Estimated reading time: 9 minutes
Key points:
- Increasing temperatures and changes in precipitation are already impacting Northwest forests, with further changes expected in the coming century.
- Northwest forests and woodlands are among the greatest carbon sinks in the United States.
- Drought, wildfire, insects, and disease pose significant risks to many types of forests and woodlands in the Northwest.
- Northwest vulnerability assessments can help managers, producers, and locals to understand how climate change is impacting forests near them.
The vast forests and woodlands of Alaska, Idaho, Oregon, and Washington support a sense of place, rural economies, cultural resources, wildlife habitat, and much more within the Northwest Climate Hub region. Near the coast, stands of Douglas-fir, western redcedar, Sitka spruce, and western hemlock tower 100 feet or more in the air, while through much of the interior, juniper and pinyon pine dot woodlands. Black spruce and aspen bow sparingly over Alaskan permafrost, while Oregon white oak grace valley woodlands with crooked shade. Roots, too, network across Northwest forests and woodlands, maintaining thick, biodiverse soil, while trees shelter bears, elk, birds, rodents, and mountain lions, to name only a few of the species that depend on these ecosystems. Northwest forests are also relied upon by humans, as they provide timber, food, over half of our water supply, recreation, bioenergy, and cultural values.
Northwest forests are among the greatest carbon sinks in the United States. Maintaining forests in Idaho, Oregon, and Washington alone could mitigate years of fossil fuel use in the western United States and enhance biodiversity. However, climate change will likely impact Northwest forests and woodlands and could threaten the ecosystems and services they provide.
How will climate change affect Northwest forests and woodlands?
Climate change projections for the end of the century suggest an increase in average annual temperature of 3.4–10°F in Idaho, Oregon, and Washington, and 2–15°F in Alaska. Precipitation projections are more uncertain than those for temperature. For Idaho, Oregon, and Washington, there is potential for a slight increase in precipitation (mainly in the winter) and more extreme precipitation events. Precipitation in Alaska is expected to increase 5–15 percent across the state, and more extreme precipitation events are likely. Lower snowpack is expected in much of the Northwest, with more precipitation falling as rain than snow, and snowpack melting earlier, resulting in longer dry seasons. Higher temperatures and longer periods between precipitation events are expected to lead to more frequent and severe droughts.
The trees and plants of Northwest forests are locally adapted to climate and tolerate moderate climatic changes. However, rapid and extensive climatic changes like those expected in the coming decades may exceed the capacity of some species to grow, reproduce, and survive. Northwest forests are changing rapidly because of increasing wildfire and insect and disease damage. As temperatures continue to increase, disturbance from drought, wildland fire, insects, and disease may drive further forest ecosystem change. Though some forests in the region could increase in productivity with increased carbon dioxide in the atmosphere and longer growing seasons, disturbances like wildfire and insect outbreaks are likely to offset those increases in productivity. Species will likely move in response to changing climate and disturbance, resulting in novel forest communities in the future. Specific responses to climate change in different forest types are discussed below.
Carbon and Northwest Forests
Though Northwest forests are considered some of the greatest carbon sinks in the United States, climate change threatens to alter the carbon balance of many of these forests. Idaho, Oregon, and Washington have some of the greatest natural terrestrial carbon storage potential within North America, and Alaska maintains 62 percent of the terrestrial carbon stocks found on federal land, as well as the two largest forests in the National Forest System. The trees in the boreal forest of Alaska have been able to amass large amounts of carbon in their roots and trunks. Similarly, permafrost, the largest terrestrial carbon sink in the world, underlays much of the soil beneath the boreal forest. As such, boreal forests account for 20 percent of the forest carbon storage globally.
Forest Types
Broadly, the forests of Alaska, Idaho, Oregon, and Washington can be categorized as temperate rainforest, dry mixed-conifer forest, subalpine forest, boreal forest, oak woodlands, and pinyon-juniper woodlands. These forests and woodlands are as varied as they are ecologically and culturally significant.
Mountain ranges in Idaho, Oregon, and Washington affect where precipitation falls and create different climates that affect forest composition. Topography and elevation also play a role in forest composition. Over 85 percent of forests in Idaho, Oregon, and Washington are dominated by coniferous trees. Douglas-fir is the most common tree in all three states. In Alaska, boreal forests cover the permafrost-underlain interior, and temperate rainforests occur in the Southeast, where it is warmer and wetter. Each forest type has a set of defining characteristics, disturbances, and responses to a changing climate.
Alaska, Oregon, and Washington are home to one of the rarest forest types in the world—the coastal temperate rainforest. The moist and temperate rainforests west of the Cascade Range in Oregon and Washington and from the Kenai Peninsula to Southeast Alaska support red alder, mountain hemlock, western hemlock, Sitka spruce, Alaska yellow cedar, bigleaf maple, and western redcedar, to name a few. Temperate rainforests are cooler but receive as much precipitation as tropical rainforests, with more than 170 inches of rain falling per year in Washington and Oregon, and 200 inches of rain per year in Southeast Alaska. High precipitation makes wildfire extremely rare, often allowing trees to reach more than 300 years in age. The primary disturbance mechanism in these forests is wind, which can topple older, large trees and create opportunistic openings in the canopy.
Climate projections suggest that iconic trees like Sitka spruce and western hemlock could be replaced by more drought resistant trees. In Alaska, trees with higher recruitment rates, like black and white spruce, might replace Sitka spruce and western hemlock. In Oregon and Washington, the persistence of Sitka spruce will depend on the persistence of coastal fog, which is uncertain. Drought-intolerant species like western hemlock will likely decrease in abundance with higher temperatures and more drought. Douglas-fir will likely continue to prosper in coastal forests. Such shifts in forest composition may have ecological and economic implications. Additionally, as the climate continues to warm, insect infestations, like those seen from the Sitka spruce beetle, could intensify.
