Environmental Engineering Reference
In-Depth Information
LITHOSERES
The development of vegetation communities over time on a fresh rock surface is called a
lithosere. In some instances the new unvegetated mineral surfaces are already
unconsolidated. Many deposits resulting from the glaciations of the Pleistocene era give
mineral landscapes which are already prepared for the invasion of land plants; they give
media such as glacial tills, fluvioglacial sands and aeolian loess silts in which plants can
readily take root. Similarly fresh volcanic ash and recently deposited river alluvium can
quickly be covered by land plants. A lithosere on a hard-rock surface, however, presents
very different and much more hostile conditions. Bare rock is classed as a xerosere, as the
surface is extremely dry, owing to the rapid flow of any precipitation. The first plants to
colonize such a surface must be able to withstand complete drought and must be able to
cling to a bare rock surface devoid of soil. This
pioneer community
typically consists of
crustose lichens which form the white, black or orange growths on stone walls,
gravestones or boulders. These lower plants gradually weather the crystals and cements
in the rock by the process of chelation; chelation is the process whereby organic
molecules, either from humifica-tion or from root secretions, can form a soluble complex
with metals, especially iron and aluminium. They can also add organic material to the
thin, raw soil, and in so doing increase its water-holding capacity and content of plant
nutrients. A
second
-
stage community
of mosses is able to establish itself, which in turn
accelerates the weathering by chelation and also
hydrolysis
. The surface is becoming less
susceptible to drought as the depth of
