Melissa Fischer (WA DNR), Glenn Kohler (WA DNR) and Karen Ripley (US Forest Service)
Fire is a natural process that many ecosystems depend upon to maintain their structure and function. 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 that are retained in the tree canopy for long periods of time. When a fire burns through a lodgepole pine stand, thousands of seeds are released as the resin seal that encloses the cones melts thereby allowing it to reproduce prolifically following a fire.
The PNW is characterized by a diversity of vegetation types and fire regimes that range 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 tree species such as subalpine fir, lodgepole pine, Engelmann spruce, and whitebark pine. Fire intervals can range greatly, (100-300 years), and are typically stand replacing.
Moderate severity fire regimes tend to occur at mid-elevation zones where dry Douglas-fir forests persist. Other tree species that may be found within this zone include grand fir, subalpine fir, lodgepole pine, red cedar, western hemlock and western larch. Moderate severity fires occur at intervals of 25-100 years and leave a mosaic of lightly burnt to severely burnt areas.
Low severity fire regimes are characterized by fires that occur at frequent intervals (1-25 years). Fuel has a limited time to accumulate and returning fires are of low intensity. Ponderosa pine forests are indicative of the low severity fire regime.
The PNW was historically subjected to fires of a variety of frequencies, intensities, and extent.
How do we know what the historic fire regimes were? Some information comes from human sources such as records of explorers, or 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 (in the early, middle, or late wood).
Figure 1. Burn scars (white arrows) in the cross section of a giant sequoia. The numbers represent the year each fire occurred. (Credit: Tom Swetnam).
Historically, dry forests of the PNW 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.
Wildfire size, severity, and frequency have been increasing, particularly in the lower elevation dry forest types. This is mainly due to past and present fire suppression efforts. Fire suppression in these forests has resulted in heavy fuel loads, species composition shifts, smaller than average tree size and multi-layered canopies that act as fuel ladders.
In addition to these ecosystem changes, aggressive fire suppression essentially ensures that most fires occur only during extreme weather conditions, such as those seen in 2015. This results in high intensity fires in areas that historically did not experience them.
In addition to wildfire size, severity and frequency, fire suppression has also affected general forest health. Douglas-fir and true firs are not as well adapted to dry sites as ponderosa pine and western larch and consequently 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. Insect outbreaks can reach epidemic proportions with the presence of stressed and dying, off-site species that offer an abundance of food to sustain populations.Climate change and fire projection models indicate that fire frequency and extent will only increase due to increased temperatures, earlier spring snow melt and longer fire seasons. So what can be done? Trends and projections of climate and fire responses suggest that there is an immediate need 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/or disease problems and fire.
Various combinations of thinning, slash treatments, and prescribed burning can be used for restoration. Mechanical thinning can reduce tree density and some fuels, but prescribed burning is usually more efficient, cost-effective, and ecologically beneficial than mechanical treatments, and the most effective means for controlling the rate of spread and severity of wildfire. Unfortunately, prescribed fire is rarely used because of liability and casualty risks, public objections to smoke, and little tolerance for management errors. In the end, fires are inevitable in these ecosystems, therefore as Hessburg et al. 2005 state, we may have to make a choice as to what type of fire and smoke we would prefer; “that associated with wildfires or that which is actively prescribed and managed”.
How you can help…
Most fires are human caused, often due to neglected campfires, sparks, irresponsibly discarded cigarettes and more often than not; debris burning. Significantly less fires may be started if increased caution is taken. For information on current fire danger and outdoor burning restrictions see: https://fortress.wa.gov/dnr/protection/firedanger/
Management of your forested property post-burn may depend upon several factors, some of which include the severity of the burn, the season of the fire, the tree species affected, and your management goals. A good place to start is to determine which trees are dead and which are alive, and the percent of live trees that may die from secondary effects such as bark beetles. If you plan to salvage harvest, the more quickly this is done, the better, as wood-rotting fungi will soon colonize fire-killed and/or damaged trees, decreasing their economic value. Additionally, numerous stressed trees can potentially lead to a bark beetle outbreak.
Will trees die from fire injuries?
How can you tell if a tree will die from fire injuries? There are five categories of injury that are useful when assessing your trees following a fire: foliage consumption, needle set, crown scorch, damage to the buds, and stem char.
Foliage consumption is basically when needles have been burned away (Figure 2A). Any foliage consumption usually indicates that there’s been sufficient heat and exposure to the fire to kill the tree and should be considered an indicator of lethal injuries.
Figure 2. A. Ponderosa pine with live needles consumed and B. Ponderosa pine sapling with needles “set” in the direction the heat and fire moved past it.
Needle set occurs when needles have been severely scorched and pushed in the direction the fire moved (Figure 2B). They will appear discolored and will be retained in an unnatural position. These needles may remain on the tree for years. Similar to foliage consumption, needle set usually indicates that there has been sufficient heat and exposure to the fire to kill the tree and should be considered an indicator of lethal injuries.
Crown scorch is caused by hot gases rising from a fire burning along the ground. Crown scorch volume indicates the level of exposure to the fire, and when integrated with information about tree species and size, provides a reliable estimate of likely tree survival.
