Spies, Thomas A. 2006. Maintaining old-growth forests. In: Haynes, Richard W.; Bormann, Bernard T.; Lee, Danny C.; Martin, Jon R., tech. eds. Northwest Forest Plan--the first 10 years (1994-2003): synthesis of monitoring and research results. Gen. Tech. Rep. PNW-GTR-651. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: 83-116. Chapter 6.