Another
aspect of woody species communities to consider is the amount
of browsing or other physical damage that has been done on
the trees and shrubs. Riparian woody species generally have
a high tolerance for being browsed. However, excessive use
(more than 50 percent of current or previous year's growth
every year) will reduce structural diversity and limit the
ability of the plants to provide shade and protection for
soil, water, and animals.
The willow
in Figure 2-26 was being browsed heavily when the picture
was taken; several years of such treatment will severely affect
its vigor. In contrast, the willow in Figure 2-27 has not
been receiving heavy use and is exhibiting good vigor and
full growth. Incidentally, these two plants are in the same
pasture, which suggests the need to consider overall conditions
rather than focusing on individual "examples."
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Figure
2-26
Occasional heavy browsing such as this is tolerable,
but if repeated oftenwill result in deterioration of
the plant. |
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Figure
2-27
If this willow were repeatedly being overbrowsed, it
would exhibit gnarled branch ends and clumpy leaf growth
rather than the full, spreading canopy it presents.
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Continued
overuse weakens woody plants, reduces their canopy cover and
root bank-holding capability, and makes them more susceptible
to disease or insect damage. Heavy browsing may prevent the
establishment of younger plants, ultimately eliminating desirable
species and resulting in a change of plant communities. Physical
damage such as trampling, uprooting, or rubbing can also adversely
affect woody plants. |