Fire and Forest Health

Although it is difficult not to think of fire from a destructive point of view, it is in fact a natural process of renewal, and a catalyst for promoting biological diversity and healthy ecosystems. Some plant species are actually adapted to fire. For instance, lodgepole pine (Pinus contorta var. latifolia) have serotinous cones (seeds are released in response to an environmental trigger such as fire). These seeds are retained in the tree canopy for long periods until a fire burns through the stand, releasing thousands of seeds as the resin seal enclosing the cones melts. This feature allows lodgepole pine to reproduce prolifically following a fire.

Fire Regimes

The Pacific Northwest includes many types of vegetation and fire regimes, from frequent surface fires to infrequent high severity fires.

High severity fire regimes are generally located in cool, wet environments at higher elevations where subalpine forests are located. These forests typically consist of subalpine fir, lodgepole pine, Engelmann spruce, and whitebark pine. Fire intervals can range greatly (100-300 years), and typically destroy entire stands.

Moderate severity fire regimes tend to occur at mid-elevation zones where dry Douglas-fir forests persist. Other tree species found within this zone include grand fir, subalpine fir, lodgepole pine, western red cedar, western hemlock and western larch. Moderate severity fires occur at intervals of 25 to 100 years and leave a mosaic of lightly burnt to severely burned areas.

Low severity fire regimes are characterized by fires that occur at frequent intervals (1-25 years). Because fuels have a limited time to accumulate in these areas, returning fires tend to be of low intensity. Ponderosa pine forests are indicative of the low severity fire regime.

giant sequoia burn scars
Burn scars (white arrows) in the cross section of a giant sequoia. The numbers represent the year each fire occurred. Photo: Tom Swetnam.

Historic Conditions

Historically, the Pacific Northwest was subject to fires of a variety of frequencies, intensities and extents. How do we know what the historic fire regimes were? Some information comes from human sources such as records of explorers or from land surveyors as they were establishing section corners. Some information comes from the forested ecosystem itself, such as the presence of charcoal layers in the soil and the even-aged character of some forests. Trees themselves record history through the growth rings that develop each year. When a fire burns through an area, the growth rings may be scarred. A fire scar tells us the year the fire occurred and may also reveal the season of fire occurrence based upon the position of the scar (photo).

Historically, the dry forests of the Pacific Northwest experienced low and mixed severity fire regimes. Low severity, frequent fires eliminated fuel ladders; elevated tree crown bases; reduced competition for site resources among surviving trees, shrubs, and herbs; promoted the growth of a low and patchy shrub and herb cover; and cycled nutrients from foliage and branches into the soil. This resulted in forests dominated by large, widely spaced, fire-tolerant ponderosa pine with little accumulation of coarse woody debris on the forest floor. Severe fire behavior and effects were uncharacteristic of dry forest-dominated landscapes.

Current Conditions

Wildfire size, severity and frequency have increased, particularly in the lower elevation dry forests. This is due in part to past and present fire suppression efforts. These forests now contain heavy fuel loads, a shift in the dominant tree species, smaller than average tree size and multi-layered canopies that act as fuel ladders. These conditions result in high intensity fires in areas that previously did not experience them.

In addition to wildfire size, severity and frequency, fire suppression efforts have affected general forest health. Douglas-fir and true firs are not as well adapted to dry sites as ponderosa pine and western larch. As a result these firs suffer physiological stress when subjected to hot, dry summers and, especially, drought. Stressed trees are more likely to succumb to insect and disease problems such as bark beetles and root disease. The presence of great numbers of stressed and dying trees offers an abundance of food to sustain insect populations and lead to insect outbreaks of epidemic proportions.

Models projecting climate change and fire patterns indicate that the frequency and extent of fire will increase due to increased temperatures, earlier spring snow melt and longer fire seasons. These projections suggest that there is an immediate need for forest managers to mitigate and adapt to increased wildfire events in order to sustain forest landscapes.

Accumulated fuels in dry forests need to be reduced so that when fire occurs, rather than becoming a conflagration that destroys the entire stand, it is more likely to burn along the surface at low-moderate intensity, consuming many small trees and restoring forest resilience to future drought, insect and disease problems and wildfire. Various combinations of thinning, slash treatments and prescribed burning can be used for restoration. Visit the Washington State Department of Natural Resources website for information about cost-share opportunities to help private landowners in eastern Washington with these tasks.

Most fires are human caused, often due to neglected campfires, sparks, irresponsibly discarded cigarettes and more often than not: debris burning. Significantly fewer fires may be started by taking greater caution. Check online for the current fire danger and outdoor burning restrictions in your county.

By Melissa Fischer, forest health specialist, Washington State Department of Natural Resources, Northeast Region,