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A plant association classification for McDonald-Dunn Forest

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  • The purpose of this study was to develop a plant association classification for the mature coniferous forests of the McDonald and Paul M. Dunn Research Forests. A secondary objective was to quantify diversity within the plant associations. This Forest is approximately 11,000 acres and is located 6 miles to the north of Corvallis Oregon. This forest is owned and managed by the College of Forestry at Oregon State University. This study, and a companion study by Leavell (1991), was funded by the Research Forest to broaden their understanding of the plant communities on the Forest. The companion study developed relationships between the plant associations and the environment. Classification was based on 108 plots, using percent cover of trees, shrubs, forbs, and grasses. This sampling density of approximately 1 plot per 100 acres is much greater than most community classifications in the area (Hemstrom & Logan, 1986; Hemstrom et al., 1987; Topik et al., 1988; Juday, 1976). A total of 117 vascular plant species were encountered; 68 of these were used in classification. Stands sampled for this classification were primarily seral, yet the most mature available. Few climax stands were available for sampling. Plant associations were developed using TWINSPAN, a two-way indicator species analysis (Hill, 1979b). Six plant associations were developed and described in this study: Tsuga heterophylla/Acer circinatum-Gaultheria shallon Abies grandis/Acer circinatum-Gaultheria shallon Abies grandis/Disporum hookeri-Thalictrum occidentale Abies grandis/Polystichum munituin Abies qrandis/Rubus ursinus-Rhus diversiloba Abies randis/Brachypodium sylvaticum One plant association, Abies grandis/Acer circinatum- Gaultheria shallon, parallels a previously described plant association described by Juday in the Valley Margin Zone (Juday, 1976). The other 5 plant associations described in this study are original descriptions. The TSHE/ACCI-GASH, ABGR/ACCI-GASH, and the ABGR/RUURRHDI plant associations are shrub-dominated. ABGR/DIHOTHOC and ABGR/POMU are forb-domjnated associations, and ABGR/BRSY is a grass-dominated plant association. Plant associations were shown to have significant differences in species richness, heterogeneity, evenness, and in vertical structure. Average species richness per plot is highest in the ABGR/ACCI-GASH and ABGR/DIHO-THOC plant associations, which also has high relative diversity (Shannon's index) and evenness. Species richness is lowest in the TSHE/ACCI-GASH plant association, probably because it was represented only by seral stands. Structural diversity included an analysis of snags. Snag number, size, and distribution were not related to plant associations. Current snag levels indicate that at most, 30 percent of the maximun populations for six woodpecker species could be supported on this Forest. A description of each plant association is given. The description for each includes: extent, location, characteristic species, species richness and percent cover within four strata (trees, shrubs, forbs, and grasses), heterogeneity (Shannon' s diversity index), and an evenness measure.
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