Abstract:
The form of a tree stem and the properties of the wood comprising
a stem may be determined by the strength requirements of
that stem. If this is true, a tree will react physiologically to
stresses which are imposed on a stem. This study was designed
to investigate the influence of static bending stress on the growth
and wood characteristics of Douglas -fir. Twenty-four nine –year-old
Douglas-fir trees from two geographic sources were divided
into three treatments. The treatments used were control, low bending
stress, and high bending stress. Trees in the low and high treatments
were bent with a horizontal force at a point 4. 5 ft. from the
base, so that a low and high bending stress, respectively, would be
produced in the trees. Bending caused another variable to be introduced
into the experiment. That variable was a transverse component
of gravity which influenced the low and high treatment trees.
The influence of stress and gravity was studied on the diameter,
radial, and leader growth of the trees and also on the specific
gravity and tracheid length of the wood which was produced.
Treatments did not cause a significant change in diameter
growth. Radial growth was increased in treatment trees on the side
of stems which was under compressive stress. The greatest increase
in radial growth occurred in the lower parts of stems from
2. 5 ft. to the base. Leader growth was significantly decreased by
the treatments with the greatest decrease in high stress trees. The
treatment thus caused a redistribution of wood formation from the
higher parts of stems to the lower parts of stems. Stress is thought
to be the factor influencing this downward redistribution of wood and
causing the increase in radial growth in the lower parts of stems.
Specific gravity of wood formed was increased by treatments,
but only on the side of sterns containing compressive stress. The
greatest increase was at 4. 5 ft. and 2. 5 ft. , with only a small increase
at the base. The increase in specific gravity was associated
with compression wood formation. The increase in specific gravity
and associated compression wood formation is believed caused by
the transverse component of gravity rather than by any stress influence.
Treatments also caused a decrease in tracheid length of wood
formed on the sides of stems containing compressive stress. This decrease in tracheid length appeared to be associated with the increase
in radial growth which occurred on treatment trees and also
with compression wood formation.
This study indicates that the function of compression wood is
not to resist stresses in trees, but rather possibly to cause a bent
or leaning tree to reorient itself in the vertical position. In order
to resist stresses in stems, trees produce increased amounts of
normal wood. In this way a greater cross sectional area of a stem
is available to resist the imposed stresses.
