Environmental Engineering Reference
In-Depth Information
Chapter
21
Vegetation development and succession
As with all elements of the natural landscape - rocks, slopes, rivers, soils - vegetation
communities have a history of development. The changes which occur in the
characteristics of a particular species over many generations are called
evolution
.
Evolutionary change takes place over millions of years and is achieved by the mechanism
of natural selection, first proposed by Charles Darwin in 1859. In this way, the modern
characteristics of plant and animal species result from a long history. The present
distribution of species is greatly influenced by past events, too. The theory of continental
drift explains why some species are widely distributed across continents now separated
by thousands of miles. The changing patterns of distribution over time are preserved in
the fossil record. The latter may take the form of hard-rock fossils or, for Quaternary peat
and lake sediments, the plant pollen grains and spores which they contain. Vegetation
communities are also subject to short-term changes over the order of, say, thirty to 100
years. Natural disturbances by floods, volcanic activity, hurricanes, disease and fire can
completely alter the vegetation of an area. The habitats of the plants are changed,
favouring a new set of communities. These short-term changes reflect the dynamic nature
of vegetation; the processes occurring in the successional development of plant
communities will be discussed in this chapter.
SUCCESSION AND CLIMAX
Ecological succession
is the term used to signify the changes in the composition of a
community over time. It refers to the sequence of communities which replace one another
in a given area. The entire sequence of stages is referred to as the
succession
or
sere
,
each temporary stage in the succession being called a
seral stage
. Plant species invade
the site when conditions are favourable, and are eliminated when the succession leads to
unfavourable local conditions. Thus seral stages are defined by the changing dominance
Figure 21.1
The classical view of unidirectional plant
succession, starting with an unvegetated surface and ending
with the climax community.
