Dry coniferous forests in the Western Oregon Cascades

Graduation date: 1981Pseudotsuga menziesii dominates the forests of the Pacific Northwest.\ud But though it is dominat, Tsuaa heteroohylla or Abies amabilis\ud is usually climax. Many researchers have studied Pseudotsuga on the\ud widespread mesic sites where it is seral, but few have examined the\ud relatively rare ecosystems in which Pseudotsuga or its associate\ud Libocedrus decurrens are the climax species. This is a study of the\ud composition, structure and successional dynamics of climax Pseudo-\ud tsuga and Libocedrus (dry site) forests in the central portion of the\ud Western Cascades in Oregon.\ud The environment of dry site forests is characterized at seven\ud reference stands (five dry sites) using predawn plant moisture stress\ud (Waring and Cleary, 1967) and temperature growth index (Waring et. al.,\ud 1972). As expected, the study type is hotter and drier than adjacent\ud Tsuga-climax sites. The data suggest that low moisture availability\ud is more critical to the occurrence of Pseudotsuga-climax habitat than\ud is high temperature.\ud Seventy-three vegetation plots are located throughout the study\ud area, 56 in dry site stands. The location, composition, and soils of\ud five plant communities, including two phases, are described based on\ud this data set. Information from fire scars and tree ages on the vegetation\ud plots indicates these forests burn at irregular intervals that\ud average 100 years. Since initiation of the oldest cohort, most stands\ud have experienced one or more fires which typically kill only a portion\ud of the trees.\ud Stand history and successional processes are investigated on two\ud intensive plots using primarily age structures and fire scars. These\ud stands have each been burned twice by fires that consumed only a portion\ud of the canopy. Regeneration following these fires was slow and\ud continued for a century or more.\ud Height growth of 40 dry site Pseudotsuga is examined and found to\ud start more slowly but continue at a greater rate later in life than\ud Pseudotsgua on mesic sites.\ud These characteristics of dry site ecosystems have several management\ud implications. A shelterwood silvicultural system is recommended\ud on dry sites. The overstory will ameliorate the hot, dry environment\ud and occupy the site during the long regeneration period. This silvicultural\ud system approximates the natural functioning of these systems\ud more closely than clear cutting.\ud Maximum mean annual increment occurs relatively later on dry sites\ud due to the slow, prolonged height growth. Relatively slow reproduction\ud further retards mean annual increment. Thus, if high volume\ud growth is a management goal, rotations must be longer than on mesic\ud sites.\ud Due to relatively linear height growth curves and reverse Jshape\ud diameter distributions on dry sites, McArdle et. al.'s (1961)\ud site index curves and yield tables are not applicable