Chapter 3: Old Growth, Disturbance, Forest Succession, and Management in the Area of the Northwest Forest Plan Thomas A. Spies, Paul F. Hessburg, Carl N. Skinner, Klaus and mitigation) are addressed mainly in chapter 2 of this J. Puettmann, Matthew J. Reilly, Raymond J. Davis, Jane A. report. Although, our effort is primarily based on pub- Kertis, Jonathan W. Long, and David C. Shaw1 lished literature, we bring in other sources where peer-re- viewed literature is lacking, and we conduct some limited Introduction analyses using existing data. We are guided by the NWFP In this chapter, we examine the scientific basis of the monitoring questions, those from federal managers and assumptions, management strategies, and goals of the our reading of the past three decades of science. Northwest Forest Plan (NWFP, or Plan) relative to Old-growth forests can be viewed through many the ecology of old-growth forests, forest successional ecological and social lenses (Kimmins 2003, Moore 2007, dynamics, and disturbance processes. Our emphasis is Spies and Duncan 2009, Spies and Franklin 1996). Socially, on “coarse-filter” approaches to conservation (i.e., those old growth has powerful spiritual values symbolizing that are concerned with entire ecosystems, their species wild nature left to its own devices (Kimmins 2003, Moore and habitats, and the processes that support them) (Hunter 2007), and many people value old growth for its own sake 1990, Noss 1990). The recently published 2012 planning (“intrinsic” values, sensu Moore 2007). Old growth also has rule has increased emphasis on land management rooted many “instrumental” or useful functions, including habitat in ecological integrity and ecosystem processes, using for native plants or animals (e.g., the northern spotted owl coarse-filter approaches to conserve biological diversity [Strix occidentalis caurina]), carbon sequestration (Har- (Schultz et al. 2013). Fine-filter approaches (e.g., species mon et al. 1990), and other ecosystem services. No single centric), which are also included in the 2012 planning viewpoint fully captures the nature of the old-growth issue rule, are discussed in other chapters. We synthesize new as it relates to federal forest management. We focus here findings, characterize scientific disagreements, identify on ecological perspectives (Kimmins 2003, Oliver 2009, emerging issues (e.g., early-successional habitat and Ruggiero et al. 1991, Spies 2004, Spies and Franklin 1996), fire suppression effects) and discuss uncertainties and many of which are overlapping conceptually and in com- research needs. We also discuss the relevance of our mon parlance. Old growth is many things at the same time; findings for management. Climate change effects on vege- for example, old growth is: tation and disturbance and possible responses (adaptation • An ecosystem “distinguished by old trees and related structural attributes. Old-growth encom- passes the later stages of stand development that 1 Thomas A. Spies is a senior scientist, U.S. Department of typically differ from earlier stages in a variety of Agriculture, Forest Service, Pacific Northwest Research Station, 3200 SW Jefferson Way, Corvallis, OR 97331; Paul F. Hessburg characteristics including tree size, accumulation is a research landscape ecologist, U.S. Department of Agricul- ture, Forest Service, Pacific Northwest Research Station, 1133 of large dead woody material, number of canopy N Western Ave., Wenatchee, WA 98801; Carl N. Skinner is a layers, species composition and ecosystem function” geographer (retired), U.S. Department of Agriculture, Forest Ser- vice, Pacific Southwest Research Station, 3644 Avtech Parkway, (USDA FS 1989). Redding, CA 96002; Klaus J. Puettmann is a professor, Oregon State University, Corvallis, OR 97331; Matthew J. Reilly is a • An ecological state resulting from interactions among postdoctoral researcher, Humboldt State University, Department of successional, disturbance, and ecosystem processes Biological Sciences, 1 Harpst Street, Arcata, CA 95521; Raymond J. Davis is a wildlife biologist and Jane A. Kertis is an ecologist, (e.g., nutrient and carbon cycles, microclimate). U.S. Department of Agriculture, Forest Service, Pacific Northwest Region, 3200 SW Jefferson Way, Corvallis, OR 97331; Jonathan • A biological condition defined in terms of life histo- W. Long is an ecologist, U.S. Department of Agriculture, Forest ries and demographics of forest plant species. Service, Pacific Northwest Research Station, 1731 Research Park Dr., Davis, CA 95618; David C. Shaw is a professor, Oregon State • A habitat for particular fauna, flora, and fungi. University, Corvallis, OR 97331. 