Juvenile development of Douglas-fir, red alder and snowbrush associations in Western Oregon

Graduation date: 1967The juvenile development of Douglas-fir, red alder and snowbrush\ud associations was investigated in western Oregon. The relationship\ud between Douglas-fir and red alder was studied and compared\ud in the Coast, Willamette Valley and Cascade regions. Snowbrush-\ud Douglas-fir stands were sampled in the western Cascades. Alder-\ud Douglas-fir relationships were studied on clearcuts supporting at\ud the same time mixtures of alder and Douglas-fir and open-grown\ud Douglas-fir. Sites where snowbrush and Douglas-fir grew together\ud were selected on the same basis. Total heights of alder and Douglas-fir\ud were expressed as cumulative one-year growth measurements.\ud Canopy height of snowbrush was measured at various ages to determine\ud growth rate. Information pertaining to moisture, vegetation,\ud slope, aspect, elevation and stand structure was recorded at each\ud sample plot.\ud The data were analyzed with the aid of a multiple regression program. Height and (height)² were treated as independent variables,\ud and annual height increment as the dependent variable.\ud Results indicate that early establishment of Douglas-fir is\ud expected to aid its dominance-takeover and allow it to evade suppression\ud by red alder. Douglas-fir trees growing on wet sites need\ud to be established earlier than those on drier habitats for the same\ud degree of suppression evasion. The rapid juvenile growth rate of\ud red alder is a major threat for the successful establishment of\ud Douglas-fir. Height growth curves of the two species intersected\ud at an earlier age on non-wet as compared to wet sites. The two\ud trees grow in direct competition up to about age 40 years on wet\ud habitats, with alder able to suppress Douglas-fir during this period.\ud Douglas-fir has virtually no chance áf survival when it is established\ud concurrently with red alder or after its appearance.\ud Snowbrush retards the growth of Douglas-fir trees during their\ud first ten years of development. Trees that are delayed more than\ud five years in establishment suffer a loss of more than 50% in total\ud height as a result of the suppressive effect of the shrub. It is expected\ud that the trees will compensate for some of this loss, but their\ud growth may never equal that of comparable open-grown Douglas-fir\ud developing under similar conditions