Graduate Thesis Or Dissertation

Two studies of Acer macrophyllum : I. The effects of bigleaf maple on soils in Douglas-fir forests : II. The ecology of bigleaf maple seedling establishment and early growth in Douglas-fir forests

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  • Bigleaf maple (Acer iaacrophyllunt) is a deciduous, persistent, sprouting species that frequently accounts for as much as 20% of the basal area in Douglas-fir forests. Because of the lack of knowledge of the role of bigleaf maple in Douglas-fir forest ecosystems and the problems that it poses as a vigorous competitor to commercial conifer seedlings, two studies were undertaken to determine its possible effects on forest soils, and to gain insight into the seedling establishment phase of its life cycle. Soil chemical and physical properties and forest floor and litterfall weights, nutrient content and forest floor turnover rates under bigleaf maple and Douglas-fir (Pseudotsuga !nenziesii) were compared on five sites on the eastern margin of the Oregon Coast Range. Litterfall weight and nutrient content were greater under maple on virtually every site for every macro and micro-nutrient. Forest floor biomass and nutrient content were extremely variable, much more so than litterfall, and there were no significant differences among the two species. However, turnover rates for forest floor biomass and nutrients were significantly faster under maple for every nutrient on every site. Bulk density of mineral soil was also highly variable with significant differences on only two sites. Soil nitrogen was generally greater under maple and there was a trend towards greater potassium under maple also. Amounts of calcium, magnesium and phosphorous showed no consistent trends. Soil organic matter content under maple was significantly greater than under Douglas-fir on 4 of 5 sites. The greater soil nutrients and organic matter under maple may be attributed to the more rapid forest floor turnover in that system. The establishment phase of bigleaf maple, a ubiquitous, deciduous hardwood in western Oregon's Douglas-fir forests, was studied in a variety of stands ranging from 1 to 150 years of age to identify those stages in Douglas-fir forest succession where bigleaf maple is most likely to become successfully established from seed. Germination rates of seed protected from rodents averaged from 30 to 40 % in all environments but typically less than 2 % of the unprotected seed germinated, indicating that seed predators play an important role in regulating seedling establishment. Seedling survival was highly dependent on light arid mortality after one growing season was particularly high in stands with greater than 90 % overstory cover. At least half of the first year mortality was due to browsing by burrowing rodents and invertebrates, with dessication as the second greatest cause of mortality during the first year. On plots that were monitored over two growing seasons, overwinter mortality was the second most frequent classification. Seedling survival was not related to soil moisture content or soil moisture tension. The highest survival rates (90 %) were in clearcuts and very open stands and the lowest (0 %) were in dense, young, conifer stands. Maple establishment in clearcuts will likely only be successful if seedlings escape shading by competing shrubs arid herbs. Optimum long term survival is most likely in Douglas-fir stands over 40 years of age.
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