A Land Manager’s Guide for Creating Fire-Resistant Forests Stephen Fitzgerald and Max Bennett Photo by Stephen Fitzgerald © Oregon State University University State © Oregon Fitzgerald Stephen by Photo EM 9087 EM 9087 • September 2013 Photograph by Emily Jane Davis by Photograph of Oregon University A Land Manager’s Guide for Creating Fire-Resistant Forests Stephen Fitzgerald and Max Bennett ildfires are a natural disturbance in Pacific Northwest forests. Historically, fire operated differently in various forest types Silviculture is the art and science (Figure 1, page 2) across the West. For example, fires were Wfrequent but less severe in ponderosa pine and dry, mixed-conifer forests. of manipulating forests to meet In higher, cooler-elevation forests, fires were less frequent but more severe; many were ‘stand-replacing’ fires because most or all overstory landowner objectives. It includes trees were killed. Over the last century and a half, forests have changed dramatically from treatments such as thinning, their pre-settlement condition. This is particularly true in the drier forests of the West, where decades of fire exclusion have resulted in a buildup of pruning, underburning, and a fuel that has increased the size and intensity of wildfires. Climate change may also be a factor in this trend toward “mega-fires.” wide variety of other practices. The emphasis today in forest management, particularly on federal lands and in wildland-urban interface areas, is on forest restoration and fuels reduction. Land managers can affect the total amount, composition (fuel sizes), and arrangement of fuels, and can thus influence the intensity and severity of a wildfire. This influence is more effective or pronounced Stephen Fitzgerald, professor and when larger areas are treated. Extension forester, central Oregon region; and Max Bennett, Extension agent, Forest This publication provides an overview of how various silvicultural and Natural Resources, Southern Oregon treatments affect fuel and fire behavior, and how to create fire-resistant Research and Extension Center, Oregon forests. In properly treated, fire-resistant forests, fire intensity is reduced State University. 2 Forest Types and Fire Regimes of the East Cascades Gradient of Potential Vegetation Types and Fire Regimes The Nature Conservancy Nature The Figure 1 and overstory trees are more likely to survive than in untreated forests. Fire-resistant forests are not “fireproof” – under the right conditions, any forest will burn. Much of what we present here is pertinent to the drier forests of the Pacific Northwest, which have become extremely dense and fire prone. Fire Behavior 101 Heat The fire triangle Oxygen Three elements are needed to sustain a fire: heat or an ignition source, fuel, and oxygen (Figure 2). Take any one of these elements away and the fire doesn’t start or goes out. For example, digging a fire line down Fuel to mineral soil, which is noncombustible, removes combustible material © OSU Fitzgerald, Stephen by Photo Figure 2 on the forest floor (surface fuel) and stops a forest fire’s progress if the fire line encircles the fire. The fire behavior triangle Fire “behavior” is primarily described by its rate of spread (in feet per hour) and its intensity (i.e., how hot it burns and how long its flame is). Once a fire ignites in forest or rangeland vegetation, its behavior Weather depends on the three factors that comprise the fire behavior triangle: the amount and arrangement of fuel, the area’s topography, and weather conditions (Figure 3). A change in any one factor during a fire alters its Topography behavior and type (i.e., whether it’s a ground, surface, or crown fire). For example, if the weather becomes hot, dry and windy, the fire will burn with more intensity and move faster across the landscape. If a Fuel fire is burning in heavy fuels and then moves into an area with light or © OSU Fitzgerald, Stephen by Photo Figure 3 discontinuous fuels, fire intensity and spread decrease. 3 Other important aspects of fire: ■ Torching. Movement of a surface fire up into a tree crown; the precursor to crowning ■ Crowning. Active fire movement through tree canopies ■ Fire whirl. Result of an upward spinning column of air that carries flames, smoke, and embers aloft; whirls often form in heavy fuels on the lee (downwind) side of ridges and, in extreme conditions, can be powerful enough to twist off entire trees. ■ Spotting. When firebrands (glowing embers) are lofted up and ahead of the main fire front, igniting multiple spot fires that then feed back into the main fire front to create very extreme and dangerous fire conditions. Photo by Bureau of Land Management Bureau by Photo Types of fires A wildfire may be composed of three different types of fire: ground, surface, and crown. The proportion of each type determines the overall severity of the fire and how much vegetation the wildfire will consume or kill. Ground fires Ground fires consume mostly the duff layer and produce few visible flames (Figure 4). Ground fires also can burn out stumps and follow