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Penobscot Experimental Forest

The year 2025 marked the 75th anniversary of the establishment of the Penobscot Experimental Forest in east-central Maine. This almost 3,900-acre experimental forest was established in 1950 as a site for long-term U.S. Forest Service research, and operational-scale research has continued on site ever since. As the only experimental forest located in the transitional zone between the eastern broadleaf and boreal forests known as the Acadian Region, the Penobscot Experimental Forest serves as an important and unique source of information about the ecology and silviculture of mixed-species northern conifers. With over 450 publications, multiple active studies, and strong university, state, and industry collaboration, research from the Penobscot Experimental Forest continues to sustain the most forested state in the nation.

Background Information and History

[image] Map of Penobscot Experimental Forest with study area boundaries.

Early experiments conducted on the Penobscot Experimental Forest were guided by regional forest management needs and questions of interest to public and private land managers, including the forest products industry in New England. The first experiments on the forest investigated various silvicultural practices such as selection, shelterwood, and other forms of partial cutting. Over time scientific investigations expanded to explore a variety of aspects of ecosystem structure and function. This included tree and understory vegetation composition, forest health, leaf area and photosynthesis, genetic diversity, wildlife-habitat relationships, nutrient cycling, and carbon storage.

Experimental forests such as the Penobscot provide unique research opportunities across time and generations. In addition to continuing studies over decades, sites of older studies have been repurposed to answer new questions or apply new technologies. 

The research conducted on the forest has proved to be very important to people who live and work in the forest economy of this region. The research also provides a lot of the foundational information about good and bad approaches to forest management (silviculture). Forest Service and university scientists on the forest work with students, host classes, and mentor graduate students to help ensure that a trained cadre of foresters is prepared to tackle issues of forest management and sustainability into the future.

About Penobscot Experimental Forest

Experimental exploitative cutting on the PEF, 1950s.

Unlike most Forest Service experimental forests, the Penobscot is not federally owned. The land was purchased by nine industrial and land-holding companies and leased to the Forest Service for research. In 1994 the landowners donated the property to the University of Maine Foundation. Today the Forest Service continues its work on the forest in collaboration with the University.

Current work by the Forest Service on the forest reflects a long-term agency commitment to research that supports sustainable management of northern conifer, or eastern spruce-fir, forests on national forests and beyond. As early as the late 1800s, the Forest Service was conducting research on the ecology and silviculture of spruce (Picea spp.) and balsam fir (Abies balsamea) throughout New England and the Adirondacks of New York. Early work in this forest type focused on restoring the production potential of stands degraded by exploitative cutting and included cutting trials in New Hampshire and New York. Those studies set the stage for later research on the forest in Maine, which evolved to address sustainability of production, composition, and structure, as well as the ecological effects of silviculture and exploitative cutting.

Ecology

Aerial view of forest at Penobscot Experimental Forest

The Penobscot Experimental Forest is in the Acadian Forest, which lies between the eastern broadleaf and boreal forests of the northeastern U.S. and adjacent Canada. From northern New England into southern Quebec and the Atlantic provinces, the Acadian Forest contains a diverse mixture of softwood and hardwood species. Red spruce (Picea rubens), balsam fir, and eastern hemlock (Tsuga canadensis) are the most common trees and occur in mixture with eastern white pine (Pinus strobus), northern white-cedar (Thuja occidentalis) and hardwoods such as red maple (Acer rubrum), birch (Betula spp.), and aspen (Populus spp.). The forest is naturally multi-aged with high structural and compositional diversity because stand-replacing natural disturbances are rare and the dominant species are long-lived and tolerant of shade. Bryophytes (e.g., Bazzania) and lichens (e.g., Cladonia) are abundant, and the Penobscot supports a number of obligate wetland species, such as northern bugleweed (Lycopus uniflorus) and three-seeded sedge (Carex trisperma). Common understory plants include Canada mayflower (Maianthemum canadense), starflower (Trientalis borealis), and wild sarsaparilla (Aralia nudicalis). Nonnative invasive plants such as glossy buckthorn (Frangula alnus) and shrub honeysuckle (Lonicera spp.), though uncommon in research areas, can be found in some parts of the forest. The Forest Service is working with the University of Maine to remove and reduce the spread of nonnative invasive plants.

[photo:] Blackman Stream, Penobscot Experimental Forest.

Soils on the Penobscot Experimental Forest are the result of glacial processes and the principal parent material is glacial till derived from fine-grained sedimentary rock. Low till ridges consist of well-drained soils including loams, stony loams, and sandy loams, while flat areas between ridges are poorly and very poorly drained loams and silt loams.

