Environmental Engineering Reference
In-Depth Information
establishes itself when the lake is reduced to about a metre in depth. This plant has the
effect of accelerating silting, both by reducing water velocity and by adding organic
debris to the lake bottom. When the depth is further reduced, a second-stage community
of bulrushes (
Scirpus lacustris
) and reeds (
Phragmites communis
) develops. Silting
continues and the water shallows enough to allow a zone of pond sedges (
Carex
spp.).
The emergence of a marsh surface above the water surface allows tussock sedges to
develop, especially the cotton grasses (
Eriophorum
spp.). Eventually higher ground
supports a deeper soil, with a better drained surface which allows tree seedlings to
survive. Alder (
Alnus glutinosa
) and willows (
Salix
spp.) can tolerate wetness and form
carr
woodland. In turn dryer conditions allow Scots pine (
Pinus sylvestris
) to grow which
eventually will be supplanted by the climax oak woodland (
Quercus robur
).
Each stage in the hydrosere is characterized by a vegetation zone where the plants
have specific tolerances of waterlogging and the degree of soil wetness. Plants which can
tolerate waterlogged soils are
hydrophytes
. Excess water excludes air (oxygen) from the
pores of waterlogged soils, and this shortage of oxygen causes problems with root
respiration. In well drained soils oxygen enters the root by diffusion from the soil
atmosphere. However, the rate of oxygen diffusion in water is about 10,000 times slower
than in air. The hydrophytes contain much more spongey tissue (
aerenchyma
) with thin
walls and large air spaces that permit air to diffuse through roots and stems. This allows
sufficient oxygen to be conducted from the atmosphere to the roots. Differences in the
rate of oxygen diffusion affect the zonal distribution of hydrosere plants; moving from
zone A to zone D in Figure 21.5 is to move through zones where oxygen moves less
freely through the plant tissues.
Another problem with waterlogged soils in hydroseres is that the soil chemistry has
undesirable characteristics. Elements such as iron and manganese are in a reduced state
(ferrous and manganous salts). In this state they are very soluble and can be absorbed in
toxic quantities by plants. Wetland species have the ability to transform the toxins into
less harmful states (ferric and manganic) by the diffusion of oxygen outward from roots.
Deposition of iron and manganese oxides around the roots of many hydrophytes is
evidence of this.
Zones E, F and G in Figure 21.5 represent the arboreal stages of the hydrosere. Alder
(
Alnus glutinosa
) is more tolerant of marshy conditions than Scots pine (
Pinus sylvestris
)
and oak (
Quercus robur
), both of which require a dry, well aerated soil for seedling
establishment. Tree seedlings are also more demanding of nutrients, and this depends
upon deeper soils with larger humus contents and more active nutrient cycles. The ability
of alder to fix atmospheric nitrogen through micro-organisms living in nodules on its
roots is a valuable input in raising soil fertility. Pine invades rapidly and forms a clear
community before it is replaced by oak, which is able to out-compete pine for light.
HALOSERES
Haloseres are found in salt marshes where silts can sediment in sheltered estuaries or
where the coastline is protected by islands, bars and spits. Dominant plants have to be
adapted to the stresses brought about by inundation by tides and additions of salt.
Continual deposition of tidal silt raises the level of the land, and a change in vegetation is
towards those species which are increasingly tolerant of prolonged exposure to the
