Graduate Thesis Or Dissertation

 

Factors limiting the distribution of the sensitive lichen Usnea longissima in the Oregon Coast Range : habitat or dispersal? Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87ps788

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  • The sensitive lichen Usnea longissima Ach., formerly a fairly common circumboreal species, has been extirpated from much of its range (e.g., Eastern Europe). Although the U.S. Pacific Northwest (PNW) remains a relative stronghold for the species, U longissima faces increasing pressure in the region from habitat loss, air pollution, and commercial harvesting. Usnea longissima has a patchy distribution at both stand and landscape levels in the PNW, which may result from a lack of suitable habitat, dispersal limitations, or both. Although the species has been researched extensively in Scandinavia, no studies have examined its habitat requirements or dispersal dynamics in the PNW. I used two approaches to assess the relative importance of habitat versus dispersal limitations as influences on the current distribution of U longissima in the Oregon Coast Range. First, I collected environmental data from sites where U longissima was present (n=75) and absent (n=75) to determine habitat associations for the species. In addition to identifying the variables that best predict habitat occupied by U longissima, analysis of the data also yielded a statistical model that I combined with raster-based GIS modeling to assess the availability of apparently suitable habitat for the species across the study area. Second, I placed 360 transplants among 12 sites in 4 habitats within the study area, and measured their growth (change in biomass) after one year. Habitats were determined from analysis of the environmental data described above, and represented a range of apparent suitability for the species, from sites of unlikely suitability where it did not occur (i.e., clear cuts on south-facing slopes), through highly suitable sites where the species was abundant (i.e., old stands on north-facing slopes). Statistical analysis of the environmental data from sites of presence and absence produced a model incorporating four of the environmental variables, in which the most significant variable ( stand age) was positively associated with U /ongissima presence. The spatial analysis results suggest that apparently suitable habitat is not limiting at the landscape level. Additionally, the transplants grew well across all habitats, and gained the most weight in sites predicted to be the least suitable habitat. Overall, the results suggest that dispersal may playa more important role than habitat in limiting the distribution of U longissima within the study area, although the potential influence of habitat on establishment of the species should be assessed to lend certainty to this suggestion. Retention of remnant trees containing U longissima will enhance its dispersal within regenerating stands, and preservation of intact stands harboring the species will increase its chances of spreading to uncolonized stands and persisting across the landscape.
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