Low areas along Blackman Stream and Chemo Pond, and in depressions, are underlain by lake and marine sediments: poorly drained silt and silty clay loams. The climate is cool and humid. Mean annual temperature is 44.2 °F, with January the coldest (17.4 °F) and July the warmest (68.6 °F). Mean annual precipitation is 40 inches, with 42 percent falling during the 156-day growing season between May and October.

Facilities Information

[photo:] U.S. Forest Service, Northern Research Station office on the PEF.

The Forest Service maintains an office at the Penobscot Experimental Forest which includes meeting space, a library, a file room, and several restrooms. This facility may be available to visiting researchers upon request. Please contact the site manager if you wish to utilize the space.

Visitor Information

[photo:] PEF sign at the intersection of Route 178 and Government Road.

The Penobscot Experimental Forest is located at 44°51′ N, 68°37′ W, off of Route 178 in Bradley and Eddington, Maine, which is 10 miles north of Bangor. Members of the public are welcome to visit. The main road through the forest, Government Road, is open to the public. Access beyond Leonard’s Mills, a nonprofit forest and logging museum accessed from the Penobscot Experimental Forest, is restricted between late November and early May. More information about the forest, and directions to nature and demonstration trails, can be obtained by visiting the Forest Service office at 54 Government Road, about 0.25 miles from the intersection of Route 178 and Government Road.

If you would like to schedule a tour of the Forest Service’s research on the Penobscot, please contact the lead scientist. Inquiries about University of Maine activities on the Penobscot, or recreational opportunities, should be directed to the University Forest Manager. Please note that campfires and camping are not allowed.

Two foresters measure the diameter of a large tree in a forest

Numerous short- and long-term studies have been conducted by Forest Service scientists and cooperators on the Penobscot Experimental Forest over the years, resulting in an invaluable research site and database. The earliest work on the Penobscot Experimental Forest was a large-scale experiment investigating the outcomes of 10 different silvicultural treatments; this work continues through the present day. Manipulative experiments initiated by the Forest Service include strip clearcutting, prescribed burning, a range of thinning studies, provenance studies, and rehabilitation treatments. These studies created a range of stand structures and compositions in proximity to one another, with stand-level replication. This means that the Penobscot Experimental Forest serves as an ideal location for observational studies requiring diverse forest structures and developmental pathways.

Current Research

Three women foresters and a dog in the forest holding timber marking paint

The largest and longest-running study on the Penobscot Experimental Forest is a silvicultural experiment called a “compartment study” covering more than 400 acres. This study is about 75 years old and consists of a dozen treatments representing a range of even- and uneven-aged silvicultural and exploitative cutting practices including variations of selection cutting, regular and irregular shelterwood cutting, diameter-limit cutting, and reference (no harvesting). Each treatment is applied to two stand-level management units, and treatments are repeated over time as appropriate. There have been more than 12 harvests in some management units since the study began.

The Penobscot Experimental Forest is also the site of two long-term Forest Service studies on the effects of precommercial and commercial thinning in naturally regenerated, even-aged stands. Forest Service investigations of biomass harvesting and prescribed burning for fuels management are among the oldest in the Acadian Forest and an experiment in rehabilitation of cutover stands is now approaching its twentieth year. In addition, there are replicates of a range-wide eastern white pine provenance study and white spruce (P. glauca) progeny tests on the Penobscot Experimental Forest.

In addition to these manipulative silvicultural experiments, the Forest Service and its cooperators have initiated hundreds of related studies on the Penobscot Experimental Forest. Many take place in the long-term experiment, which offers a range of stand compositions and structures within proximity to one another. Research focuses on effects of disturbance on the forest ecosystem, and on underlying mechanisms of tree growth. Researchers also monitor forest health, including progression of beech bark disease and beech leaf disease on plots established in the 1970s. Short- and long-term research studies include assessments of forest productivity, regeneration and recruitment, tree quality and value, wildlife-habitat relationships, nutrient cycling, understory plant communities, carbon storage, and more. In addition, the University of Maine has a large-scale, long-term experiment in ecological forestry, called the Acadian Forest Ecosystem Research Program, which complements existing Forest Service research on the Penobscot Experimental Forest.

Cross-site Research

Though initially focused on local and regional topics, the Forest Service scientists at the Penobscot Experimental Forest developed partnerships to address forest management and other issues of national and international concern. The Penobscot is currently one of 21 official experimental forests in the Northern Research Station’s Experimental Forest Network, and one of 81 in the National Experimental Forest and Range Network. Data collected at the Penobscot Experimental Forest, combined with data from other experimental forests, are used to assess carbon sequestration across the region and to determine the effects of silviculture on forest growth and yield across the United States.

