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Tech Notes No. 2

March 1994

Fire Histories: Overview of Methods and Applications

Kathleen R. Maruoka and James K. Agee College of Forest Resources, University of Washington, Seattle, Washington

Contents:

• Introduction
• Fire History Approaches
• Point Frequencies
• Area Frequencies
• Sampling and Interpreting Fire Scars
• Management Uses and Implications
• Conclusion
• Suggested Readings

Introduction

Fire has the potential to change the structure and species composition of a forest and has undoubtedly influenced the development of the forests we see today in the Blue Mountains. While we can directly observe the effects of recent fires on present forest structure and composition, we must infer the effects of previous fires using current stand structure and composition together with a record of fire occurrences.

Fire can be thought of as part of a "disturbance complex" comprising insects, pathogens, wind, and other disturbances, which contribute to landscape and species diversity. The interactions are dynamic with combinations unique to every forest. Because these factors are interdependent, removing or altering one of them changes the roles of the other interacting disturbances. For instance, it has been suggested that fire exclusion has resulted in larger western spruce budworm (Choristoneura occidentalis) outbreaks than previously recorded. In the absence of fire, the western spruce budworm plays a similar role in the forest, killing tree species and age classes that would have burned previously if wildfire ignitions had not been successfully suppressed. Because larger outbreaks increase the number of dead and weakened trees, they influence the magnitude of subsequent fire and wind events.

Determining the historic fire frequency for a stand helps us understand the role fire has played in stand development. This information is important for interpreting several of the current "forest health" issues in the Blue Mountains, and serves as base information for forming forest management strategies that incorporate natural or prescribed fire.

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Fire History Approaches

The two main approaches to developing a fire history are through analysis of point frequencies and area frequencies. Point frequencies assess fire occurrence at one location while area frequencies assess fire occurrence at the scale of the landscape. Although both yield a "fire frequency," the frequencies represent different types of information because of this difference in scale. Selecting the appropriate method depends primarily on the vegetation types and physical features of the study area, as well as the type of fire evidence present.

Vegetation types are important to consider because of different flammabilities, both in terms of standing fuel, and in rates and levels of fuel accumulation. Topography and physical features are important because changes in substrate or terrain can affect vegetation composition, cover, and continuity. Such changes can serve as effective barriers to fire spread and deserve consideration when formulating an approach to constructing a fire history.

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Point Frequencies

A point frequency represents repeated occurrences of fires at a single location and addresses the question "how often did fire burn across this point?". Point frequency methods are best used to describe the occurrence of fires in areas that experience low-severity fires, because such fires commonly leave small, datable scars on the mature trees. These fires generally consume forest litter and kill small understory trees without killing larger trees. This type of fire regime is often associated with ponderosa pine (Pinus ponderosa) forests and other lower elevation forests where pine is a codominant species.

Thick bark usually protects larger trees from being killed by low-severity fires. However, bark fissures and previous wounds offer less thermal protection and fires may kill localized portions of the cambium of a tree. Subsequent annual growth rings heal over the damaged cambial tissues from the edge of the wound inward, resulting in distinct ring patterns when viewed in cross-section (figure 1). These scars are the basis of a point frequency.

Figure 1. Fire scar formation. The cambium beneath a thin area of bark (indicated by arrow) is susceptible to damage from fire (a). Subsequent annual growth rings heal from the wound edge inward, producing distinct ring patterns (b, c). Repeated fires may create multiple scars along a single radius (d).

While a point frequency can be constructed using the scar record from one tree, a more comprehensive fire record can be compiled using data from several neighboring trees with fire scars. Because fires do not burn across landscapes uniformly, and because every fire does not scar every tree, increasing the number of sampled trees also increases the likelihood that a fire will be detected. Caution must be taken, however, to ensure that the sampled trees are close enough to represent one point on a landscape. If the trees are too distant, fires will be presumed to have burned across areas that in fact they may not have, and the frequency estimate will represent an area frequency rather than a point frequency. In general, a point estimate should not include samples from an area larger than the smallest area usually burned. In the Blue Mountains, this may be from one to several acres. Details about point frequency sampling methods and compiling fire chronologies may be found in Arno and Sneck (1977), Dieterich (1980), and Agee (1993).

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Area Frequencies

The utility of an area frequency lies not in expressing the fire frequency at one point, but across a much larger area. It is typically used in areas where fires are severe and kill most of the trees. The fire record is represented by different age classes of trees across the landscape. There are two types of approaches to determine area frequencies, the first of which is called "natural fire rotation," and the second called "fire cycle."

Natural Fire Rotation - Natural fire rotation is an area frequency technique that evaluates fire-return intervals in a study area based on total area burned over time, usually measured in centuries. It requires the reconstruction of all past fire events during the period of study. Unlike the point frequency method, which relies on the presence of fire-scarred trees, the area frequency method uses stand ages as well as any fire scars to recreate the extent of past fire occurrences. Although fire scars are useful when present, stand ages and charcoal are often the only evidence remaining after severe fires. In the Blue Mountains, the natural fire rotation method can be applied to lodgepole pine (Pinus contorta), mountain hemlock (Tsuga mertensiana), subalpine fir (Abies lasiocarpa), and moist grand fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) sites because these forest types are often associated with stand-replacing fires.