In the United States, the boreal forest stretches south from Alaska’s Brooks Range to the northern edge of the Kenai Peninsula and covers 60-70 percent of the forested land area in Alaska. Because 6-8 months out of the year have temperatures below freezing, only a few tree species, such as black and white spruce, can survive in boreal forest. However, rising temperatures and the increasing frequency of large fires in Alaska are causing a shift from coniferous to deciduous trees, with older spruce forest giving way to trees like birch and aspen. Additionally, wildfires threaten to release carbon amassed in the trunks and roots of boreal trees, as well as the thawing permafrost beneath them. Invasive species such as earthworms, mountain pine beetle, and Siberian moth could move farther north and spread quickly in a warming climate, threatening boreal forests. Even native species like spruce beetle are becoming problematic as the climate warms. As a result, some studies suggest the boreal forest could transition from a carbon sink to a carbon source in years to come.
The inland Northwest (east of the Cascade Mountains) receives limited precipitation in comparison to forests along the coast. As a result, forests in the inland Northwest are comprised mostly of shade-intolerant trees that do well with less moisture, like ponderosa pine, lodgepole pine, Douglas-fir, western larch, grand fir, and quaking aspen.
These forests are accustomed to frequent low- or mixed-severity fires, and in some cases, need fire for tree seed dispersal (e.g., ponderosa pine and lodgepole pine). However, for much of the last century, fire suppression was the predominant management action in these forests, resulting in denser forests that are more susceptible to insect infestations, disease, and intense wildfires. In the past few decades, as understanding of these forests’ relationship with fire improves, land managers are working to bring fire back to portions of dry mixed-conifer forests.
In a warming climate, some dry forest species may shift to more suitable habitat, including higher elevation areas. Water availability will be a key determinant of where these forests establish and how successfully they grow in the future. Compounding stresses, like insects, disease, and drought, could lead to widespread mortality in the current range of dry forests.
The subalpine forests of the Northwest typically exist in high-elevation locations throughout Idaho, Oregon, and Washington. Dominant tree species include limber pine, whitebark pine, Engelmann spruce, mountain hemlock, and subalpine fir. These hardy trees sit just below tree line and often endure strong winds, high snowpack, and cold temperatures.
With higher temperatures and decreasing snowpack in a changing climate, growth of trees in subalpine forests may increase. Trees from lower elevations may become more competitive in these higher elevation forests. Though subalpine forests are protected by snow for much of the year, wildfire can act as a significant disturbance to these ecosystems. In the rare events when wildfire occurs in subalpine forests, it often occurs as a stand-replacing event, with significant damage to existing trees. Currently, subalpine forests are burning at a higher rate than any other forest type in Idaho, Oregon, and Washington. Under a changing climate, these forests could be particularly susceptible to vegetation shifts due to snowpack loss, drought, wildfire, and other disturbances, such as insect outbreaks.
Western juniper woodlands are widespread in central Oregon, south-central Washington, and southwestern Idaho. Western juniper, the dominant tree of these woodlands, prefers drier habitats. There are a number of old-growth juniper stands in the region that exceed 1,000 years in age.
A number of factors, including fire exclusion and removal of fine fuels by grazing livestock, have allowed Western juniper to colonize sagebrush steppe areas in southeastern and central Oregon and southern Idaho, creating issues for sagebrush-reliant species like sage grouse. Juniper encroachment has also been linked to increased erosion and reduced spring and streamflow, impacting water quality. It is unclear, however, whether juniper encroachment is related to the impacts of climate change in the Northwest, since its effects were documented over 130 years ago. The increase in post-settlement juniper woodlands is likely to interact with the increased likelihood of wildfire, and could cause an increase in high-severity crown fires than previously seen in these areas. In warmer and drier sites, juniper are exhibiting die-off, making these areas more susceptible to invasion by annual grasses like cheatgrass.
The oak woodlands of Washington and Oregon support Oregon white oak, ponderosa pine, California black oak, and increasingly, Douglas fir. They occur in dry, lowland sites and act as transitional zones between prairies and conifer-dominated forests. Oregon white oak is accustomed to hot and dry conditions and is highly drought tolerant. Since the cessation of Indigenous burning in the mid-1800s, many oak woodlands have experienced degradation and habitat loss to encroaching conifers and shrubs. As such, oak woodlands have been designated as a habitat of greatest conservation need by the Washington Department of Fish and Wildlife and as a significant habitat for Oregon species of greatest conservation need.
As the climate continues to change, Oregon and Washington are expected to experience hotter, drier summers and wetter winters, both of which the Oregon white oak can withstand. Increases in fire frequency are also likely to favor oaks. As such, oak woodlands may expand with climate change in the Northwest. However, loss of this habitat is associated with conversion to other land uses, such as agriculture and timber, invasive species, and fire suppression. Much of the remaining oak woodlands are in private ownership, meaning that cooperative approaches are crucial to conservation, particularly because oak woodlands require active management.
Additional Resources
To learn more about Northwest forest vulnerability to climate change, view the resources below to find the climate change vulnerability assessment specific to your area.:
Colville and Okanogan-Wenatchee National Forests
Columbia River Gorge National Scenic Area and Mt. Hood and Willamette National Forests
Intermountain region Part I and Part II
Northern Rockies Part I and Part II
Assessing vulnerabilities and adapting to climate change in northwestern U.S. forests is a scientific publication that reviews climate impacts, vulnerabilities, and management actions for western resource managers.
Upcoming Vulnerability Assessments: