United States Department of Agriculture Forest Service Potential Vegetation, Disturbance, Pacific Northwest Plant Succession, and Other Aspects of Region Umatilla National Forest Ecology Forest F14-SO-TP-09-00 May 2000 David C. Powell The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of multiple use management of the Nation’s forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and man- agement of the national forests and national grasslands, it strives – as directed by Congress – to provide increasingly greater service to a growing nation. The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orienta- tion, and marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabili- ties who require alternative means for communication of program information (Braille, large print, audio- tape, etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720- 5964 (voice or TDD). USDA is an equal opportunity provider and employer. ii Potential Vegetation, Disturbance, Plant Succession, and Other Aspects of Forest Ecology David C. Powell U.S. Department of Agriculture, Forest Service Pacific Northwest Region Umatilla National Forest 2517 SW Hailey Avenue Pendleton, OR 97801 Technical Publication F14-SO-TP-09-00 May 2000 iii AUTHOR DAVID C. POWELL is a silviculturist, U.S. Department of Agriculture, Forest Service, Umatilla Na- tional Forest, 2517 SW Hailey Avenue, Pendleton, Oregon 97801. ABSTRACT Historically, a dominant perspective in ecology viewed forests as stable ecosystems existing in harmony and balance with their environment. Many land managers were educated during an era when college in- struction emphasized this steady-state view of the natural world. The last three decades saw a period of rapid ecological change for literally millions of acres in the Blue Mountains. Some of this change was related to normal growth and maturation (e.g., plant succession), but much of it resulted from insect and disease impacts. Initially, the high impact levels caused land managers to question the health of ecosys- tems, but eventually they began to realize that these disturbance processes were not producing the catas- trophic results that would have been predicted by their education and training. With the advent of ecosys- tem management in 1992, and as a result of changing attitudes about disturbance processes, land manag- ers began to think about plant succession, competition, and other aspects of forest ecology in a different way. The primary objective of this document is to describe the historical perspective (an equilibrium or balance-of-nature philosophy) and compare it with the current paradigm (a non-equilibrium or dynamic equilibrium concept). Keywords: Ecological amplitude, tolerance, competition, disturbance, plant succession, Blue Mountains, old-forest structure, relay floristics, initial floristics, seral status, vegetation classification, potential vege- tation, management implications, historical range of variability, landscape ecology, forest health. TITLE PAGE PHOTOGRAPH The Last Roundup. Each fall during the second week in September, Pendleton, Oregon is home to the Pendleton Round-Up rodeo. During Round-Up week, Pendleton holds several parades in which the Uma- tilla National Forest traditionally entered a float. In 1975, the Forest’s parade float commemorated two disturbance events – an outbreak of Douglas-fir tussock moth in mixed-conifer forests, and a mountain pine beetle infestation in lodgepole pine forests. Over 350,000 acres of the Umatilla National Forest were rapidly defoliated by tussock moth between 1972 and 1974, at which point the outbreak collapsed on its own after a native virus spread through the insect population. In 1974, DDT was applied to some of the defoliated area, although a relatively small proportion of the Umatilla’s outbreak was treated (32,706 acres). On the parade float, the small grave in the lower right portion of the photograph has a tombstone that reads “Tussock Moth R.I.P.” When this photograph was acquired, mountain pine beetle was just as- suming preeminence as a landscape-level disturbance agent; an outbreak that began in 1968 on the Wal- lowa-Whitman National Forest (near Johnson Rock Lookout) grew steadily from there – by the fall of 1975, almost one and half million acres of lodgepole pine in the Blue Mountains were infested with pine beetles. This Round-Up float reflects prevailing attitudes of that era – disturbance processes were viewed as an aberration and it was predicted they would diminish in importance once forest management success- fully transformed overmature (decadent) stands into vigorous, pest-free communities. Subsequent devel- opments – an intense spruce budworm outbreak and numerous landscape-scale wildfires – suggest that these predictions may have been optimistic, eventually causing managers to reevaluate their preconcep- tions about the role of disturbance processes in forest ecosystems. ACKNOWLEDGEMENTS This report benefited from reviews by PAUL HESSBURG (Pacific Northwest Research Station, Wenat- chee Forest Sciences Laboratory) and CHARLIE JOHNSON (Area Ecologist for the Malheur, Umatilla, and Wallowa-Whitman National Forests). iv TABLE OF CONTENTS Introduction......................................................................................................................................1 Ecological amplitude .......................................................................................................................1 Tolerance and competition...............................................................................................................6 Vegetational variation......................................................................................................................7 Disturbance ......................................................................................................................................9 Old forest structure in dynamic landscapes .............................................................................14 Plant succession .............................................................................................................................17 Equilibrium paradigm ..............................................................................................................18 Non-equilibrium paradigm.......................................................................................................21 Relay floristics .........................................................................................................................23 Initial floristics.........................................................................................................................25 Seres and seral stages.....................................................................................................................27 Vegetation classification................................................................................................................35 Management implications..............................................................................................................37 Historical range of variability ........................................................................................................39 Forest health.............................................................................................................................43 Landscape ecology.........................................................................................................................44 Scale.........................................................................................................................................47 Fragmentation ..........................................................................................................................50 Glossary .........................................................................................................................................52 Appendix 1: Plant names ...............................................................................................................58 Appendix 2: Life history information............................................................................................60 Literature Cited ..............................................................................................................................72 FIGURES Figure 1–Effect of slope and aspect on solar radiation at 50° north latitude ............................................... 2 Figure 2–Variations in cone serotiny for lodgepole pine............................................................................. 3 Figure 3–Extreme environmental events have an important influence on ecosystems................................ 4 Figure 4–Vegetation zones of the Blue Mountains...................................................................................... 5 Figure 5–Idealized temporal and spatial relationships among selected disturbance regimes ...................