Graduate Thesis Or Dissertation

Logs as sites of tree regeneration in Picea sitchensis-Tsuga heterophylla forests of coastal Washington and Oregon

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  • Logs are seedbeds for trees in many Picea sitchensis-Tsuga heterophylla forests of Washington and Oregon. Factors affecting this close association, including seed retention and seedling establishment on logs and successional changes in log characteristics, are explored In this study. Field and laboratory experiments indicated that competition with vegetation on the forest floor favored tree seedling development on logs. Reciprocal transplants of soil and log blocks revealed differences in substrate quality and not position effects, such as standing water, produced the "nurse-log" phenomenon. Clearing vegetation significantly increased survival of planted and natural conifer seedlings above survival on uncleared plots. Experiments on the effects of predation and soil pathogens indicated these interactions were of minor importance. Logs are therefore sites where competitive effects are sufficiently small to allow abundant seedling recruitment. Log surfaces vary widely in their ability to retain seeds and needles. Moss- and litter-covered surfaces retained 48-98% of seeds and needles, whereas sound and rotten wood and bare bark retained 0-8%. A model of seedling establishment on log surfaces in Picea-Tsuga forests, which incorporated the effects of seed retention and seedling survival rates, indicated 1.3% of the seed crop would produce one-year-old seedlings on logs but only 0.02-0.18% on undisturbed forest floor. Surficial litter accumulations enable Picea and Tsuga seedlings to establish and grow rapidly on slightly decayed logs. Survival rates for the first two years of both species increased asymptotically with litter biomass with a maximum of 62% for Picea and 34% for Tsuga. Seedling growth was fastest when canopy openness and litter biomass were high, but slow when either factor was low. Successional development influences tree seedling recruitment and survival on logs. Changes in bark, bryophytes, humus and trees were examined for a chronosequence of Picea, Pseudotsuga and Tsuga logs. Bark fragmentation was a critical process; it removed plants and humus, reinitiated succession and was responsible for major differences in successional patterns among log species. Although a large proportion of tree seedlings establish on logs in Picea-Tsuga forests, high mortality rates caused by competition, bark fragmentation and toppling from logs indicated long-term survival on logs was very low and possibly equivalent to the forest floor.
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