95 GENERAL TECHNICAL REPORT PNW-GTR-966 We define old-growth forests based on live and O’Hara et al. 1996, Spies 2006, and Davis et al. 2015 for dead structure and tree species composition (see below). more discussion of old-growth or old-forest definitions). Old-growth forests in the NWFP area differ with age, According to Spies and Franklin (1988), old-growth is part forest type, environment, and disturbance regime (Reilly of a structural and compositional continuum of successional and Spies 2015, Spies and Franklin 1991). The variability stages that varies by environment. According to O’Hara et and complexity of site conditions, forest succession, and al. (1996), speaking of frequently disturbed environments, disturbance processes make defining old-growth difficult old forest is a part of the successional continuum that varies or impossible under a single definition. Under the U.S. by environment and disturbance processes, which have the Department of Agriculture (USDA), Forest Service ability to advance or retard succession. (USDA FS 1989) definition (above), the only features To operationalize the successional continuum con- distinguishing old-growth from other forests, across all cept of old-forest development, Davis et al. (2015) created forest types, are the dominance or codominance of old, an old-growth structure index (OGSI) to characterize the large, live and dead trees (multiple canopy layers are not degree of old-growth structure (“old-growthiness” cali- necessarily a defining characteristic). For example, in brated by potential vegetation type) that occurs in a stand fire-frequent historical forest types, old-growth forests of any age or history, for use in mapping and monitoring have large old live and dead trees, but amounts of dead- in the Plan area. Two definitions for late successional/old wood are low, canopies are generally open, and areas with growth were then created: OGSI 80 (structural conditions multiple canopy layers are uncommon (Dunbar-Irwin and commonly found in forests that are 80 years and older) Safford 2016, Safford and Stevens 2016, Youngblood et al. and OGSI 200 (structural conditions that are represen- 2004) (fig. 3-1). tative of forests containing trees that are more than 200 In the NWFP, “older forests” were defined as “late-suc- years of age). These classes roughly correspond to the cessional/old-growth” based largely on stand developmental definitions used by FEMAT, the Forest Ecosystem Man- and successional patterns of Douglas-fir/western hemlock agement Assessment Team (FEMAT 1993), for mature (Pseudotsuga menziesii/Tsuga heterophylla) forests (Frank- trees (80 to 200 years old) (e.g., “late- successional” in lin et al. 2002) (fig. 3-2). This multilayered closed-canopy the NWFP) and old growth (>200 years) but have the old growth (e.g., canopy cover >80 percent) was the focal advantage of being structure based and calibrated to dif- point of old-growth conservation during the development of ferent potential vegetation types. Also, given that this is a the NWFP, but as we shall argue, old growth is far more continuous index, other age/development thresholds (e.g., diverse than that and functions quite differently across the 120 years) could be used for mapping and monitoring. range of the northern spotted owl. “Older forests” in the We note that the structure index and definitions used original NWFP includes mature forests, 80 to 200 years of in the monitoring program are based on current forest age—a pre-old-growth stage, known somewhat confusingly conditions from forest inventory plots, which means that in as “late-successional”2 in the Plan), and old-growth forests. fire-frequent dry zone forests, the structure and composition Old-growth has been defined in the NWFP and elsewhere as of old growth is a product of 100 years or more of fire forests containing large and old, live and dead trees, a exclusion and highly altered forest development processes. variety of sizes of other trees, and vertical and horizontal Inventory definitions for dry, old forests based on densities heterogeneity in tree clumps, gaps, and canopy layering (see of large-diameter fire-tolerant trees have been developed for the eastern Washington Cascade Range (Franklin et al. 2007a). However, definitions and indices of dry, fire- 2 Most of the time in this document, we use the term “late suc- cessional” to refer to vegetation that is in the later stages of forest dependent, old-growth forest structure at stand and land- succession where age, height, and biomass are near maximum and scape scales are still needed for the larger NWFP area (see shade-tolerant species are the primary understory or overstory tree species. This broad class would include old growth according to below for further discussion). classic definitions in textbooks (Barnes et al. 1998). 96 Synthesis of Science to Inform Land Management