Environmental Engineering Reference
In-Depth Information
'slacks' occur in the dune zones (often caused by wind 'blow-outs'), marshy areas of
rushes (
Juncus
spp.), sedges (
Carex
spp.) and willows (
Salix
spp.) show that the water
table is near the surface. It is possible to get local hydroseres or haloseres, depending on
inundations by the sea, around the ponds of these 'slacks'.
As one moves inland the soil changes in psammoseres on sand dunes are striking.
Acidity increases and pH declines as the influence of sea shells and salt spray lessens,
and leaching remains strong through the coarse sand. Organic matter in the topsoil
increases with distance inland, as the more varied and more abundant plant cover gives a
larger litter input to the soils. Micro-organisms are more abundant and produce more
humus, which in turn is able to hold more moisture, to provide more nutrients, and to
give greater stability to soil surfaces. Eventually a deep, humus-rich soil with a thriving
faunal population of earthworms, snails and insects will result. In this manner the original
sterile area of sand has been transformed completely by the succession of communities.
HYDROSERES
All lakes are ephemeral features of the landscape because they are gradually filled in with
sediments. This silting process occurs independently of plant successions, but the process
is considerably speeded up on lake margins by the development of hydroseres. The
littoral zones of ponds and lakes are shallow sediment-receiving zones where the shallow
water allows concentric zones of aquatic vegetation to develop, with one community
replacing another as the depth of water changes in space and time. Figure 21.5 shows a
transect of vegetation zones which one would meet in passing from the open water of the
centre of the lake on to surrounding dry land.
The pioneer community of water lilies (
Nymphaea alba
)
Figure 21.5
Vegetation zones in a hydrosere from open water
to terrestrial forest where the climax vegetation is oak
woodland.
