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Updated 12 October, 2003

US National Assessment of
the Potential Consequences
of Climate Variability and Change
Educational Resources
Regional Paper: Pacific Northwest


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Evergreen coniferous forests dominate the landscape of much of the Northwest. West of the Cascade crest, coniferous forests cover about 80% of the land, and include some of the world's largest trees and most productive forests and about half the world's temperate rainforest. Belying their lush appearance, on most sites these forests are constrained by lack of moisture during the warm dry summers. The lack of moisture limits seedling establishment and summer photosynthesis, and creates favorable conditions for insect outbreaks and fire. Forests of the dry interior operate under an even more severe summer soil-moisture deficit. The summer dryness is the most important factor controlling the species distribution and productivity of forests throughout the region.

Forests throughout the Northwest have been profoundly altered by human activities over the past 150 years. West of the Cascades, forests have been cleared for conversion to other land uses or clear-cut for timber and replanted, replacing massive old-growth forests with young, even-aged managed forests. By various estimates, 75 to 95% of original old-growth forests have been logged, and much of what remains is in small fragmented stands. Those changes are estimated to have released to the atmosphere 2 billion metric tons of carbon over the century.

East of the Cascades, decades of intensive grazing, fire suppression, and selective harvesting of mature trees have transformed the former open, park-like forest of ponderosa pine, Douglas fir, and western larch into a dense mixed forest overstocked with shade-tolerant pines and firs. The new-forest mix is highly susceptible to insect outbreaks, disease, and catastrophic fire. While the former open forest structure was maintained by frequent, low-intensity fires, fire suppression activities here have allowed large accumulation of fuel (trees and brush). Under times of moisture stress, these high fuel loads increase the risk of extreme fires over large areas, as occurred in the summers of 2000 and 2001. During the 20th century the effects of these human activities on Northwest forests have been much greater than the climatic effect. However, during the 21st century forest management programs are highly likely to face the impacts of climate effects.

Tree growth can show a clear effect of past and present climate variability, which is why tree rings can provide a useful record of past climate. The most pronounced effects are in stands near their climatic limits, e.g., at the upper (cold) or lower (hot and dry) timberline. Tree growth and forest fires depend on the PDO. Unusually warm and dry winter and spring climate, as is associated with the warm phase of PDO, usually means less tree growth and greater risk of fires. However, near the upper timberline, where trees are not moisture-limited but where late-melting snowpack limits growth, tree growth is best during the warm phase of PDO.

Tree Growth and Inter-Decadal Climate Variability

Tree Growth and
Inter-Decadal Climate Variability

Trees near their climatic limits show strong signals of interdecadal climate variability. Those near the upper treeline grow best in warm-PDO years because snowpack is lighter, while those near the dry lower treeline grow worst in warm-PDO years, because of summer moisture deficit.

Source: Peterson and Peterson, 2000.

This is because the warm phase of PDO tends to have lighter snowpack and consequently the trees have an earlier start to the high-elevation growing season, promoting tree growth and upward expansion of forests to colonize sub-alpine meadows. Near the lower timberline, the opposite relationship is present. Tree growth shows a negative relationship with PDO, because the warm dry winters and light snowpack of positive-PDO periods increase summer drought stress, which limits the growth at these lower elevations.

The principal effect of climate on the majority of Northwest forests has come not through direct effects but indirectly, through changes in disturbance by fire, insect infestation, and disease. Major disturbances can reset forests to their establishment stage, when trees are the most sensitive to adverse environmental conditions like heat and drought. For insect and disease mortality, small-scale studies have shown strong relationships of both bark beetle and defoliator bug outbreaks with severe drought conditions. For fire, total Northwest forest area burned shows a significant region-wide association with PDO and with the Palmer Drought Severity Index, especially before the introduction of widespread fire suppression. The effect of ENSO on fire in this region is less clear.

Pacific Northwest Summer Soil Moisture Change, 21st Century

Environmental Impacts

For available projections of how the climate is likely to change, evaluation of the potential effects on Northwest forests need to consider a number of complex interactions that exist between several key factors. Some of the factors vary with location, others with the time of year. The direct effect of projected warmer summers without a substantial increase in rainfall would be to increase the occurrence of summer drought. Summer drought can result in reduced tree growth, increased stress and tree mortality, and decreased seedling survival. Reduced snowpack in the mountains has different effects at different sites. Reduced snowpack extends the growing season and seedling establishment is easier where snowpack is heavy (e.g., near the present upper tree line). Less snowpack reduces available growing-season moisture and consequently increases drought stress in dry areas in lower elevations. One early study used observed associations between existing forest communities and local climate to project the effects of future climate change on Northwest forests. This study projected that forested area in the Northwest would contract, principally through forest dieback and sagebrush-steppe expansion at the dry lower treelines east of the Cascades.

It is likely, however, that the effects of increased summer drought will be offset to some degree by wetter winters and by the direct effects of higher atmospheric carbon dioxide (CO2) concentration (some plants use water more efficiently with higher CO2). Although drought stress (ranging from mild growth constraints to severe stress, even death) typically peaks in late summer, the severity of drought stress depends mostly on summer temperature and on winter and spring precipitation. This is because forests rely on moisture stored in deep soil layers to offset lack of summer water. Under climate model projections, the forest growing season begins several weeks earlier in the spring than at present, allowing forests to take advantage of increased winter precipitation for early growth. Beyond the water that is put to use immediately, wet winters could help to offset summer drought stress, if the soil has enough storage capacity to hold the additional water until it is needed in the late summer. Such storage, however, would have little benefit for shallow-rooted seedlings.

