GAP SIZE, WITHIN-GAP POSITION AND CANOPY STRUCTURE EFFECTS ON CONIFERSEEDLING ESTABLISHMENT

1 Emergence, establishment and growth of Abies amabilis, Pseudotsuga m enziesii and Tsuga heterophylla were studied for 2 years in variously sized canopy gaps created in four stands on the west slope of the Casc ade Range in central Oregon and southern Washington, USA. Seedlings or iginating from seeds sown on controlled microsites were compared with natural seed rain. 2 Seedling establishment was greater in gaps than i n closed-canopy areas, but was relatively low in portions of large gap s exposed to direct solar radiation, particularly for Tsuga. Some evid ence of gap partitioning by regenerating seedlings was found, though a ll species were most abundant in shaded portions of gaps. 3 Seedling s ize increased with gap size, and was greatest at gap centres, Pseudots uga 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 st ands 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 avail ability of shade-tolerant tree seeds will also control the rate and sp atial pattern of canopy development.