Graduate Thesis Or Dissertation


Synecology of the white-oak (Quercus garryana Douglas) woodlands of the Willamette Valley, Oregon Public Deposited

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  • Quercus garryana dominated plant communities are found in the interior coastal valleys and on foothills from southeastern Vancouver Island, British Columbia, south to San Francisco, California. They occur as savannas, intermixed with a variety of conifers, and in almost pure stands. The diversity of habitats occupied by the species and the variety of vegetation associated with it provide an opportunity for a basic synecological investigation of plant communities having a common, important member. The study was designed to describe the floristic composition and structure of the Quercus garryana communities, to determine their ecological relation to the physical environment, and clarify successional status. It provides a framework for future autecological investigations of the component species and is directed at furnishing a fuller understanding of the synecology of an important segment of the vegetation of western Oregon. Two levels of sampling were used during the study. The first was a rapidly applied, qualitative method. This reconnaissance technique was designed to provide an assessment of dominance, size-class distribution, and composition of the species comprising the vegetation. Information on influential habitat factors was also obtained. Sampling at this level enabled the examination of many stands, provided a record of each stand, and served as the basis for the selection of stands for more quantitative sampling. One hundred forty stands were examined during the reconnaissance phase of the study. Subjective evaluation of these stands supported by an analysis using marginal punch cards determined that only a few species were dominating the tall shrub and low shrub-herbaceous layers of the understory and that certain combinations of these dominant species were being repeated geographically through the study area. The second sampling phase was based on the reconnaissance information and was designed to provide a quantitative record of the species complexes or plant communities delineated during reconnaissance. This method was applicable by a single investigator and measured the dominance, frequency, size distribution, and density of tree species; and the percent coverage and percent frequency of shrubs, herbs, and grasses. Soil profile descriptions were made in each stand where the vegetation was quantitatively sampled. Salient features of the physical environment were also recorded as were indications of past and present land uses. Forty seven stands were sampled. Quercus garryana was the cverstory tree dominant in all of these stands. The understory pecies were found to fall into four major communities. From mesic to xeric these were: the Corylus cornuta/Polystichum munitum community, the Prunus avium/Symphoricarpos albus community, the Amelanchier alnifolia/Symphoricarpos albus community, and the Rhus diversiloba community. These communities were named for the species usually dominating the tall shrub and low shrub layers. Seven soil series served as the substrate of these communities; Steiwer, Carlton, Peavine, Dixonville, Nekia, Olympic, and Amity. The understory plant communities of the Quercus forest show considerable variation and intergradation. Changes are a matter of shifts in species dominance rather than alterations in species composition. The absence of environmental extremes in the Willamette Valley and heavy present and historical land use have increased this intergradation. The presence of large, open-form Quercus garryana trees surrounded by smaller forest-form trees indicates that the present Quercus forests have developed from a savanna. The cause of this change in gross physiognomy is probably the control of the repeated ground fires which swept the pre-settlement savanna. Mature Quercus trees are not harmed by ground fires, but such fires would tend to keep dense reproduction from occurring. The present abundance of Rhus diversiloba in the under story may be directly related to heavy livestock grazing. The interconnected root system of ground cover and liana-form Rhus provides the species with a grazing resistance mechanism. The liana Rhus is out of reach of grazing animals. Photosynthate transferred from the liana to the ground cover plants would aid the latter in retaining vigor even under heavy grazing pressure. Thus it would have an advantage over other plants that were also being grazed, eventually becoming the ground layer dominant. Both Pseudotsuga menziesii and Acer macrophyllum appear to be successional to Quercus garryana. Acer is better adapted to mesic sites than Quercus. Pseudotsuga will succeed Quercus on less mesic sites. The successional trends are promoted by livestock grazing which opens up the ground layer and facilitates seedling establishment especially for Pseudotsuga. On some sites Prunus avium, an introduced species, is becoming part of the overstory canopy. It reproduces vigorously in its own shade and will become an important member of the Quercus forest in the future. Quercus garryana reproduction was more abundant on drier, exposed sites and the species seems to be able to perpetuate itself on these locations.
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