- Tree seedling establishment and growth were studied in experimental canopy gaps to determine the effect of structural and environmental heterogeneity on species dynamics within mature and old-growth Douglas-fir forests in the Cascades of central Oregon and southern Washington. Factors examined included forest age, gap size, within-gap position. substrate, year of germination, density of understory vegetation, shading by woody debris, light, moisture, and surface temperature. Four gap sizes, with gap diameter to tree height ratios ranging from 0.2 to 1.0, were created in the fall of 1990. Seeds of Abies amabilis, Pseudotsuga menziesii, and Tsuga heterophylla were sown in controlled microsites and germination, survival, and growth were monitored for two growing seasons. Seedlings established from natural seed rain were also monitored in natural microsites. Seedling establishment was greater in gaps than in closed-canopy areas. but was relatively low in portions of large gaps exposed to direct solar radiation, particularly for Tsuga. Shade from woody debris facilitated establishment in exposed areas, while shade from understory vegetation aided establishment of Pseudotsuga but not of Tsuga. Establishment in shaded portions of gaps declined with vegetation cover, with a greater effect from herbs than from shrubs. Seedling establishment was greatest on decayed wood in closed-canopy areas and small gaps, was similar on mineral soil, litter, and wood in shaded portions of largest gaps, and was lowest on litter in exposed portions of large gaps. Species establishment patterns differed between years, apparently due to
differences in weather. Seedling growth increased with gap size, and was greatest in gap centers. The growth response to increasing light was greatest for Tsuga.
Some evidence for gap partitioning by seedlings was found, although all species
were most abundant in similarly shaded portions of gaps. Establishment and growth of
the different species was related to seed size and interacting gradients of above- and
below-ground resource levels. Heterogeneity at the seedling scale often over-rode
environmental gradients associated with gap size and within-gap position. While gaps
may accelerate development of old-growth characteristics in these forests, establishment of Tsuga heterophylla is particularly sensitive to microsite quality.