Conifer seedling response to northern temperate forest gaps

Studies of gap dynamics have contributed significantly to our understanding of the role of small-scale disturbance in forest ecosystems, but have been little used by foresters for predicting tree responses to partial cutting. Fifth year growth and survival of five commonly planted tree species were contrasted in canopy gaps (single-tree to 5000 m(2)), in the forest understory and in the open conditions of a clear-cut. There were strong and consistent trends in growth response among the tree species as gap size increased. Growth of all the species increased rapidly from small single-tree gaps to about 1000 m(2) gaps, but thereafter, showed little change up to 5000 m(2). Tree size and current growth rates for all species were highest in full open conditions. Performance of lodgepole pine (Pinus contorta), the most light-demanding species, exceeded that of all other species in large gaps (1001-5000 m(2)) and clearcuts. In large and medium gaps (301-1000 m(2)), the largest trees of all species were found in the middle gap position and there was little difference between the sunny north and shady south positions, except for lodgepole pine which clearly grew poorly in the south position. Total size and growth rates of all species were nearly identical in small gaps (20-300 m(2)) and again in the forest understory. The light advantage expected off the north end of high latitude gaps was not a benefit for tree growth, suggesting that below-ground effects of canopy edge trees have an important influence on seedling growth in these forests. Differences in mortality among the tree species were evident, with the most light demanding species having the greatest early mortality in shaded areas of medium gaps, in small gaps and in the forest understory. Careful matching of tree species to gap size and gap position can minimize early mortality and maximize growth rates. Opening sizes need not be very large (0.1-0.2 ha or larger) in order for most tree species to achieve growth rates similar to those found in the open conditions of clearcuts. (C) 2000 Elsevier Science B.V. All rights reserved.