Invasion by trees into a grassy bald on Marys Peak, Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/cn69m736g

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  • Grassy balds are meadows that occur on or near the summits of montane peaks and ridges. The sites on which balds are located generally fall within the climatic tolerance ranges of associated tree species. This study evaluates the stability of a forest-meadow transition zone of such a bald on Marys Peak in the Oregon Coast Range. The habitats for invasion by trees are described in terms of ecotone types based on forest tree species and dominant meadow taxa. Four Abies procera ecotone types, the Festuca/Carex (FC), the Festuca/Elvmus/Agrostis (FHA), the Festuca/Smilacina/Viola (FSV), and the Agrostis/Festuca (AF), were identified using ordination and clustering analysis. Active invasion by trees into the adjacent meadow vegetation is occurring along locations occupied by all four Abies procera ecotone types. Three Pseudotsuga menziesii ecotone types were also defined. The trees in the Pseudotsuga menziesii ecotone types either are not encroaching into the meadows or occur in such limited extent they pose no immediate threat to the meadows. The Abies procera ecotone types are distributed along gradients of light, moisture, and growing season length. There is a trend toward decreasing light in the order FC > FSV > AF > FEA, and for moisture availability in the order FEA > FSV > FC > AF. Meadow vegetation responds to these gradients both in terms of species composition and the relative importance of competition among species. The anticipated importance of competition for the meadow vegetation of these types is ranked in the order FC > FSV>̲ FEA > AF. Tree establishment into these meadow habitats is controlled in part by abiotic factors and in part by competitive interference from, and physical barriers created by, meadow vegetation. The intensity of tree invasion (numbers and sizes of trees) and vigor (growth rate) of trees suggest rates of forest encroachment into the meadows in the order FEA >̲ FSV > AF > FC. Microsites that permit seedling establishment, weather conditions that enhance microsite availability, and coincident mast seed years facilitate tree seedling recruitment. Safe-sites for tree seedling establishment in the FC are associated with interruptions of the heavy sod layer and reduced herbaceous competition. Peak establishment years in the FC are related to heavy spring snowpack, low June rainfall, and occasionally to high August rainfall. These weather conditions reduce the competitive effects on Abies procera seedlings of the graminoid sward which dominates the FC type. Sites for tree seedling establishment in the AF are generated in part by natural disturbance to the grass sward, in the form of soil creep, shading, and root competition from dominant forest trees. Peak establishment years appear related to increase in total summer precipitation. Peak recruitment for the FEA and FSV types occurs when spring snowpack is low. Implications of these results are 1) that tree invasion along FC ecotones requires reduction in competition from dense grass sedge- dominated vegetation, 2) that competition is naturally ameliorated by soil-creep disturbances in the grass-dominated AF, and increased moisture facilitates establishment/and 3) that invasion of both the FEA and the FSV requires increased growing season length and is not competition limited. Thus, forest expansion is extremely slow because it requires the coincidence of available safe sites for tree seedling establishment, years of heavy seed production, and ecotone type-specfic weather patterns which facilitate seedling recruitment.
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  • File scanned at 300 ppi (Monochrome, 24-bit Color) using Capture Perfect 3.0.82 on a Canon DR-9080C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2013-07-17T21:55:19Z (GMT) No. of bitstreams: 1 MageeTeresaK1985.pdf: 6546665 bytes, checksum: a9163d3c684e16331323771aa5cd8051 (MD5)
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