and burn decaying roots and decayed logs in the soil. A fire burning in tree roots often goes undetected except when it follows a root near the soil surface. Then, it can emerge, ignite surface fuels, and become a surface fire. Ground fires can often smolder for days and weeks, producing little smoke. Dashed line represents Slash piles containing too much soil can allow a base of smoldering duff. ground fire to smolder for weeks or months (called © OSU Fitzgerald, Stephen by Photo a “hold-over” fire), then re-emerge later and ignite Figure 4 surface fuels, causing a wildfire. To prevent this, a skilled tractor operator should use a brush blade to create clean slash piles, or use a hydraulic excavator to stack and pile slash without adding soil to the pile. 4 Photo by XXXXXXXXXXXX Photo by BLM by Photo Surface fires Surface fires produce flaming fronts that consume needles, moss, lichen, herbaceous vegetation, shrubs, small trees, and saplings (Figure 5). Surface fires can ignite large woody debris and decomposing duff, which can then burn (glowing combustion) long after surface flames have moved past. Surface fire severity can be low to high. High-severity surface fires can kill most trees (up to or more than 75 percent) as a result of crown and bole scorch, but can be highly variable, leaving scattered individual trees and patches of green trees. Surface fires with flame lengths less than 4 feet can be controlled by USGS Brennan, Teresa by Photo ground crews. Surface fires can develop into crown fires if “ladder fuels” Figure 5. Surface fires consume connect surface fuels to crown fuels, fuel moisture is low, or weather needles, moss, lichen, herbaceous vegetation, shrubs, small trees, and conditions favor torching and crowning. saplings. Wildlandfire.com Figure 7. Active crown fires are intense and stand-replacing blazes influenced by wind, topography, and crown density. Crown fires Crown fires are either passive or active. Passive crown fires involve the torching of individual trees or groups of trees (Figure 6). Torching is the precursor to an active crown fire. Crown fires become active when enough heat is released from combined crown and surface fuels to preheat and combust fuels above the surface, followed by active crown fire spread from tree crown to tree crown though a canopy (Figure 7). Crown fires are usually intense and stand-replacing, and are strongly influenced by wind, topography, and tree (crown) Service Park Scott Isaacson, National by Photo density. Figure 6. Passive crown fires involve individual trees or groups of trees. 5 Photo by Stephen Fitzgerald © OSU Fitzgerald Stephen by Photo Photo by BLM by Photo Figure 8. Surface fuels and height to canopy are two factors that Figure 9. Reducing surface fuelds and increasing the height to affect the transition from a surface fire to a canopy fire. the crown base increase the odds larger proportions of a forest will survive a fire. Four factors influence the transition from a surface forest fuels so that fire acts and plays out in a more fire to crown fire (Figure 8): natural way and pattern, particularly in ponderosa pine and drier, mixed-conifer forests. ■ Surface fuel and foliage moisture content ■ Surface flame length (affected by fine surface fuel There are five principles of creating and maintaining loading, wind, and slope) fire-resistant forests: ■ Height to the base of tree crowns (i.e., height of the ■ Reduce surface fuels canopy) ■ Increase the height to the base of tree crowns ■ Density of tree crowns (degree of overlapping of tree ■ Increase spacing between tree crowns crowns) ■ Keep larger trees of more fire-resistant species The common denominator is fuel ■ Promote fire-resistant forests at the landscape level We have little or no control over most factors in the By following these principles we can: fire and fire behavior triangles. For example, we can’t ■ Reduce the intensity of a fire, making it easier for control the wind, topography or oxygen, or stop every firefighters to suppress fire ignition. However, one element we can control is ■ Increase the odds that larger proportions of a forest will fuel. We can alter fire behavior by reducing the amount survive a fire (Figure 9). Small trees, shrubs, and other and changing the arrangement of fuel before a wildfire understory vegetation may be injured or killed, but erupts. Recent examinations of large wildfires in the larger trees in the stand will only be scorched, and soil West show fire intensity and severity were usually damage also will be reduced significantly reduced when fuels had been reduced ■ Reduce extent of post-fire restoration activities needed, beforehand. such as replanting. Principles of Fire-Resistant Forests Reduce surface fuels Reducing surface fuels, such as slash and small shrubs, A fire-resistant forest has characteristics that make impairs potential flame length and fire intensity, making crown fires unlikely and allow the forest to survive fires easier to control and less likely to reach into surface fire without significant tree mortality in the tree crowns.