Forest development in canopy openings in old-growth Pseudotsuga forests of the western Cascade Range, Oregon

Size and age structure, and spatial pattern analysis were used to study the population dynamics of old-growth Pseudotsugamenziesii (Mirb.) Franco forests of the western Cascade Range, Oregon, U.S.A. Tree population structures confirmed that in theabsence of fire, Pseudotsuga was replaced by Tsuga heterophylla (Raf.) Sarg. and (or) Abies amabilis (Dougl.) Forbes.Regeneration patterns of these species reflected establishment in canopy openings or under Pseudotsuga, but not under Tsuga.Abies amabilis occurred more frequently as seedlings and saplings in openings than did Tsuga because of favourable aspects of itslife history. The composition and structure of the pioneer forest developing after fire was a determinant of subsequentregeneration. If Pseudotsuga dominated, Tsuga and Abies invaded the stand at an early stage. Regeneration in stands whereTsuga was a large component, however, was limited by dense canopies and the occurrence of canopy openings. These patternsdemonstrate the importance of small-scale disturbances such as tree falls in the dynamics of Pacific Northwest conifer forests.