Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests managed for timber in western Oregon frequently lack structure and diversity associated with old-growth forests. We examined thinning effects on overstory and understory development for 8 years after treatment. Three 30- to 33-year-old Oregon Coast Range plantations were partitioned into four overstory treatments: unthinned (~550 trees/ha) and lightly (~250 trees/ha), moderately (~150 trees/ha), and heavily (~75 trees/ha) thinned. Within each overstory treatment, two understory treatments were established: underplanted with Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.) or not underplanted. Thinning increased overstory stem growth, crown expansion, and retained crown length. Thinned overstory canopies began to close rapidly the third year after thinning, decreasing % skylight by approximately 2%/year, whereas % skylight in unthinned stands increased slightly. All seedlings planted in unthinned stands died, whereas eighth year survival in thinned stands averaged 88%. Natural regeneration densities and distributions were highly variable. Understory shrub cover was reduced by harvesting disturbance but recovered by the fifth year. Thinning increased understory plant species diversity, and no shrub species were lost. Thinning to low densities and underplanting has the potential to accelerate development of multilayered stands characteristic of old-growth Douglas-fir forests.

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An index was developed that takes into account the combined influence of air and soil temperatures, light, and the availability of soil moisture upon photosynthesis by a unit area of fully exposed foliage of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco). The index is derived from a summation of daily estimates of carbon assimilation (milligrams CO2 assimilation per decimeter per day) for an entire year. In a comparison of forest environments in western Oregon the index was correlated to a measure of forest productivity (r2 = 0.99). Furthermore, it suggests that much of the annual carbon fixation occurs during the mild winters characteristic of the region. The ability to assess the effects of frost, soil drought, and other variables separately was valuable in explaining differences between coastal, valley, and mountainous sites.

The influence of various amounts of shrub and herb cover on microclimate and on survival and growth of Engelmann spruce (Piceaengelmannii Parry) and lodgepole pine (Pinuscontorta Dougl.) was examined in plots established in the Engelmann Spruce – Subalpine Fir zone of south central British Columbia. The planting sites varied from those with undisturbed vegetation to bare plots with exposed mineral soil. After three growing seasons, survival of both conifer species was greater than 95%, except at the highest levels of vegetation cover, where survival was 76 to 80%. Diameter was the conifer growth measure most responsive to reduction in adjacent vegetation. In the absence of adjacent vegetation, mean diameters of spruce and pine seedlings were 44 and 84% greater, respectively, than those of seedlings planted in undisturbed vegetation, and mean diameters increased still more when mineral soil was exposed. Height growth was less responsive to reduction in adjacent vegetation. Soil water potential, never more negative than −0.01 MPa, was the same at all levels of vegetation cover. Midday and predawn xylem pressure potential of spruce did not vary with the amount of vegetation. Although soil water was ample, moderate water stress occurred, most likely because uptake was restricted by low soil temperatures. Results suggest that soil temperature, air temperature, and light level are the primary factors controlling conifer seedling performance in an undisturbed vegetation community.

The leader and cambial growth of sapling Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) from both inland and coastal varieties followed a definite pattern in four western Oregon environments. Generally, buds became active first and cambial growth became active soon after. Leader growth stopped in August, long before cambial growth, which continued into October. Phenology, total seasonal growth, and growth pattern for trees from coastal sources from Vancouver Island, B.C., to southern Oregon were more similar than for trees from inland sources from British Columbia to Idaho and Arizona. Most of the differences among populations in one season's growth were related to growth rate rather than growth duration.