A
primary characteristic of vegetation is
its three-dimensional structure, sometimes
referred to as its physiognomy, or architecture.
Most people have an understanding of this
idea through their familiarity with terms
like "jungle", "woods",
"prairie" or "meadow";
these terms conjure up a mental image
of what such vegetation looks like. So,
meadows are grassy and open, tropical
rainforests are dense, tall, and dark,
savannahs have trees dotting a grass-covered
landscape, etc. Obviously, a forest has
a very different structure than a desert
or a backyard lawn. Vegetation ecologists
discriminate structure at much more detailed
levels than this, but the principle is
the same. Thus, different types of forests
can have very different structures; tropical
rainforests are very different from boreal
conifer forests, both of which differ
from tempeate deciduous forests. Native
grasslands in South Dakota, Arizona, and
Indiana are visibly different from each
other, low elevation chaparral differs
from that at high elevations, etc.
Structure
is determined by an interacting combination
of environmental and historical factors,
and species composition. It is characterized
primarily by the horizontal and vertical
distributions of plant biomass, particularly
foliage biomass. Horizontal distributions
refer to the pattern of spacing of plant
stems on the ground. Plants can be very
uniformly spaced, as in a tree plantation,
or very non-uniformly spaced, as in many
forests in rocky, mountainous terrain,
where areas of high and low tree density
alternate depending on the spatial pattern
of soil and climatic variables. Three
broad categories of spacing are recognized:
uniform, random and clumped. These correspond
directly to the expected variation in
the distance between randomly chosen locations
and the closest plant to such locations.
Vertical distributions of biomass are
determined by the inherent productivity
of an area, the height potential of the
dominant species, and the presence/absence
of shade tolerant species in the flora.
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