|Abstract or Summary
- The growth potential of Douglas-fir, grand fir and western hemlock advance regeneration in the forest understory of Douglas-fir-dominated forests was investigated to detennine the feasibility of selection cutting systems in northwest Oregon. This study compared the growth of Douglas-fir, grand fir and western hemlock advance regeneration along a light gradient with hemispherical photography, modeled the relative importance of light, competition, subject tree and stand characteristics on growth and reported observations on the growth response of advance regeneration to stand manipulation. Western hemlock, grand fir and Douglas-fir advance regeneration was found at 5, 6, and 7 % of diffuse skylight in selection thinned stands respectively, indicating that all three species are capable of survival and growth in low light and, therefore, are suitable for uneven-aged management. Growth of advance regeneration in terms of height, diameter and volume followed the order: Western hemlock > Douglas-fir> grand fir, although there was seldom a significant difference between Douglas-fir and grand fir. Overtopping shrub vegetation significantly reduced growth and light availability in comparison to trees that were not overtopped. Height, radial and volume growth of all three species increased with increasing light. Regression analysis revealed that along the measured light gradient (6 44 % of diffuse skylight) i; growth of western hemlock was superior (different intercepts) to Douglas-fir at one interior Coast Range site but there was no significant difference in the relationship of growth to increased light levels (equal slopes) ii; Douglas-fir advance regeneration growth was superior (different intercepts) to grand fir at three sites but there was no significant difference in the relationship of growth to increased light levels (equal slopes). Growth modeling explained 36 to 67 % of the variation in Douglas-fir, western hemlock and grand fir advance regeneration growth and revealed that tree characteristics were better predictors of advance regeneration growth than environmental variables. Independent of species, live crown ratio was the most important variable explaining variation in absolute height growth. The best-growing trees were in general taller than competing shrub vegetation and characterized by higher live crown ratios. Western hemlock was particularly sensitive to shrub cover and grew best when shnib cover was low (< 35 %). Therefore, where possible, select future crop trees with a live crown ratio of at least 50 % for Douglas-fir and grand fir and 60 % for western hemlock and remove competing vegetation. Studying the release of midstory and understory trees at the four sites in northwestern Oregon after thinning revealed in general that Douglas-fir, grand fir and western hemlock grew better following release. However, the vigor of response differed for the three species and increased in the order of western hemlock, grand fir and Douglas-fir. Douglas-fir generally released more rapidly and more vigorously than grand fir, but also decreased more rapidly. To keep a significant proportion of the advance regeneration in a very vigorous state, thinning cycles of 6 to 10 years appear necessary. Since advance regeneration of Douglas-fir and grand fir released to thinning, thinning cycles of 10 to 15 years are possible for keeping a large proportion of the advance regeneration in stable condition. Even highly suppressed Douglas-fir were capable of release after thinning.