Environmental Engineering Reference
In-Depth Information
Chapter
20
Vegetation systems
Vegetation clothes Earth and provides the vital link between the sun and Earth's
ecosystems. The fixation of the incoming solar radiation 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 and biological factors. Plants live together in communities, and so they
are influenced by a range of 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 which are used in the study of
vegetation communities.
UNITS AND SCALE OF STUDY
Vegetation is only one component of the world's landscape 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 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 plants which make up natural vegetation do not exist just as individuals - they
also live in communities. In the idea of
plant community
is the concept of all the
relations between plants. These interrelationships are mostly beneficial to all parties;
these are called
obligate symbiotic relationships
. The driving force in developing these
mutual bonds is
co-evolution
, the development over time of mutually beneficial
connections between organisms in a particular environment. Plants and animals have
evolved these obligate ties by living together, and together they form an organically
interdependent community. Thus an ecosystem such as an oak woodland can be
understood and explained only by referring to the functions and processes involved in the
interaction of its component parts, including climate in the air layer, the soil layer and the
living organisms of both.
The second property of plant and animal communities is that they are collections of
organisms whose common denominator is tolerance of the particular environment that
they share. Some ecologists believe that this is the most important feature of
communities, and any other relations are optional rather than obligatory
facultative
relationships
. This is the
individualistic community
concept of the American ecologist
Gleason, put forward in 1939. It is in contrast to the
organismic community
concept of
