Lichens are an important part of the biota in western Oregon forests, where they perform valuable ecological roles and contribute significantly to biodiversity. Lichens in western Oregon are threatened by a number of factors including air pollution and land use practices. If we wish to maintain the persistence of lichens in future landscapes it is critical that we understand the responses of lichen communities and individual lichen species to the environment and forest structure. This dissertation explores factors that are related to differences in lichen community composition and the distributions of individual lichen species in the western Cascades of Oregon, using a large landscape scale data set. I sought to relate major gradients in lichen community composition to environmental factors, and describe differences in lichen communities with respect to forest age (Chapter 2). I found three major gradients in lichen communities at a landscape scale in the western Oregon Cascades. These gradients were related to climate as expressed by elevation and annual temperature, air quality, north-south position, the richness of epiphytic macrolichens, and forest age. I developed a rarity score, which can be used to identify hotspots of rare species diversity at a landscape scale (Chapter 3). I then built descriptive models of this rarity score to identify abiotic and biotic factors associated with the occurrence of rarity hotspots. I found that models of rarity score that used explanatory variables based on lichen community composition performed better than models that used explanatory variables based solely on environmental factors. I narrowed my focus to the level of individual species responses to the environment and forest structure by developing habitat models for 11 lichen species in the western Cascades (Chapter 4). We selected these species because they performed important ecological roles, were rare across the landscape and associated with old growth forests, or because their distributions were poorly understood. These models can be used to increase the efficiency of landscape level surveys for rare species, predict the response of these species to forest management practices, and understand factors associated with the distributions of these lichens. / Graduation date: 2006