Environmental Engineering Reference
In-Depth Information
susceptibility of the rock to weathering and, second, the weathering intensity as
influenced by climate. Soil-forming processes in other primary successions involve either
stabilization of sand (psammoseres) or siltation of water bodies (hydroseres and
haloseres). These physical processes are generally much more rapid, and thus the
corresponding succession to climax is much quicker. In Britain typical sites of lithoseres
are landslides, scree slopes and some cliffs. Where bare rock surfaces have been formed
by human activity, e.g. quarrying, the scars formed on quarry faces or on discarded rock
waste remain visible for a long time. In such cases, if reclamation is required, it is usual
for the slow natural processes to be speeded up by planting, stabilizing loose surfaces,
and even importing topsoil. By these means the primary succession can be speeded up
tenfold. A typical lithosere is found on the hard and prominent Carboniferous limestone
pavements, referred to in Chapter 1 in Upper Wharfedale in the Pennine uplands. The
bare rock surface of this karst landscape is colonized by
crustose lichens
initially, which
commence the processes of physical and biochemical weathering. Several generations of
these organisms provide humic remains, which in turn can be invaded by
prostrate
mosses
. Once deeper depressions have been hollowed out, thick
cushion mosses
can fill
them and start the processes of soil formation. Fine mineral particles are released by
weathering, and often also blown or washed into the site. Over time a granular Ah
horizon is formed beneath a grass turf, and a lithosol is formed, which over limestone
would be a rendzina soil (FAO Calcaric Leptosol) (Plate 21.3).
As well as vegetation development and soil deepening, the lithosere is characterized
by invasions of animals and micro-organisms which increase in abundance and variety as
the succession proceeds. Humus that accumulates consists mainly of the droppings of
small arthropods (mites), collembola and insect larvae. These all play an important role in
breaking residues. On base-rich limestones the chemical conditions are favourable for
earthworms, which will invade the thin soil and thoroughly mix mineral and organic
particles. In more acidic situations earthworms are inhibited, and the undecomposed
organic matter accumulates as a peaty ranker.
PSAMMOSERES
The psammosere is a succession which starts its development on bare, loose sand, either
on sea or lake shores. The beach itself has no plant cover, as the waves continually move
the sand, whose abrasive action will destroy any rooted plants (Plate 21.4). Flora are
restricted to microalgae and diatoms, often attached to sand grains. Some organic matter
will be brought in on each tide, and decomposition will be carried out by organisms such
as lugworms living in the sand. A considerable population of microorganisms,
nematodes, cocepods and worms provides food for large predatory worms, and many
species of filter feeders are found within the sand.
In coastal regions with strong and consistent winds, sand is moved inland and
deposited as dunes. The first deposition of
embryo dunes
is initiated by the deposition of
sand around pioneering plants such as saltwort (
Salsola kali
) and sea rocket (
Cakile
maritima
). Continued growth of the dune depends upon stabilization of the shifting sands
by grasses, especially marram grass (
Ammophila arenaria
) in Europe. This species is
salt-tolerant and drought-tolerant and spreads vegetatively by rhizomes, laterally growing
underground stems. Rhizomes and extensive root systems help to bind the sand, and to
