Fire in the Umatilla National Forest

By: Sarah Hilbert


What types of forests were present before settlement? 

            When settlers arrived they found open pine forests on dry, south facing slopes and ridges, which contained primarily ponderosa pine trees.  They also found mixed conifer forests that were dominated by pine, but also contained other species.  These forests looked almost exactly like pine-dominated forests but contained scatterings of Douglas fir, grand fir, larch, Englemann spruce, and lodgepole.  This type of forest grew on moist sites such as north facing slopes and along streambeds.  Although these two forests types appeared to be very similar, they grew in areas with different water availability.  Both the ponderosa pine and mixed conifer communities experienced light ground fires every 10-12 years and stand replacing fires at intervals of several hundred years

            In addition to the south facing slope pine-dominated and the mixed conifer communities, two other types of forests existed in the Blue Mountains.  The first is known as the north-slope fir-larch type forest, a dense, dark forest in which Douglas fir and grand fir grew in thick stands.   These communities also have a scattering of larch, lodgepole, spruce and ponderosa pine.  Although this type of forest had the same species as the mixed conifer forest, the proportions of the species of trees were different.  Instead of ponderosa pine dominating the communites, Douglas fir and grand fir were the most abundant species.  The north-slope fir-larch forest experienced low to medium intensity fire in patches every 40-80 years and stand replacing fires every 150-250 years. The fourth, and last type of forest was the sub-alpine fir communities that grew at high elevations.  These forests experienced patchy light fires every 20-40 years and intense fires every 80-200 years.

Fires in the forests of the Blue Mountains were initiated both by lightning, and set by Native Americans to maintain open forests, which facilitated hunting on horseback and also created grassland for their animals.  



Why were fires suppressed?

            The Forest Service came to the Blue Mountains in the first decade of the twentieth century.  To early foresters, fires were seen as unnatural and destructive because they were thought to destroy humus, deplete soil nutrients, kill young growth, damage timber, and create an opportunity for disease and wind throw.  These processes were viewed as detrimental to the health of forests.  Foresters wanted to manage the forest for the most valuable species, ponderosa pine.  Fires were a threat to the beautiful park-like all pine forests they loved.  They saw that pine seedlings were destroyed during fire, so fire was believed to inhibit future growth of the stands.

What the foresters did not see is that frequent fires ensured that the forests would remain open and pine-dominated because pine had evolved and adapted to a frequent fire regime. Ponderosa pine could reproduce in a fire regime because of its thick, extremely fire and insect resistant bark that forms when a tree is 2 inches in diameter.  Ponderosa pines can reach this size in about 10 years, which is about the average time between frequent fires.  Ponderosa pine also has enclosing needles and thick bud scales which shield its meristems (growing buds).  These two adaptations enable the species to grow and reproduce despite frequent fire.  In the mixed conifer stands, other species are able to grow if the time span between fires is long enough for saplings to put on protective layers.  For example, Douglas fir grows a protective bark layer at 4 inches in diameter.  Although some conifer trees can mature and survive the fires, the forest stays open and pine dominated as most other species were burned off.  In the absence of fire, conifer species less adapted to fire are able to grow in thick stands and compete with the pine.

The concept of competition, an integral aspect of forest health and vitality, lead early foresters to manage forests in ways that would increase density so as to increase competition for water, light and nutrients.  They thought competition was what led to strong vigorous trees, thus overcrowded stands were believed to be a good thing.  It was believed that fire lessened competition by killing trees and thinning stands.

However, early ideas of competition leading to strong vigorous trees were not correct.  In reality, western conifers (ponderosa, western larch, Douglas fir, lodgepole) do not follow foresters’ traditional “stand development theory”.  Weaker trees did not die as a result of competition leaving only the healthiest trees, as was originally thought.  Without fire, no individual tree species become dominant, the undergrowth becomes a thicket of same aged trees whose growth rates slow significantly.


Changes in forest dynamics due to suppression:

            A change in fire regimes often results in a change in species composition and density.  The open south facing pine forests remain pine because the slopes are too dry for Douglas Fir and other species to grow.  Pine trees are sun loving and have long taproots, which give them an edge over Douglas Fir on the hot and dry south slopes.  Exclusion of fire in this type of forest does not result in a species composition change, but does result in forests with increased density.

            The mixed conifer stands do experience a shift in species composition with fire suppression.  The open pine-dominated forests become dense stands with fewer pines, because the forests are taken over by dense stands of Douglas fir, grand fir, larch and lodgepole.  In the absence of fire, firs grow faster than pines and soon dominate the forest.  These dense stands of trees may experience nutrient and water limitations. During droughts the dense firs can succumb to insect epidemics. Grand fir and Douglas fir are favored by the tussock moth and the budworm.  Although all tree species have pest problems, insects associated with ponderosa pine are usually less severe and more easily mananged (Mitchell 1990). Insect epidemics lead to the weakening or death of trees, and results in an increase in flammable biomass, which make forests very vulnerable to intense fire.

