flow laterally

Fig. 14.5 Flooding grassland can increase the accessibility of soil invertebrates to breeding waders such as lapwings by suppressing vegetation growth. If pools remain into late spring and summer, they are rapidly colonized by aquatic invertebrates and provide important feeding areas for wader chicks. (Malcolm Ausden and RSPB Images)

Fig. 14.5 Flooding grassland can increase the accessibility of soil invertebrates to breeding waders such as lapwings by suppressing vegetation growth. If pools remain into late spring and summer, they are rapidly colonized by aquatic invertebrates and provide important feeding areas for wader chicks. (Malcolm Ausden and RSPB Images)

from these ditches through the soil into the field. On soils with lower rates of water transmission, movement of water into the field from surrounding ditches becomes insufficient to replace water lost from the field by evapotranspiration in late spring and summer. On undamaged peat, a water table of 20—30 cm below the soil surface is recommended for breeding Snipe (RSPB, EN and ITE 1997). Again, variation in surface topography is helpful in maintaining high water levels, especially if shallow ditches help to irrigate areas around them.

Was this article helpful?

0 0

Post a comment