Because of the projected increases in temperature and precipitation, it is likely that the forests of the region will benefit by experiencing a short-term increase in productivity. Over the longer term, however, there are indications that increased evapotranspiration (loss of water from the soil both directly by evaporation and through plants by transpiration) under warmer temperatures could overwhelm the projected amounts of increased precipitation and any water-use efficiency gains from increased amounts of carbon dioxide. Such an outcome would be likely to result in large forest diebacks, generally late in the 21st century for moderate to high warming levels. Results are now available that would support the investigation of whether increases in the water use efficiency (from higher concentrations of carbon dioxide) of this region's forests would have a large or small effect on future forest growth.

Changes in disturbance are likely to lead to the largest effects of future climate variability or change on Northwest forests. Ecosystem models project significant increases in the number of trees and the amount of forest area burned in the Northwest interior by 2100. Changes in other disturbances, such as wind, insects, and disease, are also possible under a changing climate but the exact character of these disturbances is as yet uncertain. For insects, general warming is likely to encourage northward expansion of the range of southern insects, while longer growing seasons are highly likely to allow an increase in the number of insect generations in a season. Forests that are moisture stressed are more susceptible to attack by sap-sucking insects such as bark beetles. It is also possible that drought stress encourages attack by foliage eaters such as spruce budworm.

Very little is understood, however, about the interactions between multiple disturbances, e.g., between insect attack and fire, under projected climate change and the changes in forest character that follow. Finally, it is crucial to note that ecosystem models only project potential vegetation, the vegetation that would be present on a site in the absence of human intervention. In the Northwest, forest management and land-use change are presently, and are likely to remain, predominant factors shaping the structure, species mix, and extent of forest ecosystems. Interactions between these human-driven factors and the many possible pathways of climate influence on Northwest forests are as yet uncertain.

Societal and Economic Impacts

The effects of predicted changes in climate on the forests of the Pacific Northwest could have profound social impacts -- impacts that could be a mix of positive and negative depending on how much and low long forest growth and health benefit from changes in climate and CO2. Many people are drawn to this region because of its natural beauty and the lifestyle its diverse ecosystems offer. Should climate change compromise those ecosystems, much of the aesthetic and recreational value of the Pacific Northwest lifestyle could be changed. While the impacts at this time are only speculative, the tourism and recreation industries could be affected if climate change altered the ecosystems that attract people to the region. Service industries that support tourism and outdoor recreation-hotels, inns, restaurants, and travel services could also be affected.

Forest ecosystems also offer other services to society that could be affected from continued human activities as well as projected climate changes. For example, loss of forest cover, road building, and timber harvesting are compromising the ability of forests to provide wildlife habitat, clean air, sequester carbon, and regulate availability and quality of the region's water, a resource that is essential to the health of other natural resources and ecosystems.

Although people move freely within the Pacific Northwest, the same is not true for many species of plants and animals. Species adapted to some of the extreme conditions found in this region, for example, the temperate rainforest, cannot expand their range easily or quickly. Moreover, in some parts of this region, human activity has cut off migration corridors through development, and natural impediments such as mountains prevent the movement of those species that could adapt to new habitat ranges.

In contrast to water and salmon, where management already reflects at least limited awareness of the effects of climate variability, a survey of Northwest forest managers suggests they regard climate change as unimportant. The reasons they gave included: because mature stands are resilient to wide climatic fluctuations; and because 40-70 year timber harvest schedules average out the effects of shorter-term climate variation. Long-term climate trends are highly likely to make these assumptions invalid. Trees are likely to mature in a climate substantially different than when they were planted, possibly requiring changes to many aspects of forest management beyond those recently adopted due to addressing concerns regarding endangered species. Long-term planning could be important because forestry still plays a vital economic and cultural role, particularly in the western region of the Pacific Northwest. The Northwest produces about a quarter of the Nation's softwood lumber and plywood. There are expected to be short-term positive benefits of climate change for forests in the region, while the long term negative environmental impacts on forest productivity and reduced forest range in the region is likely to decrease the timber yields by 2100.

Strategies to Address Potential Impacts on Forests

Strategies that could be useful in avoiding or mitigating climate-change impacts on forest resources and ecosystems vary in their approach.

Available options could include:

  • planting species best adapted to projected rather than present climate (e.g., planting Douglas fir on suitable sites in the silver fir zone), or with known broad climatic resilience;
  • implementing additional measures to restore and maintain complexity of forest structure and composition within intensively managed areas;
  • managing forest density for reduced susceptibility to drought stress; and
  • using pre-commercial thinning, prescribed burning, and other means to reduce the risk of large, high-intensity disturbances and to facilitate adaptation to changed climate regimes.

Managing forests effectively under changing conditions is very likely to require:

  • increased capacity for long-term monitoring and planning;
  • recognition of the increasing importance of seasonal and interannual climate variability when forests are also stressed by long-term trends; and
  • application of increased understanding of climate variability and predictive skills to factor projected periods of drought stress and fire risk into short-term forest-management decisions such as timing and species of planting, and use and timing of prescribed burning.

Maintaining forest ecosystem services and biological diversity is also likely to grow increasingly challenging under climate change. Options for doing so would include:

  • increasing establishment of protected areas, incorporating the maximum possible diversity of topography and landscape;
  • active measures to promote species migration and maintenance of diversity, even in non-commercial forests; and
  • further reduction in the intensity of commercial harvest.

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