Gap Size, Within-Gap Position and Canopy Structure Effects on Conifer Seedling Establishment

1 Emergence, establishment and growth of Abies amabilis, Pseudotsuga menziesii and Tsuga heterophylla were studied for 2 years in variously sized canopy gaps created in four stands on the west slope of the Cascade Range in central Oregon and southern Washington, USA. Seedlings originating from seeds sown on controlled microsites were compared with natural seed rain. 2 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. Some evidence of gap partitioning by regenerating seedlings was found, though all species were most abundant in shaded portions of gaps. 3 Seedling size increased with gap size, and was greatest at gap centres. Pseudotsuga growth was relatively low except in the largest gaps, Tsuga growth increased dramatically with gap size, and Abies growth responded the least to gap size. 4 Seedling establishment and growth differed among the four stands. Establishment in closed-canopy areas was lowest in stands with dense conifer canopies relatively close to the forest floor. Natural establishment of Tsuga in gaps was very low in mature stands but abundant in old-growth stands, reflecting differences in seed rain. 5 Silviculturally created openings may accelerate the development of multiple canopy layers in mature forests, but gap size and the availability of shade-tolerant tree seeds will also control the rate and spatial pattern of canopy development.