The natural fire rotation is calculated as the quotient between the total time period considered and the total proportion of the landscape burned during that time period. Because some areas may have burned more than once, the total proportion of the study area burned may exceed one. For example, if 20,000 acres burned in a I 0,000 acre study area over a period of I 00 years, the natural fire rotation calculation would be: 100 years/ [20,000 acres burned/10,000 acre study area] = 50 years.

A natural fire rotation does not imply that every stand in the study area burns with the same frequency within the calculated time period. Some stands used in the calculation will have burned more than once and some not at all. If the study area is topographically or vegetatively fragmented, it may be useful to divide the study area into smaller areas to reduce the amount of spatial variability. Similarly, considering time periods with known differences in human or climatic influences as separate periods may reduce temporal variability. Inferences drawn from areas or periods with very different features may be misleading.

Compiling information to calculate a natural fire rotation begins with identifying stand ages across the study area. This can be accomplished using aerial photos and field reconnaissance. Once the stands have been identified, establishment periods are documented by coring several trees within each stand. Stand perimeters can then be more clearly delineated. Sampling density depends on the age, size, species, and topographic uniformity of the stand.

Establishment and regeneration periods differ with tree species, location, and fire severity, and may continue for several decades following a fire. However, establishment periods are usually much shorter than the fire-free intervals, so using differences in stand establishment dates serves as a proxy for detecting different fire events. Because there is often a lag time associated with tree establishment following a fire, the earliest tree establishment date in a stand is assumed to be a conservative estimate of the year that fire occurred in the stand; the fire actually may have occurred several years prior to the year that the sampled tree became established.

Stands are then combined into a stand age mosaic. Topographic features such as ridges, streams, rivers, and swamps, as well as vegetative features such as large areas of discontinuous fuel are often natural fuel breaks and are used in conjunction with the stand age mosaic to interpret the fire history of the study area. For example, in areas without significant fuel breaks, stands with similar establishment dates separated by a younger stand most likely originated after the same fire event. The younger stand also burned in the earlier fire but was created by a subsequent fire burning through a portion of the earlier burn.

Two aspects of the calculation promote a conservative estimate of the natural fire rotation period. First, we may be unable to detect light burns that leave little evidence. Second, the evidence of some past fires may have been obliterated by more recent fires. Both may result in a longer estimate of natural fire rotation than actually exists.

Fire Cycle--The fire cycle is a statistical model of fire history based on current stand ages (Johnson and Van Wagner 1985). The model is based on the negative exponential or Weibull distributions, and best represents fire frequency in large areas where a small propor- tion of stands burn intensely each year, resulting in a mosaic of stands with different ages. Certain assumptions are made in fire cycle calculations regarding ignition patterns, fire size, stand flammability, topographic uniformity, and stable climate. They assume ignition patterns are random, that only a small portion of the landscape burns each year, and that the fire cycle has been relatively constant over time. These assumptions limit applications in topographically fragmented terrain, or in areas where the largest fires are a significant portion of the total study area. The fire cycle best approximates fire occurrence when very large study areas (200,000-300,000 acres or more) are used, so that the average fire size is only a small proportion of the total landscape. The fire cycle model is widely used in boreal landscapes but has not been applied as much in temperate forest landscapes.

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Sampling and Interpreting Fire Scars

Fire scars are basal wounds which may extend over I0 feet up the bole of a tree. Large fire scars are often referred to as cat-faces. Once a tree has been scarred, it is likely to be scarred along the same radius in subsequent fires. Trees with multiple scars provide the most information about fire occurrences.

There are several ways to sample fire scars. Collecting a complete cross-section from a scarred tree is the most definitive method for identifying fire scars but destroys the tree. Another method is to collect a cross-section of one-half of the scarred surface. This can be accomplished by cutting a wedge containing the scars. Alternatively, the tree can be sampled by making two parallel horizontal cuts and two plunge cuts, and extracting the section (Arno and Sneck 1977). These sampling techniques have the advantage of, allowing the tree to survive, but may predispose it to windthrow or pathogens. Thus, it may be important to consider safety precautions as well as scar record when selecting trees in certain areas.

In instances where collecting sections is not possible, several increment cores of a fire-scarred tree may be taken (Barrett and Arno 1988). One core sample should contain the fire-scarred ring and the growth ring immediately before it. Another core taken from an unscarred portion of the tree can then be used as a reference to match the ring sequences of the other cores and deduce the year of scarring. This method is the least intrusive, but generally provides a record limited to the most recent scar. Stumps with fire scars are another source of information. If a stump is sound, an entire cross-section can simply be sliced through the stump. After the scar samples have been collected and airdried, they are sanded with successively finer grades of sandpaper until the ring patterns become clear. Counting rings from the bark edge inward using a stereoscopic microscope results in a single record of fire occurrences. The average number of years between every pair of sequential scars is the mean fire-return interval contained in the scar sample. The mean fire-return interval and the variability of the mean fire-return interval are important descriptors of fire frequency.