Scorched needles will have been changed to a dull orange or grey-brown color, they are not burned away. To determine crown scorch volume, you want to estimate what percent of the volume of previously living crown is now scorched. Let us use Figure 3 to estimate tree survival using crown scorch volume. The tree on the left appears to have approximately 40% crown scorch (needles are reddish in color), while the tree on the right appears to have approximately 75% crown scorch.
Two additional pieces of information you will need to know is the species of tree you are assessing and the DBH. DBH stands for “diameter at breast height” and is a measurement of the diameter of the tree bole at 4.5 feet from the ground. The two trees in the picture are both ponderosa pine. The tree on the left has a diameter of 20 inches and the tree on the right has a diameter of 14 inches.
Figure 3. Two ponderosa pine showing different degrees of crown scorch.
Figure 4 shows the probability of fire-induced mortality for ponderosa pine of different sizes and differing levels of crown scorch volume. You can see from the chart that a ponderosa pine with a DBH of 20 inches and 40% crown scorch (our tree on the left, Figure 3) has a 10% probability of fire-induced mortality, while a ponderosa pine with a DBH of 14 and 75% crown scorch (our tree on the right, Figure 3) has an approximate 70% probability of fire-induced mortality. Because the tree on the right has such a high probability of mortality, it may be a tree you would want to remove.
Figure 4. Chart from the book After the Burn by Yvonne C. Barkley showing the probability of fire-induced mortality for ponderosa pine using DBH (diameter at breast height, inches) and crown scorch volume (%).
A major factor determining whether a conifer with crown scorch can survive is the damage to the buds. Determine whether the buds were killed by slicing through one to examine its interior. If the interior is brown and punky, the bud is dead. If it’s green and succulent, it is alive. Even if all the needles on a branch were scorched, if the buds survive that’s a good sign.
occurs when the trunk of the tree has been burned. Char is not simply the surface of the bark blackened by smoke or soot; the tree trunk tissue has been altered. It may be burned away or changed to a black, Styrofoam-like crumbly tissue. Stem char happens when the bark catches fire alone or in close proximity to other burning material like fallen logs. It indicates a significant exposure to injurious heat, but is not always lethal. To evaluate injury, sample the cambium (inner bark) in as small an area as possible on four equally spaced locations around the trunk and within three inches of the ground line to minimize wounding. Dead cambium is darker in color, often resin soaked, and hard or gummy in texture. Live cambium is lighter in color and rather pliable. If inner bark is destroyed on more than 50% of bole circumference, survival is unlikely Figure 5. Evaluation of cambium injury.
There are a few additional factors to keep in mind when assessing your trees after a fire: 1. trees tend to be more sensitive to fires that occur before August 1st than after because early in the growing season trees are more metabolically active; 2. trees on poor sites (rocky, sandy, dry) are more sensitive than trees on good sites; 3. trees with thin bark (i.e. grand fir) are more sensitive than trees with thick bark (i.e. ponderosa pine) because thin bark provides less insulation for the metabolically active tissues; 4. trees with small buds (Douglas-fir) are more sensitive than trees with large buds (ponderosa pine) because small buds have more exposed surface area and less insulating mass; and 5. trees in poor health are more sensitive than trees in good health because weak trees have fewer reserves to recover from injury.
Most long term studies have shown that fire related mortality is greater during the second growing season following a fire than that observed during the first. Why would this be? One reason is that some species of bark beetles are very attracted to fire damaged trees and begin to attack these trees following fire.
Bark Beetles and Fire
Beetle epidemics following wildfires are a possibility, but not a given. Several factors have to be in place in order for an outbreak to occur. For one, fires must occur at a time of the year when beetles are in the adult stage and can quickly infest the tree. In line with that, there must be a population of beetles within a reasonable distance. Additionally, there must be a sufficient supply of undamaged phloem. For this reason, trees in areas that have experienced moderate burns are at the greatest risk of infestation, not necessarily the severely burned trees. This is because moderate burns provide bark beetles with a large supply of stressed trees that still have intact phloem. In severely burned trees, the phloem will be too dry and therefore uninhabitable.
Beetle-induced mortality to fire injured trees occurs primarily in the first year or two following fire. Additional mortality may occur when a high beetle population develops in the fire-injured trees and attacks the fire survivors the following year. The extent and duration of this subsequent mortality depends on the size of the bark beetle populations and vigor of remaining trees.
General post-fire bark beetle management includes salvaging fire damaged and infested trees before the next beetle flight. Life cycles vary among the different bark beetle species, therefore you may want to contact your local forest health specialist to determine which species you are working with. Maintaining a low stand density may be helpful as this will increase the vigor of the residual trees. Vigorous trees with plenty of access to water, light, and nutrients are best equipped to defend against bark beetle attack. Trees need these resources to maintain pitch flow and defensive chemicals. Pesticides can be used to prevent attack on high value trees, but this technique is expensive and could negatively affect natural insect predators if it is not used in a manner which will best preserve them. Remember to always read and follow the label when using pesticides.
In light burns, the amount of bark beetle attraction depends mostly on the amount of root collar damage. Most thick barked species such as mature Douglas-fir, western larch and ponderosa pine will have lower mortality and not attract beetles unless smoldering duff significantly damages roots or root collars. Thin barked species such as true firs can tolerate little damage at ground level without significant stress, making them much more susceptible to bark beetle attack. Look for trees that have little apparent bole or crown damage, but may be completely girdled at the root collar (can use stem char method).
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