            Long-term fire suppression has altered the forest ecosystems of the Blue Mountains. The dense stands that result from suppression have greatly increased above ground biomass, so the partitioning of nutrients has changed.  Nutrients such as nitrogen, phosphorous, and sulfur are now more concentrated in foliage, small branches and debris, which makes these nutrients more vulnerable to volatilization (transition from a solid or liquid to a gas) during fire.  Plants cannot readily utilize gaseous forms of these nutrients.  The loss of these nutrients to the air has reduced the amount of nutrients in the soil, and the quantity of nutrients available for plant growth.

            Root structures have changed due to the accumulation of litter on the forest floor.  Increased litter cools the microclimate near the forest floor, which results in increased soil moisture.  In dry soils, roots must grow deep to acquire needed water, but as surface soil moisture increases root structures change with more roots clustering close to the surface.  These shallow roots are more susceptible to fire.  Even a light fire could singe tree roots, possibly destroying huge old trees if soil moisture is low during a fire event.  Shallow roots also make the trees more susceptible to wind throw and flooding because they are less stable.


Benefits of frequent fire:

            Light fires (every 6-12 years) keep the forest stand open, kill off young trees, reduce fuel loads, release nitrogen into soil stimulating the growth of grasses and nitrogen fixing shrubs such as ceanothus.  Frequent fires keep stands well thinned, but shouldn’t be too frequent so a few young trees do survive to form future stands.

            Reducing biomass with frequent fires is important so that when fires do burn, they are not as intense.  A build up of fuels leads to much higher fire temperature.  Hot, intense fires are more likely to destroy tree roots (and cause tree mortality) leading to soil erosion.  High temperatures of fire can sterilize the soil, killing soil organisms and reducing nutrient availability, increasing the recovery time of a forest.  The removal of forest debris that occurs with light burning may also be an important factor in decreasing soil acidity that occurs from the build up of acidic conifer needles on the forest floor. 

            Frequent fire can aid in the reproduction of trees in several ways.  By breaking down plant litter, fires can aid in the release of essential mineral elements needed for plant growth.  The ash and nutrients can create suitable soil conditions for the growth of large seed supplies, especially those opened by fire.   The reduction of competition for moisture, nutrients, heat and light by temporarily eliminating the overstory should favor growth of young trees in forests that previously had difficulty competing with other forest species.


Prescribed fire:

After almost a century of fire suppression, foresters realize the importance of fire to ecosystems of the northwest.  Fire is being employed by the Forest Service and others in an attempt to return forests to a pre-fire suppression condition.  Reasons for prescribed burning include:

            Controlling the species composition of vegetation

            Maintaining ecological diversity

Reducing wildfire hazard by reducing fuels around areas of heavy human activity and broad expanses of wildlands where natural fires are difficult and expensive to control (Norris 1990)

Stimulating the growth of native grasses and plants for wildlife and domestic livestock

Keeping insects and disease at minimum levels by maintaining healthy stands of trees

Releasing minerals and nutrients in the soil for immediate use, thus preparing the ground for tree regeneration (Fire in the Blues- Pamphlet)



Most of this information (except where indicated) came from:

Langston, L. 1995.  Forest Dreams, Forest Nightmares: The Paradox of Old Growth in the Inland West. University of Washington Press, Seattle, Washington. 368p.


Other references include


Mitchell, R.G. 1990.  Effects of prescribed fire on insect pests.  In Walstad, J. D., S. R. Radosevich and D.V. Sandberg (eds.) Natural and Prescribed Fire in Pacific Northwest Forests.  Oregon State University Press, Corvallis, Oregon. 317p.


Norris, L.A. 1990.  An overview and synthesis of knowledge concerning natural and prescribed fire in Pacific Northwest forests, p. 7-22.  In Walstad, J. D., S. R. Radosevich and D.V. Sandberg (eds.) Natural and Prescribed Fire in Pacific Northwest Forests.  Oregon State University Press, Corvallis, Oregon. 317p.


Fire in the Blues: A Natural Force, A Powerful Tool.  Pamphlet produced by:

The United States Department of Agriculture

The Forest Service Pacific Northwest Region

The Blue Mountain Natural Resources Institute

Malheur, Ochoco, Umatilla, Wallowa-Whitman National Forests.


USDA Forest Service:


Information on Prescribed fire, and before/after pictures of Mill Creek prescribed fire:


Umatilla National Forest:


***Fire Effects Information:

                        Includes fire effects/adaptations for many plant and animal species





Courtesy of BLM