CENTERPIECE OF RESEARCH ON THE PENOBSCOT EXPERIMENTAL FOREST: THE U.S. FOREST SERVICE LONG-TERM SILVICULTURAL STUDY John C. Brissette and Laura S. Kenefic Abstract.—Established between 1952 and 1957, the U.S. Department of Agriculture, Forest Service experiment comparing several silvicultural treatments is not only the centerpiece of research on the Penobscot Experimental Forest in Maine, it is also one of the longest-running, replicated studies of how management techniques influence forest dynamics in North America. Ten treatments representing even- and uneven- aged silvicultural systems and exploitative cutting are replicated twice on operational- scale experimental units averaging 21 acres in size. Treatments are applied uniformly to experimental units in accordance with prescriptions designed to direct both stand structure and composition. In some treatments harvests are scheduled at intervals (e.g., 5, 10, or 20 years); in others, harvests are triggered by stand conditions. Each experimental unit, or compartment (most recently termed management unit), has an average of 18 permanent sample plots (PSPs) for measuring attributes of trees ≥0.5 inches in diameter at breast height. Tree regeneration and other vegetation are measured on multiple subplots within each PSP. Measurements are taken before and after harvests and, in many treatments, at intervals between harvests. Over the past 60 years, this long-term experiment and associated short-term studies have generated fundamental knowledge about forest ecosystems and silvicultural guidelines for the northern conifer forest type, and, in a more general sense, have contributed to our understanding of mixed-species forest science and management. INTRODUCTION research has proceeded on the PEF with periodic harvests and regular re-measurement of treatment Between 1952 and 1957 the U.S. Department of effects on tree and stand growth and other response Agriculture, Forest Service established a long- variables. term silvicultural experiment on the Penobscot Experimental Forest (PEF) in Maine. It is currently A series of study plans has guided the long-term titled Silvicultural Effects on Composition, Structure, silvicultural experiment on the PEF. The most recent and Growth of Northern Conifers in the Acadian plan, by J.C. Brissette and L.S. Kenefic, was approved Forest Region: Revision of the Compartment January 2008 and was an update and revision of one Management Study on the Penobscot Experimental submitted by R.M. Frank, Jr. and approved in May Forest. This experiment was one of a series of similar 1975. Frank’s study plan superseded the original plan studies on experimental forests across the United of January 1953 by T.F. McLintock and subsequent States. These experiments were called “compartment revision by A.C. Hart in June 1962. Each of the management studies” because they were designed revisions updated the long-term study to adjust to around large, essentially operational-scale, changing research priorities, build on what had been experimental units (≈20-40 acres) (metric conversions learned thus far, and ensure the relevance of the are in Appendix I) known as compartments. Very experiment for future scientists and managers. Results few of those studies were continued as planned, but 60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 31 from the first 40 years of this study were summarized timeliness of treatment application, and the quality of by Sendak et al. (2003). This paper focuses on the the long-term database (Brissette et al. 2006; Kenefic experiment as it is being carried out under the current et al. 2006; Russell et al., this volume). We feel that study plan. Details about changes that have occurred these qualities more than make up for the shortcoming over the years in treatment structure and response in experimental design. variables can be found in metadata associated with the measured data (Brissette et al. 2012). The primary objective of the study is to quantify tree and stand response to silvicultural treatment. Response Much has changed in the 60 years since this study variables are regeneration; species composition; and was first conceived. Social and political ramifications tree and stand growth, productivity, and quality. These of forest management have brought debate about data provide information about the interaction of appropriate silviculture into the public arena. Logging natural and human disturbances and their effects on systems have advanced from hand felling and horse stand dynamics. To meet this objective, the hypotheses skidding to cut-to-length processors and forwarders. address some of the important unanswered questions However, many of the fundamental issues that about managing mixed northern conifer stands in the prompted installation of the study remain the same. region. For example: Do responses vary between… Spruce budworm (Choristoneura fumiferana) is still … managed and unmanaged stands? a threat and discussions continue about the role of … stands managed with clear silvicultural silviculture in reducing impacts during outbreaks. objectives and stands exploited for current timber Diameter-limit harvesting is still practiced and its production with no concern for future composition, long-term effects debated. For social, economic, and structure, or condition? biologic reasons, natural regeneration remains the predominant method of establishing new trees and … stands managed for one or two cohorts and stands in the northeastern United States, but many stands managed for multiple cohorts? questions about ensuring adequate regeneration of … stands that once regenerated are left to develop desired species are yet unanswered. Because of the naturally and stands that receive tending treatments silvics of the major species in the northern conifer such as cleaning or thinning? forest of which the PEF is representative—red spruce (Picea rubens Sarg.), balsam fir (Abies balsamea [L.] Because of the range of response variables measured, Mill), eastern white pine (Pinus strobus L.), eastern this experiment not only answers questions about hemlock (Tsuga canadensis [L.] Carr), paper birch whether treatments differ but also addresses how (Betula papyrifera Marsh), and red maple (Acer treatments differ and what about them differs. rubrum L.)—both even- and uneven-aged silvicultural systems can be used and no one system has achieved Defining hypotheses to test is an important part universal acceptance. Questions remain about the of study planning. But in a long-term experiment entire array of silvicultural options available to natural such as this one, the most enlightening outcomes resource managers. cannot be planned for; that is, an important aspect of this experiment is studying the unpredicted The long-term study on the PEF has experimental and unexpected. Although the unexpected cannot design limitations that cannot be corrected, the most be articulated in a hypothesis statement, it can be serious being only two replicates of the treatments (see stated that this study addresses questions about the Frank and Kenefic, this volume) and separation of the uncertainty inherent in any silvicultural treatment control from the rest of the experiment (Kenefic et al. because of the long-term nature of stand development 2005b). However, the study is unique because of its and the unpredictability of sporadic natural disturbance longevity, integrity of the original treatment structure, 32 GTR-NRS-P-123 Penobscot Experimental Forest: events and the likely prolonged effects of climate and closely related Great Lakes-St. Lawrence Forest change. In addition to understanding the various Region extends west through southern Quebec and pathways of stand development initiated by particular Ontario (Rowe 1972). The Boreal Forest Region lies silvicultural manipulations, managers need to know north of the Acadian and Great Lakes-St. Lawrence the likelihood of achieving their desired objectives regions. Maine juts into eastern Canada, with New along those pathways. Such knowledge is best attained Brunswick to the east and north, and Quebec to through long-term monitoring, where understanding the north and west. The Laurentian Mixed Forest increases incrementally with every measurement cycle. Province, Warm Continental Division (McNab and Avers 1994) north of Portland, Maine, has been A secondary objective of this study is to provide a identified with the Acadian Forest (Braun 1950). variety of forest structures at one location to be used as the framework for short-term experiments in ecology The PEF is located in the southern extent of the and silviculture (see Appendix II for some examples). Acadian Forest Region, in the towns of Bradley and The long-term experiment can best be described as Eddington in east-central Maine (44°54' N, 68°38' W) empirical; the short-term studies are often process- (Fig. 1). The dominant conifers are shade-tolerant oriented and thus can address why treatments differ. and regenerate well under canopy cover. Advance regeneration is prolific (Brissette 1996), and without Ultimately, results from this long-term experiment and it regenerated stands are converted to a hardwood associated short-term studies generate fundamental composition (Hart 1963). Balsam fir and spruce knowledge about forest ecosystems and science- species are the principal commercial softwoods. based management guidelines for northern conifers Though the amount and early growth rates of fir and associated species in the Acadian Forest Region