Communities
with high productivities and in which
at least one shade tolerant tree species
is present, have high levels of biomass
because of their high foliage densities
throughout a large vertical distance.
Although this discussion centers on biomass,
it is difficult to measure in practice.
Ecologists thus often measure a surrogate,
plant cover, which is defined as the percentage
of the ground surface area that has plant
biomass (especially foliage) vertically
above it. If the vertical distribution
of the foliage is broken into defined
height layers, cover can be estimated
for each layer, and the total cover value
can therefore be over 100; otherwise the
values range from zero to 100. The measure
is designed to be a rough, but useful,
approximation of biomass. In some vegetation
types, the underground distribution of
biomass can also discriminate different
types. Thus a sod-forming grassland has
a more continuous and connected root system,
while a bunchgrass community's is much
less so, with more open spaces between
plants
(though
often not as drastic as the openings or
spacings in the above-ground part of the
community, since root systems are generally
less constrained in their horizontal growth
patterns than are shoots). However, below-ground
architecture is so much more time-consuming
to measure, that vegetation structure
is almost always described in relationship
to the above-ground parts of the community.
Like all biological systems, plant communities
are temporally and spatially dynamic;
they change at all possible scales. Dynamism
in vegetation is defined primarily as
changes in either or both of species composition
and vegetation structure. Temporally,
a large number of processes or events
can cause change, but for sake of simplicity
they can be categorized roughly as either
abrupt or gradual. Abrupt changes are
generally referred to as disturbances;
these include things like fire, high winds,
landslides, floods, avalanches and the
like.
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