Basic Forest Tree Biology
Trees and other plants have often been called factories
because they convert light energy into chemical energy.
Through the process of photosynthesis, leaves capture
solar energy by converting carbon, hydrogen, and oxygen
into complex sugars. One of these complex sugars
is cellulose, the main ingredient of wood fiber.
Although a complete knowledge of photosynthesis is not
essential to understanding how trees grow, it is
important to realize that, in theory, the faster and
more efficiently a tree carries on photosynthesis, the
faster it will grow. Availability of light and water are
two factors that can be controlled by harvesting.
When a portion of the growing stock is removed, the
photosynthetic material is reallocated to the remaining
trees. In a young stand, it is usually only a matter of
5 to 10 years before the crowns of residual trees grow
into the spaces left by those taken out. Trees are
principally composed of four main parts: roots, stem,
branches, and leaves. Other specialized structures, like
flowers and seeds, develop periodically for purposes of
reproduction. However, virtually all of the important
physiological processes in trees involve one or more of
the four main parts.
Roots
About 20 % of the mass of a forest-grown tree is devoted
to roots. In addition to anchoring the tree, roots
gather mineral nutrients, take up water, and
store the products of photosynthesis. Forest tree roots
are much more extensive than they appear. For example,
the root system of a sugar maple may extend as much as 2
to 5 times beyond the spread of its crown. Most of these
roots, known as fine or feeder roots, are within a few
inches of the soil surface.
Though the fine roots may account for only 14 % of the
total root mass, 80 % of the total root length is in
fine roots. Consequently, roots are everywhere in the
forest and the ones that are most sensitive to damage
are also the most susceptible.
Stem
The main stem usually makes up about 60 % of a tree's
weight. It supports the branches and leaves and serves
as the main plumbing system, with vessels to transport
water and nutrients up to the leaves and with other
cells to transport photosynthesis sugars to living
tissue throughout the tree. The growing
portion is only a thin layer of cells surrounding the
main stem. Each year this thin sheath of cells puts down
a new layer of sapwood. The rate at which it
does this determines how fast a tree grows in girth.
Branches & Leaves
A tree's branches support leaves in a
configuration that maximizes light availability or that
protects them from excessive exposure on harsh sites.
Branches also serve as the second-order plumbing of the
main stem. The leaves carry on photosynthesis and
exchange important gasses, such as oxygen and carbon
dioxide, with the atmosphere. Combined, branches and
leaves make up about 20 % of the tree's total weight.
Although all trees have roots, stems, branches, and
leaves, the form of each of these components differ
among species - and within a species in some cases.
Some characteristics, such as the size and shape of
leaves from the top of the crown versus those on shaded
lower branches, even differ on the same tree. For this
reason, leaf size and shape are said to be plastic
because they are characteristics that mold themselves to
the circumstances. This is one of the reasons why
learning trees solely by their leaves is so unreliable
and frustrating.
Environmental Factors
Although many of the obvious differences among trees
within a species are random, some are not. Important
differences in a tree's form and function may be caused
by environmental factors. For example, the root systems
of most trees tend to be more extensive on drier sites.
Another example: open-grown trees tend to have short
stems and wide, deep crowns, while forest-grown trees of
the same species, in their struggle to obtain crown
space, tend to have long stems and short, irregular
crowns that fit the available space in the canopy.
Important genetic differences between species have
evolved over millions of years.
Although not all
structural differences are due to adaptations that make
one species a better competitor than another on a given
site, many of them are. For example, many conifers have
adapted to become better competitors on dry sites than
most hardwood species. Though most conifers will do well
on better sites, their natural habitat is defined by the
limits of tolerance of other species. White pine is a
good example. It grows extremely well on moist,
protected, "hardwood" sites, but it is nearly impossible
to get a new stand of pine started using natural
regeneration. Hardwood species, such as sugar
maple and yellow birch, are much better competitors in
the understory.
On a drier site, the reverse may
be true - white pine can compete more effectively than
most hardwoods. Each species has a range of environments
in which it will grow. These extend from circumstances
where it is a minor component, poorly formed and slow
growing, to situations where the tree is able to take
full advantage of a site and grow to its maximum
biological potential. One of the most common silvicultural errors in forest management is trying to
grow a species on a site where it can achieve only a
fraction of its growth potential.
[This information has been excerpted from the
Introduction to Forest Ecology and Silviculture
Thom J McEvoy, Extension Forester, published by the
University of Vermont in 1995] |