When drained and fertilised, many former fens became so productive that they have to be mown regularly throughout the year or be used as pastures. However, meadows mown up to two or three times a year, even with some enrichment of nutrients, can support a good number of perennial species: Polygonum bistorta, Trollius europaeus, Cardamine pratensis or Fritillaria meleagris are some striking examples (Figures 8.9 and 8.10). The higher the level of nutrients and the greater the frequency of mowing cycles, the poorer the species diversity will be because nutrient-demanding species are more rampantly growing and are more competitive (‘C-Strategists’, see below) than species from nutrient – poor sites (‘S-Strategists’, or ‘stress tolerators’). Ecological strategies are discussed in greater detail in Chapter 4.
Vegetation on exposed mud and disturbed or uncultivated wetland
The ground of drained ponds or the disturbed soil surface of ditches and ploughed wetland has a ruderal character. Such sites will be populated first by Short-lived Rush (Juncus) species, annual Bidens species or Alopecurus aequalis, followed by Carex canescens, Ranunculus flammula, Epilobium spec. and other tussock plants with a rich seed-bank. These fast-growing ruderals (R-strategists—Grime 1986) can become annoying weeds on the shore zone of artificial ponds. During the early years after completion, such a site is effectively disturbed and therefore prone to long-term invading weeds. These can be problematic in the gaps between planted individuals at high nutrient level. Without regular maintenance, the vegetation of fertile wet meadows will develop into a willow shrubland or a reed bed with Phragmites australis or to a tall forb community. The latter stage is populated mostly with Filipendula ulmaria, Lysimachia vulgaris and Lythrum salicaria. These species produce a spectacular flowering effect, but are only recommended where space is not limited. In North America, Lythrum has become a notorious invasive neophyte, so it is not recommended to use it there in planting design.
Exposed mud on the edge of water bodies is of particular value to invertebrates, whilst periodically flooded herbaceous vegetation around the edge of ponds and lakes is also beneficial to amphibians. It is therefore important, where space allows, that room is given for very shallow margins in places that enable fluctuations in water levels to produce periods of exposed mud and also flooded vegetation.