Graduate Thesis Or Dissertation
 

Stand development and individual tree morphology and physiology of young Douglas-fir (pleudotsuga menziesii) in association with tanoak (Lithocarpus densiflorus)

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  • Effects of tanoak (Lithocarpus densiflorus) on early growth, morphology, and physiology of Douglas-fir (Pseudotsuga menziesii) were studied to determine developmental characteristics of young stands and mechanisms of competition. Growth, leaf area, and biomass of Douglas-fir, tanoak, and shrubs / herbs were monitored for six years after establishing a gradient of tanoak cover with and without suppression of shrubs / herbs. Growth trajectories for Douglas-fir and tanoak were related to initial tanoak cover, a variable that can be predicted using inventory data from the pre-harvest stand. Morphology, duration, and relative rates of growth of Douglas-fir were studied for three growing seasons to determine how the accumulating effects of competition caused tree size to vary. Tanoak competition reduced the size and number of buds, internodes, and needles for Douglas-fir shoots. In a given year, Douglas-fir growth was limited by competition in previous years via reduced potential growth (i.e., internode number for shoot growth and stem circumference for diameter growth) and by competition in the current year via reduced attainment of potential growth (i.e., expansion of internodes and basal area growth per unit stem circumference). To describe mechanisms of competition, variables of microclimate (air / soil temperature, light, and soil water) and physiology (xylem pressure potential, conductance, and photosynthesis) were measured in each of eighteen consecutive months on Douglas-fir of different competitive status. Throughout the year, Douglas-fir shaded by tanoak had light-limited rates of photosynthesis. During the growing season, both shaded and unshaded Douglas-fir in tanoak-dominated areas experienced elevated leaf temperatures, lower relative humidities, and reduced soil water availability, resulting in limited rates of photosynthesis.
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