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Maine’s forests are shifting as a result of climate change

12,000 years ago, spruce was a major tree species in Maine. By 8,000 years ago, it had largely disappeared, replaced mainly by white pine and oaks. Not until about 1,000 years ago did spruce once again rise to prominence in Maine. What happened? The climate changed, with cooler, moister conditions favoring spruce and warmer, drier climates conducive to more temperate hardwoods. This teaches us an important lesson: when the climate changes, the forest changes too.

Are forest species expected to change?

Because the species composition of forests in a region is largely determined by temperature and moisture, forests around the world are already changing as these two key climate factors shift. In northern New England, for example, American beech is increasing and sugar maple, red maple, and birches are declining. Most projections for the future predict that spruce and fir, which are more northern species, will become less abundant, and more southern species, such as oaks, hickories, and white pine will become more abundant across the state – much like 8,000 years ago, as described above.

Maine supports a forest that is transitional between the eastern temperate deciduous to the south and the more coniferous boreal forest to the north. Many of Maine’s plant and animal species are at either their northern or southern boundary. Red and white spruce are northern species near their southern limit in Maine. White oak and shagbark hickory are more southern species at their northern limit in southern Maine today. The transitional nature of Maine means that the makeup of our forests are vulnerable to large shifts as climate change takes hold. These changes will make Maine forests less northern (e.g., less spruce and moose) and more temperate (e.g., more oaks and deer).

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Source: (Left) Red Spruce Forest near the St. John River in northern Maine. (Right) White Oak-Shagbark Hickory forest on Mt Agamenticus in southern Maine. Photos by Andrew Barton

Check out The Climate Change Atlas. This resource allows you to simulate how the distribution of most tree species in Maine (and elsewhere in the USA) are likely to change in the future. There’s one for birds too. Below are future projections for two of the many Maine tree species: red spruce (declining) and white oak (spreading). These projections are based on an optimistic climate warming scenario (4.5), which assume substantial cuts in emissions.

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How will climate change effect forest physiology?

Climate change will alter temperature, moisture, snowpack, growing season, and much more, all of which will affect the physiology of trees and functioning of forests. These fundamental impacts–on tree photosynthesis, water exchange, and growth–underlie how forests will change, for better or worse, in terms of their ability to take up and “sequester” carbon, provide wildlife habitat, and produce economic benefits. Sequestering (or storing) carbon is a critical aspect of solving the climate crisis: when forests store carbon in their plants and soils, they are keeping carbon dioxide (CO2) out of the atmosphere.

CO2 emitted into the air by the burning of fossil fuel (oil, natural gas, coal) is the main culprit in climate change, but it’s also the lifeblood of plants, which take up CO2 through tiny holes in their leaves. In the presence of sunlight and water, they convert CO2 into plant food: photosynthesis! In some situations, the extra CO2 humans have emitted into the air might have a “fertilizing” effect: more CO2, more photosynthesis. However, most plants can’t actually use this extra carbon because more growth outstrips water and nutrient supplies. Besides, in many areas, hotter conditions will also lead to more drought, slowing plant growth and therefore diminishing their ability to use extra CO2.

In fact, climate change will have negative and positive effects on the physiology of plants and functioning of forests. Longer growing seasons and warmer temperatures might be good for some trees. Drier conditions, heat waves, and more intense wildfires will be bad for many tree species. We have a lot to learn about the balance of negative and positive impacts for Maine forests, but there are reasons for serious concern.  If emissions aren’t drastically reduced, the negative impacts on both natural and agricultural plants would be catastrophic for most regions.

Source: Videography courtesy of Reed McLean, Ordway Grove, Norway, Maine

Will climate change intensify forest pests and natural disturbances?

Climate change will also influence other stresses on forests, such as forest pests and natural disturbances. Some pests, such as the native spruce budworm, might be reduced, but others, such as non-native hemlock woolly adelgid (see below) may become a more serious pest. Relegated to the warmer coast today, this species will likely spread to much of the state as the climate warms. This would seriously impact hemlock populations in Maine, one of the more common trees in our state. There’s also been concern that the southern pine beetle, which feeds on hard pines such as pitch pine, would eventually migrate into Maine. In fact, in just the past few years, small numbers of this serious forest pest have been documented in far southern Maine.

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Besides the pest problem, Maine forests would also face intensified natural disturbance. Storms get their energy from warm air, and thus global warming will intensify these natural disturbances. Maine is already experiencing storms that are dumping more rain over short, intense periods than before the climate change era. Wildfire is…well…a wildcard in the sense that whether Maine sees more fire than today will depend on the balance of drier, hotter conditions vs. more precipitation, both of which are predicted to increase. As a result, the state will need to be prepared for a higher incidence of wildfire.

Forest Physiology

How will the forest product industry will need to adapt to climate change?

Changes in Maine’s climate and forests will present challenges to the industry. The industry depends to some extent on harvesting on winter days when the ground is frozen and less subject to damage from heavy machinery. A reduction in such days has already been documented in Wisconsin, a state with climate and forests similar to those in Maine. Accommodating these kinds of operational changes will increase the cost of harvesting, unless alternative approaches are developed.

Just as important, the Maine forest products economy will need to respond to changes in the abundance of different tree species. As an example, less red spruce, the major species used for the paper industry, and more oak, a key species used in the production of lumber, will require changes in forest management and mill operations.

Forest Product Industry

How is forest management important for dealing with climate change?

The state has a large stake in the future of the forest products industry, both because it is an important economic driver and because forest management will have a large impact on the extent to which carbon will be stored in forests rather than released into the atmosphere. Forests have great potential as a “natural climate solution.” Recent calculations by the University of Maine and Colby College reveal that the amount of CO2 taken up by Maine forests (through photosynthesis) equals 60% of the total CO2 emissions from the entire state in a year (see figure below). In other words, our forests are playing a major role in keeping CO2 out of the atmosphere, where it would warm the planet. Managing Maine forests to increase the uptake and storage of carbon even further is becoming a key priority. For information on this, see the final report of Maine Climate Council Task Force on Creation of a Forest Carbon Program.

SCIENTIFIC ASSESSMENT OF CLIMATE CHANGE AND ITS EFFECTS IN MAINE This was a major source for much of the information in this summary of changes to Maine forests.

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Scientific Assessment of Climate Change and Its Effects in Maine. A Report by the Scientific and Technical Subcommittee (STS) of the Maine Climate Council (MCC). See Pages 230-247 on forests (image above, page 232). 2020. Augusta, Maine. 370 pp.

Forest Management

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