Cross-dating fire scars refines and extends fire scar records. Cross-dating is accomplished by matching growth ring patterns from two or more samples. It allows us to distinguish between closely-spaced fire scars and to determine the actual years that fire burned. Stump samples must be cross-dated unless the exact year the tree died is known.

Cross-dating fire scars can be accomplished two ways. Unique ring patterns can be identified by comparing two or more scar samples (figure 3).

Figure 3. Cross-dating fire scar samples. These hypothetical samples were taken in 1992. Marker years are identified by an asterisk (*). Fire scar years are indicated using heavy arrows. Note that combining scar records results in more fire occurrences than represented on each individual sample.

These marker ring patterns can be used to calibrate the samples with each other. Alternatively, a master ring chronology can be compiled using increment core samples from several nearby trees against which ring patterns from the scar samples can be compared. The advantage to using a master chronology is that missing rings can usually be detected and ring counts can be adjusted accordingly. It should be noted that the ring patterns near the scars are often distorted and may confound cross-dating efforts.

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Management Uses and Implications

Interpreting current stand structure with fire history information can provide a more complete picture of stand development than stand structure alone. Age cohorts, spatial distributions, and species distributions of trees within a stand or across a landscape may reflect fire frequency, extent, and severity.

An area in the Burns Ranger District in the Malheur National Forest near Myrtle Creek provides a good example of how stand structure and fire history data can be combined. A sample of fifty trees representative of the species and age structure at this site shows that most of the older trees are ponderosa pine, while the younger trees are primarily grand fir (figure 4).

Figure 4. Tree establishment dates in 10-year-interval classes and historic fire occurrences (T) at Myrtle Creek. PSME = Douglas- fir, PIPO = ponderosa pine, ABGR = grandfir. Note the increase in grandfir after frequent fires ceased.

During the period of 1752 to 1890, there were I0 fire scars at this site for a mean fire-return interval of 15.3 years and an interval range from 5 to 23 years. The absence of fires prior to 1752 is probably more related to the absence of fire evidence than the absence of fire. Fires were frequent between 1752 and 1890, and were probably low-intensity. Ponderosa pine was able to withstand the fires, while thinner-barked seedlings and saplings of grand fir and Douglas-fir were killed. When the fire frequency decreased in the late nineteenth century, trees that were unable to survive under a frequent fire scenario were able to establish and survive. Examining stand development in the presence and absence of fire may foreshadow changes which may have future management implications. For instance, extended periods without fire in forests that historically burned frequently may result in significant changes in stand composition and structure and have the potential to change the severity of future fire events from low to moderate or high. Structural changes might also influence the magnitude or types of other disturbances in the forest, such as insect and pathogen outbreaks.

Similarly, fire history studies can be applied to prescribed burning programs. Mean fire-return intervals are often used as guidelines for fire frequency in prescribed burning programs. However, the temporal and spatial variability of fire events within and between forest types may have more important implications thansimply a mean fire-return interval and should also be considered. Using the Myrtle Creek example and assuming a desired prescribed fire plan to burn the forest at a "natural" fire return interval, the 15.3-year mean fire-return interval would be recreated by simulating both the mean and its variances Rather than fires exactly every 15.3 years, a variable set of intervals with the same mean is desirable. For example, fire return intervals of 9, 20, 22, 5, 12, 20, and 19 would produce a mean fire return interval of 15.3 years and mimic the variability of historic fire frequency.

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Conclusion

A fire history documents past fire events, which occurred under different climatic and floristic conditions than current conditions. Thus, applying fire history to forest management should be done with caution after considering these influences. Nevertheless, a fire history of an area can provide valuable information about the influence of fire on forest development. This understanding can be used by managers interested in using fire to achieve management goals.

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Suggested Readings

Agee, J.K. 1993. Fire ecology of Pacific Northwest Forests. IslandPress, Washington, D.C. 493 p.

Arno, S.F. and K.M. Sneck. 1977. A method for determining fire history in coniferous forests of the mountain west. General Technical Report INT42. U.S. Dept of Agric., Forest Service, Inter-mountain Research Station. Ogden, UT. 28 p.

Barrett, S. and S.F. Arno. 1988. Increment-borer methods for determining fire history in coniferous forests. General Technical Report INT-244. U.S. Dept. of Agric., Forest Service, Intermountain Research Station, Ogden, UT. 15 p.

Dieterich, J.H. 1980. The composite fire interval- A tool for more accurate interpretation of fire history. In: Stokes, M.A, and J.H. Dieterich, tech. coords. Proceedings of the fire history workshop. General Technical Report RM-8 1. U.S. Dept. of Agric., Forest Service, Rocky Mountain Research Station. Fort Collins, CO. p. 8-14.

Johnson, E.A. and C.E. Van Wagner. 1985. The theoryand use of two fire history models. Can. J. For. Res. 15:214-220.

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TechNotes are produced in cooperation with the Blue Mountains Natural Resources Foundation and Institute Partners.