Graduate Thesis Or Dissertation
 

Biodiversity in managed forests of western Oregon : species assemblages in leave islands, thinned, and unthinned forests

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/08612q92q

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  • Both leave islands, or green tree retention clusters, and thinning prescriptions have been proposed as alternative silvicultural strategies designed to sustain the structural and biological diversity of managed forests. However, the relationship of the physical structure of leave islands and thinned forests to their associated microclimates, flora, and fauna remain largely unknown. We evaluated habitat and biota after forest thinning from 600 to 200 trees per hectare with three sizes of leave islands. Specifically, we used analysis of variance, species occupancy pattern assessments, and community analysis methods to examine differences in habitat and vascular plant, arthropod, amphibian, and mollusk abundance and diversity with respect to thinning and leave island size in four western Oregon managed forest stands. We found multiple treatment effects of thinning and leave island size relative to microclimate and vascular plant diversity and ground cover. The microclimate and vascular plant species composition differed between thinned and unthinned forest while conditions within leave islands approximated conditions in unthinned forest. Proportions of exotic and early-successional species and species ground cover were higher in thinned forest than unthinned forest and higher in small leave islands than larger leave islands. Treatment effects on arthropod, amphibian, and mollusk density were mixed. Of 118 parameters analyzed, negative effects of thinning on faunal species were detected for five arthropod species, low-mobility arthropod captures, one salamander species, one salamander family (Plethodontidae), amphibian species richness, and one mollusk species. Of 83 parameters assessed, positive effects of leave island size were found for arthropod species richness, overall density, density within six functional group measures, and for six species groups. Treatment effects of leave island size were mixed for amphibians and mollusks with positive effects of leave island size for overall mollusk density, snail density, and density within three mollusk species groups. Indicator species analyses identified seven vascular plant and two arthropod species indicative of thinned forest, 0.2 ha and 0.4 ha leave islands. Assessments of species occupancy patterns revealed insights regarding the potential utility of managing the forest matrix for habitat heterogeneity. For example, 71(19%) taxa occurred only in leave islands and 139 (37%) taxa occurred only in leave islands and unthinned forest. These patterns may indicate occurrences of rare species and do not necessarily indicate associations with these unthinned forest types. Community analyses highlighted the importance of addressing multiple spatial scales in forest management prescriptions by identifying distinct biotic assemblages occurring at forest type, study site, and mountain range scales. Our results suggest that leave islands may provide refugia for some low-mobility, ecologically sensitive species in managed forests of the Pacific Northwest.
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