Geoscience Reference
In-Depth Information
20
Principles of
Vegetation clothes Earth and provides the vital link
between the sun and Earth's ecosystems. The fixation of
the photosynthetically active radiation (PAR) by the
process of photosynthesis in the leaves and stems of plants
provides the organic molecules which support all life on
Earth. Without plants there would be no organisms
and no human life. The distribution of plants on Earth is
not random or haphazard. Plants are governed in their
distribution by a range of physical, chemical and bio-
logical factors. Plants live together in communities, and
so they are influenced by mutual relations with other
plants, just as human beings in human societies are ruled
by sets of relationships. This chapter explores the nature
of plant-environment relations, and the key concepts in
the study of vegetation communities.
As with all elements of the natural landscape - rocks,
slopes, rivers, soils - vegetation communities have a
history of development. The changes in the characteristics
of a particular species over many generations are called
evolution
. Evolutionary change takes place over time scales
from hundreds to millions of years, and is achieved by the
mechanism of
natural selection
, first proposed by Charles
Darwin in 1859. The distribution of species is greatly
influenced by past events, too. Continental drift explains
why some species are widely distributed across continents
now separated by thousands of miles. Clues to past
patterns of distribution are preserved in hard-rock fossils
and plant remains including pollen grains and spores in
Quaternary peat and lake sediments. Vegetation com-
munities are also subject to short-term changes over the
order of thirty to 100 years. Natural disturbances by
floods, tsunami, 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, and processes occurring in
these plant successions will also be discussed in this
chapter.
Vegetation is only one component of the world's land-
scape and of global ecosystems, but special importance is
attached to it, as it is the basis of productivity. It fixes
carbon through photosynthesis, builds up organic matter
in soil, provides food and shelter for animals, stabilizes
soils and influences microclimates and the hydrological
cycle. In short, vegetation provides the life-supporting
properties of the biosphere; it supports the food webs of
herbivores, carnivores and decomposers which make up
the world's fauna on land and in the sea.
The community concept
The plants which make up natural vegetation do not
exist just as individuals - they also live in communities.
In the idea of
plant community
embraces all the relations
between plants. These interrelationships are mostly
beneficial to all parties and are called
mutualistic symbiotic
relationships
. The driving force in developing these
mutual bonds is
co-evolution
, i.e. the development over
time of mutually beneficial connections between organ-
isms in a defined environment. Plants and animals have
