United States Department of Agriculture True Fir Spacing and Forest Service Yield Trials—20-Year Pacific Northwest Research Station Update Research Paper PNW-RP-590 Robert O. Curtis May 2013 D E E P R A U R T LT MENT OF AGRICU Non-Discrimination Policy The U.S. Department of Agriculture (USDA) prohibits discrimination against its customers, employees, and applicants for employment on the bases of race, color, national origin, age, disability, sex, gender identity, religion, reprisal, and where applicable, political beliefs, marital status, familial or parental status, sexual orientation, or all or part of an individual’s income is derived from any public assistance program, or protected genetic information in employment or in any program or activity conducted or funded by the Department. (Not all prohibited bases will apply to all programs and/or employment activities.) 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All Other Inquiries For any other information not pertaining to civil rights, please refer to the listing of the USDA Agencies and Offices for specific agency information. Author Robert O. Curtis is an emeritus scientist, Forestry Sciences Laboratory, 3625 93rd Avenue, SW, Olympia, WA 98512-1101. Abstract Curtis, Robert O. 2013. True fir spacing and yield trials—20-year update. Res. Pap. PNW-RP-590. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 32 p. This report updates data and comparisons from previous reports (Curtis and others 2000, Curtis 2008) on a series of precommercial thinning and yield trials in high-elevation true fir–hemlock stands, using data from the 12 replicates for which 20-year data are now available. The stands were varying mixtures of Pacific silver fir (Abies amabilis (Douglas ex Loudon) Douglas ex Forbes), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and noble fir (Abies procera Rehder). Trends noted in the 2008 report continue. Increment in top height was somewhat reduced at wide spacings. Volume increment and basal area increment increased as the residual number of trees increased. There was a small increase in diameter increment of the 80 largest trees per acre at wide spacing. Relative stand densities on the unthinned plots, as measured by stand density index (SDI) and relative density (RD), are still increasing; it appears that the maximum (not yet attained) is probably at least 20 to 25 percent greater than values observed in Douglas-fir. Keywords: Precommercial thinning, increment, yield, Abies amabilis, Tsuga heterophylla, Abies procera, true firs, noble fir, Pacific silver fir. RESEARCH PAPER PNW-RP-590 Summary This report updates data and comparisons from previous reports (Curtis and others 2000, Curtis 2008) on a series of precommercial thinning and yield trials in high- elevation true fir–hemlock stands, using data from the 12 replicates for which 20-year data are now available. The stands were varying mixtures of Pacific silver fir (Abies amabilis (Douglas ex Loudon) Douglas ex Forbes), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and noble fir (Abies procera Rehder). Treatments included five levels of the residual number of trees plus an unthinned treatment. Trends noted in the 2008 report continue. Increment in top height was somewhat reduced at very wide spacings. Volume increment increased but at a decreasing rate as the residual number of trees increased, similar to trends observed in other thinning studies. Basal area increment increased with the residual number of trees at wide spacings, but was nearly constant for 430 and 700 trees per acre. There was an increase in diameter increment of the 80 largest trees per acre at wide spacing. Relative stand densities on the unthinned plots, as measured by stand density index (SDI) and relative density (RD), are still increasing. Maximum SDI and RD (not yet attained) will probably be at least 20 to 25 percent greater than corresponding values for Douglas-fir. Differences in height growth patterns between true firs and Douglas-fir, combined with the ability of true fir–hemlock stands to develop high basal areas, imply that true fir–hemlock stands of a given site index will be consid- erably more productive than Douglas-fir stands of the same numerical site index. Continuation of these trials for the next several decades should provide reliable information on development patterns and silvicultural responses that is needed as part of the process of developing models to guide management decisions. ii True Fir Spacing and Yield Trials—20-Year Update Contents 1 Introduction 2 Previous Research 2 Past Management 3 The Study 4 Objectives 4 Study Area 4 Study Design 6 Plot Establishment and Measurement 9 Data Summarization 9 Height Estimates 9 Volume Estimates 10 Site Index Estimates 10 Species Composition 11 Analyses 12 20-Year Increment Analyses 18 Yield Analyses 23 Relative Stand Density Limits 25 Discussion 28 Acknowledgments 28 Metric Equivalents 29 References 32 Glossary of Symbols and Acronyms iii RESEARCH PAPER PNW-RP-590 iv True Fir Spacing and Yield Trials—20-Year Update Introduction From 1987 to 1994, 18 precommercial thinning trials were established in true fir–hemlock stands along the west side of the Cascade Range in Washington and Oregon and in Washington’s Olympic Mountains. Establishment and 5-year results were reported by Curtis and others (2000), and 10-year results by Curtis (2008). The stands studied are within the Coastal True Fir–Hemlock type (Society of American Foresters type 226: Eyre 1980), primarily within the Abies amabilis zone (Franklin and Dyrness 1973) but also with two in the Tsuga mertensiana zone. The principal tree species are Pacific silver fir (Abies amabilis (Douglas ex Loudon) Douglas ex Forbes), noble fir (Abies procera Rehder), and western hemlock (Tsuga heterophylla (Raf). Sarg.). Other minor associated species include western redcedar (Thuja plicata Donn ex D. Donn), yellow cedar (Chamaecyparis nootkatensis (D. Don) Spach), mountain hemlock (Tsuga mertensiana (Bong.) Carrière), subalpine fir (Abies lasiocarpa (Hook) Nutt.), western white pine (Pinus monticola Douglas ex D. Don) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). True fir–hemlock forests at higher elevations in the Cascade Range and Olym- pic Mountains play major roles as protective cover for watersheds and as scenic and recreational areas, and have in the past been important as timber-producing forests. Large areas of Abies amabilis–zone forest exist in the Mount Baker–Snoqualmie (MBS), Gifford Pinchot (GP), Olympic (OLY), Mount Hood (MH), and Willamette (WIL) National Forests. There are large additional areas in the national parks in the region, and limited areas on Washington Department of Natural Resources (WDNR) lands and on private ownerships. There are also large areas of similar forests that are important timber-producing areas on Vancouver Island and in the This report is an update Coast Range of British Columbia. of the 10-year results This report is a partial update of the 2008 publication, with additional data and published in 2008. some differences in procedure. It repeats much of the background information given in the 2000 and 2008 reports and then presents some summaries and comparisons based on the 12 installations for which 20-year data are now available. Complete 20-year measurements (at 0, 5, 10, and 20 years from establishment) from all 18 installations will not be available for several years. It seemed desirable to carry out an interim analysis to (1) present currently available information, (2) provide guid- ance for later workers who may analyze the complete set, and (3) utilize the author’s acquaintance with the study from its inception. Any future analyst will not have this direct knowledge, and future capabilities may also be handicapped by prospec- tive personnel and funding limitations. 1 RESEARCH PAPER PNW-RP-590 Previous Research Although there is a considerable body of information on stand development, yields, and site evaluation for low-elevation hemlock, little information is available on the true firs and high-elevation true fir–hemlock mixtures. Herman and others (1978) and Hoyer and Herman (1989) developed height growth and site index estimates for noble fir and Pacific silver fir, based on stem analyses of trees from unmanaged old-growth stands.