Conducting Research

Scientists who wish to conduct research on the Penobscot should contact the University Forests Office at the University of Maine. The University Forests manager will direct your request to the appropriate organization (e.g., the Forest Service) if you wish to work in the long-term study area. Note that destructive sampling is limited within the Forest Service’s long-term study and prohibited on permanent sample plots.

Long-term Monitoring and Data

People in hard hats kneel on the forest floor amidst ferns and small marker flags

All data collected by the Forest Service in its long-term (1950s to present) silvicultural experiment on the Penobscot Experimental Forest are maintained in a relational database. These data have been collected on permanent sample plots before, after, and at intervals between treatments. Each experimental unit (compartment, or management unit) is about 20 acres in size; permanent sample plots cover 15 percent of the area and consist of nested circular plots. Tree measurements are taken on sample trees as small as 0.5 inches in diameter at breast height, and include species, diameter, and condition; seedlings have also been measured by species and height class since the 1960s. Since the 1970s, individual sample trees have been assigned identification numbers and tracked over time. Since the 1990s, a subsample of tree heights, crown widths, crown lengths, and tree locations is taken; ground cover is recorded; and trees continue to be measured after they die. This intensive sampling has resulted in more than one million tree measurements, which are available to researchers.

Most data from the Forest Service’s long-term silvicultural experiment on the Penobscot is available with metadata and supporting documentation through our Data Catalog. Requests for additional data should be directed to the Lead Scientist.

Data Catalog

We maintain a number of data sets associated with our long-term studies. Those that are published and available online are described below. The links in this section open new windows containing the data publication information, which include links to the actual data or data web site.

Softwood forest on the left, and regenerating hardwood forest on the right

Principal findings of Forest Service research on the Penobscot Experimental Forest can be summed up in two words: silviculture matters. If managers wish to manage stands dominated by shade-tolerant conifers in a way that maintains species composition and productivity, then they must focus on retaining desired species and establishing regeneration in the partial shade of the overstory.

If, instead, trees are harvested opportunistically without consideration of the residual stand or new cohort, then stands are likely to convert to sprouting and shade-intolerant hardwoods. This distinction is well-illustrated by comparing diameter-limit and similar exploitative treatments to selection and shelterwood. Exploitative treatments resulted in undesired shifts in composition, quality, and structure, whereas the latter are well-stocked with vigorous conifers. The outcomes of this work have motivated changes in forestry practice and policy throughout the region and beyond.

[photo:] Poorly stocked, poor-quality stand resulting from experimental exploitative cutting on the PEF.

Another important finding from Forest Service research on the Penobscot is that results change over time, particularly when harvests occur multiple times in the same stand over decades or longer (as they often do in managed forests). Effects of individual treatments on ecosystem components, and similarities and dissimilarities between treatments, rarely remain constant over time. In addition, external factors like extreme weather events and availability of markets for certain tree species or wood products can change on-the-ground outcomes, financial returns, and whether a chosen management pathway is deemed successful and sustainable. Results from studies at the Penobscot Experimental Forest help forest managers expect the unexpected and minimize losses.

Though the Penobscot Experimental Forest studies are long-term from a human perspective, they cover a small fraction of the lifespan of dominant tree species. We already know that some treatments have undesired outcomes, but it is unknown which are most productive or sustainable over the long-term.

To read the latest long-term silvicultural results, check out Managing Your Woodland: Forestry Research Translated for Landowners.

Tour U.S. Forest Service research at the Penobscot Experimental Forest and see how different kinds of silviculture and harvesting have changed the forest over more than half a century. Learn from experts at the University of Maine, Maine Forest Service, Maine Audubon, and others around the state about how to manage your woodlot for the future, considering timber production, climate change, pests and disease. View an extended version of this video: Forestry for the Future: Lessons in Sustainable Management from Maine.

There are few places in the region where you can see the long-term outcomes of different types of forest management in one location or interact with researchers working cooperatively on natural resource questions. The Penobscot Experimental Forest is one such place and has a long history of technology transfer. Scientists and cooperators host numerous tours each year, and provide education and research opportunities to college students in natural resource fields.

Woman standing at the edge of a forest with an orange cap and orange forester's vest, holding timber marking paint

Undergrad Resources

The Forest Service works with our University partners to provide employment and training opportunities for undergraduate students in the fields of forestry and natural resource management. Students participate in field data collection, data entry, timber marking, treatment application, laboratory work, infrastructure maintenance, and archiving efforts.

