The juvenile development of Douglas-fir, red alder and snowbrush
associations was investigated in western Oregon. The relationship
between Douglas-fir and red alder was studied and compared
in the Coast, Willamette Valley and Cascade regions. Snowbrush-
Douglas-fir stands were sampled in the western Cascades. Alder-
Douglas-fir relationships were studied on clearcuts supporting at
the same time mixtures of alder and Douglas-fir and open-grown
Douglas-fir. Sites where snowbrush and Douglas-fir grew together
were selected on the same basis. Total heights of alder and Douglas-fir
were expressed as cumulative one-year growth measurements.
Canopy height of snowbrush was measured at various ages to determine
growth rate. Information pertaining to moisture, vegetation,
slope, aspect, elevation and stand structure was recorded at each
sample plot.
The data were analyzed with the aid of a multiple regression program. Height and (height)² were treated as independent variables,
and annual height increment as the dependent variable.
Results indicate that early establishment of Douglas-fir is
expected to aid its dominance-takeover and allow it to evade suppression
by red alder. Douglas-fir trees growing on wet sites need
to be established earlier than those on drier habitats for the same
degree of suppression evasion. The rapid juvenile growth rate of
red alder is a major threat for the successful establishment of
Douglas-fir. Height growth curves of the two species intersected
at an earlier age on non-wet as compared to wet sites. The two
trees grow in direct competition up to about age 40 years on wet
habitats, with alder able to suppress Douglas-fir during this period.
Douglas-fir has virtually no chance áf survival when it is established
concurrently with red alder or after its appearance.
Snowbrush retards the growth of Douglas-fir trees during their
first ten years of development. Trees that are delayed more than
five years in establishment suffer a loss of more than 50% in total
height as a result of the suppressive effect of the shrub. It is expected
that the trees will compensate for some of this loss, but their
growth may never equal that of comparable open-grown Douglas-fir
developing under similar conditions
