Abstract:
This study examined the abundance, size, growth, and age of advance regeneration
Douglas-fir, beneath an eighty year-old overstory at a single site on plots subjected to different overstory thinning treatments. Treatments consisted of keeping overstory basal area within upper and lower limits for periods of 12-17 years which, depending upon the replication, ended 17-24 years prior to this study. Of the three thinning levels, the
heaviest thinnings (overstory basal area kept between 22.5 and 29.25 m2/ha) averaged the
most, tallest, oldest, and fastest growing seedlings, while the light thinnings (overstory basal area kept between 36.0 and 45.0 m2/ha) had the fewest, shortest, youngest, and slowest growing seedlings in 1994. Control plots had almost no regeneration. Among the seedling characteristics that were measured, treatment differences in seedling density were the most significant. Both the magnitude and the significance of differences in seedling density were greater in 1994 than they were in 1977 when overstory treatment differences were greater and more significant. Seedling height differences among treatments were somewhat less significant, while age differences were not significant. Seedling density also showed the greatest block differences. Present (1994) treatment differences in seedling density were well explained by both the relative density of the overstory in 1966 (the year by which each block had been thinned twice) and the cumulative reduction in overstory relative density through 1966. For both these explanatory variables, 1966 values explained 1994 seedling density treatment differences better than values from 1977 (the year the last block received its final thinning) or 1991 (the most recent year for which data are available). Block differences were explained by both the present and past competition from the shrubs Oregon grape and salal. They were also partially explained by the cumulative reduction in relative density through 1966.
Height differences were also explained by 1966 values of both relative density or the
cumulative reduction in relative density but seemed unaffected by shrub competition. The major herb and shrub species in 1974 and 1994 were Oregon grape and bracken fern. Since 1974 the percent ground cover of bracken fern has decreased while that of Oregon grape has increased.
