DOES RIPARIAN FOREST RESTORATION THINNING ENHANCE BIODIVERSITY? THE ECOLOGICAL IMPORTANCE OF LARGE WOOD 1 Michael M. Pollock and Timothy J. Beechie 2 ABSTRACT: Intact riparian ecosystems are rich in biological diversity, but throughout the world, many have been degraded. Biodiversity declines, particularly of vertebrates, have led to experimental efforts to restore riparian forests by thinning young stands to accelerate creation of large diameter live trees. However, many ver- tebrates depend on large diameter deadwood that is standing as snags or fallen to the forest floor or fallen into streams. Therefore, we reviewed the sizes of deadwood and live trees used by different vertebrate species to understand which species are likely to benefit from different thinning treatments. We then examined how ripar- ian thinning affects the long-term development of both large diameter live trees and deadwood. To this end, we used a forest growth model to examine how different forest thinning intensities might affect the long-term pro- duction and abundance of live trees and deadwood. Our results suggest that there are long-term habitat trade- offs associated with different thinning intensities. Species that utilize large diameter live trees will benefit most from heavy thinning, whereas species that utilize large diameter deadwood will benefit most from light or no thinning. Because far more vertebrate species utilize large deadwood rather than large live trees, allowing ripar- ian forests to naturally develop may result in the most rapid and sustained development of structural features important to most terrestrial and aquatic vertebrates. (KEY TERMS: riparian ecology; rivers/streams; restoration; forests; fish; fluvial processes; forest management; forest structure; large wood.) Pollock, Michael M. and Timothy J. Beechie, 2014. Does Riparian Forest Restoration Thinning Enhance Biodiversity? The Ecological Importance of Large Wood. Journal of the American Water Resources Association (JAWRA) 50(3): 543-559. DOI: 10.1111/jawr.12206 INTRODUCTION Throughout much of the world, old, complex, and biologically diverse forested riparian ecosystems have been cleared and replaced by young, structurally sim- ple forests (Naiman et al., 1993; Sala, 2000; Strom- berg et al., 2004). In the Pacific coastal states of North America, extensive degradation of riparian for- ests has led to the decline of numerous aquatic and terrestrial species that depend upon them, a number of which are now listed under the United States (U.S.) Endangered Species Act (ESA) (Nehlsen et al., 1991; USDA and USDI, 1994). In such forests, much of the decline in biodiversity is due to the loss of four major structural features; large live trees, large snags, large down wood on the forest floor, and large down wood in streams (Bauhus et al., 2010; Bunnell and Houde, 2010; Marcot et al., 2010). Notable among the ESA-listed species that utilize riparian ecosys- 1 Paper No. JAWRA-13-0084-P of the Journal of the American Water Resources Association (JAWRA). Received April 3, 2013; accepted March 14, 2014. © 2014 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the USA. Discussions are open until six months from print publication. 2 Research Scientists, Northwest Fisheries Science Center-Watershed Program, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. E, Seattle, Washington 98112 (E-Mail/Pollock: michael.pollock@noaa.gov). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA 543 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 50, No. 3 AMERICAN WATER RESOURCES ASSOCIATION June 2014