Grad Resources

The Forest Service’s study area and data are available to graduate student researchers, and are in constant demand for a wide range of investigations. More than 80 theses in the fields of silviculture, wildlife biology, entomology, remote sensing, forest health, ecophysiology, and more reflect the diversity of research topics pursued within the long-term silvicultural experiment on the forest. Students and faculty who wish to pursue studies in collaboration with the Forest Service on the Penobscot Experimental Forest must submit a proposal through the University Forests Office.

A group of foresters stand in a circle in the forest listening to a speaker
Photo Credit
Courtesy photo by Logan Johnson

During this field tour, participants observed and learned of the results of shelterwood with precommercial and commercial thinning at the Penobscot Experimental Forest.

Professionals

Forest Service and University of Maine staff are available to give tours of the research to land management professionals and discuss results and their implications.  Although field tours can be arranged anytime, access during the winter is often limited.  In addition, researchers working on the Penobscot are always available for one-on-one consultation with clients.  Results of research on the forest are regularly presented at regional, national, and international professional and scientific meetings.

 

Though the Penobscot Experimental Forest was established by the Chief of the Forest Service to address research issues of regional concern, our partnership with the University of Maine and attention to questions at larger scales are critical to our long-term success.  Over time, our collaborations and scope have grown, resulting in greater applicability of results.

Partnerships

Log landing with a large stack of logs and logging equipment

Collaboration with the University of Maine is the cornerstone of the Forest Service’s research program on the Penobscot. The largest study on the Penobscot aside from the Forest Service’s long-term silvicultural experiment is the University’s Acadian Forest Ecosystem Program, which explores the outcomes of ecological forestry. That study was initiated in the mid 1990s and has generated numerous findings outside the scope of the Forest Service experiment. The University Forests Office is responsible for managing areas not currently in research, and for overseeing administration of the forest. A Research Operations Team consisting of Forest Service scientists and University faculty coordinates activities and reviews proposals for new research installations.

Forest Service scientists at the Penobscot Experimental Forest work with practitioners to co-develop research that meets the needs of forest managers on national forests and beyond. These collaborations with foresters and other natural resource managers from forest industry, corporate landowners, conservation organizations, state and federal agencies including the National Forest System, and tribal nations ensure that research and technology transfer from the Penobscot Experimental Forest is relevant to those working in the forest in New England and beyond. 

In addition, many scientists from other agencies and institutions in the U.S. and Canada commonly use the Penobscot Experimental Forest for their work.

Biomass Harvesting and Prescribed Fire

This research collaboration with the USDA Northeast Climate Hub, Maine Forest Service, University of Maine, and University of Massachusetts repurposed a slash disposal study established on the Penobscot Experimental Forest in 1964-65 to answer new questions about long-term effects of biomass harvesting on site and stand productivity. Work is continuing today with repeated harvesting, burning for fuels mitigation, and productivity assessment. Funding is provided by the U.S. Forest Service, Northern Research Station, Maine Forest Service, University of Maine, and Northeastern States Research Cooperative. Related research in collaboration with the National Forest System and University of Maine paired experimental sites at the White Mountain National Forest and Penobscot Experimental Forest to determine the effects of different levels of overstory retention on fuels availability for prescribed burning in temperate forests.

Advisory Committees

The Research Operations Team, a committee consisting of researchers from the Forest Service and University of Maine, works with the University Forests Office to manage activities on the Penobscot. The Research Operations Team reviews proposals for new studies outside the existing experiments, approves the expenditure of a portion of the funds earned from forest management, and works with the University Forest Manager to set policies for the forest.

Key Personnel

Lead Scientist

  • Person
    Laura S. Kenefic, PhD

    Laura S. Kenefic, PhD

    Team Leader and Supervisory Research Forester
    https://research.fs.usda.gov/about/people/laura.kenefic

Site Manager

  • Person
    USDA Forest Service Logo

    Skylar Roach

    Forester
    https://research.fs.usda.gov/about/people/skylar.roach

Collaborators

  • Keith Kanoti, University of Maine, School of Forest Resources. Keith is the University Forest Manager.

Videos: Research at Penobscot Experimental Forest

Videos: Penobscot Experimental Forest 70th Anniversary

Related Links

Understory Publications

Selected Publications

Download a comprehensive list of all publications that use data collected from the Penobscot Experimental Forest by Forest Service scientists and other researchers.

Last updated August 10, 2026
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