Conserving forest biodiversity while meeting global demand for wood products is an ongoing challenge for 21st century forest management. Naturally regenerated and regulated forests are typically heterogeneous but forest management intended to maximize wood production often homogenizes forest structure. Retaining green trees and dead wood at the time of forest harvest, termed structural retention, is a common practice for promoting biodiversity in forests managed intensively for wood production. Despite widespread implementation of structural retention and substantial body of research on effects of retention on forest biodiversity, there remains a need to evaluate alternative tree retention practices on biodiversity in specific forested regions and silvicultural systems. Further, ecologists are increasingly interested in the impacts of management actions on functional trait diversity, but few studies have quantified the effects of alternative retention practices on functional diversity of animal communities. I used a manipulative study with five experimental retention treatments to quantify effects of differing size, number, and location of retention tree patches on two animal taxa in clearcut forests within the U.S. Pacific Northwest. In my first chapter, I quantified the effects of experimental retention treatment on ground beetle species (Coleoptera: Carabidae) and functional (morphological) richness and variation in community composition between retention and clearcut areas. Although I did not find strong effects of retention treatment on either species or functional richness, I found less variation in community composition between retention and clearcut areas in the treatment that contained several small patches. This finding indicates that ground beetle communities supported by small, dispersed retention patches are redundant to those found in surrounding clearcut areas in managed forest stands. In Chapter 2, I quantified effects of retention treatment on abundance of three common small mammal species and found that generalist and forest species were more abundant in smaller isolated retention tree patches, and for one species this effect was related to higher levels of downed trees in small patches. For this species, smaller retention tree patches provided more structure than larger patches, partially related to increased probability of retention tree blowdown in the smaller patches. In Chapter 3, I related small mammal species and functional richness to retention treatment and found that species, but not functional richness was slightly higher in patches connected to forested riparian buffers compared to isolated upland patches. I conclude from these results that retention placement does not strongly influence the number of small mammal species present or their contribution to forest ecosystem function, but small increases in species richness can be obtained by grouping all trees adjacent to riparian zones. In Chapter 4, I analyzed abundance data from eleven small mammal species to look for evidence of competitive interactions between species. Contrary to expectations, I found that when the effects of retention treatment and environmental variation were accounted for, most species abundances were positively correlated. This finding suggests that environmental factors and not inter-specific competition structured small mammal communities in early seral forest plantations. Results from my research indicate that no single retention strategy is optimal among taxa for biodiversity conservation in clearcut forests of the Pacific Northwest, and implementing a combination of retention treatments among harvested stands favors diversity and abundance of the two animal groups I studied. My results further indicate that community responses to fragmentation of retained forest patches in clearcuts differs between taxonomic groups, highlighting the importance of clearly articulated biodiversity objectives (as opposed to a broad goal of conserving biodiversity). Lastly, my study demonstrated the importance of accounting for animal and plant communities that occur in areas surrounding patches when measuring the conservation value of small forest patches.
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