- During the past 20 years, repeated thinning of a Douglas-fir forest, now 65 years old, has allowed desired limits of basal area to be maintained. In the process, the understory environment has altered sufficiently to allow Douglas-fir reproduction to establish in abundance. The forest is located on the east side of the Coast Range, north of Corvallis, Oregon.
A study was undertaken to examine growth patterns of understory trees in relation to microenvironment, physical attributes of the
tree and time. This information would help determine how long a tree can be maintained at a given level of shade and still function properly. This information also would add to the understanding of modification of growth strategies for survival. To characterize the understory environments of the three thinning
intensities (23 to 30, 30 to 37 and 37 to 44 m2/ha basal area), measurements were made of plant moisture stress, evaporative demand,
air and soil temperature, light and vegetative cover. Light measured at seedling crown best separated the environments; light was found to increase with a decrease in stand basal area, averaging 5, 8 and 12 percent of daily sun for the three thinning levels. Trees, randomly selected, ranged in height from 10 cm to 1.5 m and in age from 4 to 15 years. Parameters measured included leader growth for the past two seasons, diameter growth, above-ground biomass, height and age. Bud phenology and beginning of cambial activity were also observed.
Development of buds in the spring was inversely related to light while renewed cambial activity was directly related to tree size and light. Increments of height and diameter growth were found to increase
with light and tree height. Linear differences in average leader and diameter growth and above-ground biomass production were found between the three intensities of thinning, with trees under the heaviest thinning growing most. Average growth is predictable for at least two years. However, rates of leader growth and biomass production, in relation to growth of the previous year and leaf biomass, were not different for individual trees from the three understory environments. A balance of foliar to stem bioniass was maintained by
understory trees. Physical characteristics of the tree accounted for most of the variation in growth. As thinning of young Douglas-fir becomes a widespread management
practice, it can be expected that, in similar vegetation types, Douglas-fir seedlings will establish naturally in the understory. If the new forest is established at final harvest several years in lag time may be saved; regeneration problems often associated with clear-cutting will be alleviated, planting costs reduced or eliminated, and the forest gene pool maintained.