Abstract:
The response to various levels of stand density by natural Douglas-fir regeneration,
shrub and sprouting hardwood species was studied in the mixed-evergreen-forests of
southwest Oregon. Partial-cut old growth (harvested 22-31 years prior) and thinned evenaged (harvested 9-17 years prior) stand types were used as surrogates for intentionally managed uneven-aged stand types, which were not available for this study. A lower number of clearcut and uncut stands were also sampled in order to capture the full range of overstory densities.
Harvest preceded the establishment of: >95% of seedlings (trees 15-140 cm height) in thinned (970 seedlings/ha) and partial-cut (1300 seedlings/ha) stand types and, 82% of saplings (trees 1.4-5.0 m height, w/ conical crown shape) in partial-cut (370 saplings/ha) stands. These results indicate that harvest stimulated natural regeneration of Douglas-fir. However, regeneration was not evenly spaced throughout the plots. For
example, seedling frequency averaged 0.45 in thinned and 0.46 in partial-cut stands (0-1.0
scale). Frequency and density of regeneration tended to increase as stand density index (SDI) decreased. Height growth of regeneration generally increased as SDI decreased. Sapling height growth in partial-cuts was greatest on steep, south facing slopes compared to other slope and aspect combinations. The fastest growing third of individual seedlings had an average annual leader growth of 10 cm/yr. and saplings 38 cm/yr. Shrub cover averaged 61% and increased with time since harvest and on north facing slopes (adj. r2 = 0.33, p < 0.001). Shrub cover was not related to density or frequency of Douglas-fir
regeneration but it was negatively related to height growth of regeneration. Results from this study indicate that natural regeneration of Douglas-fir in partially
harvested stands is sufficient to maintain a significant component of this species in the
stands with no further management, i.e. shrub control or future harvest. However,
reductions in stand density and shrub cover could likely increase frequency, density, and
growth rates of regeneration in an uneven-aged management scenario.
