Revisiting Fire History Studies

Revisiting Fire History Studies

One of the cornerstones of current forest policy is the assumption that western forests are outside of their “normal” density and appearance or what is termed “historic variability” due a hundred years of mismanagement that included logging of old growth, fire suppression, and livestock grazing. This idea has been used to justify logging public lands to “restore” forests to their pre-management era appearance and resiliency. Due to this past mismanagement we are told that forests are “overgrown” “decadent” and ready to burn.

Not to dismiss effects of the kind of management that largely continues unabated today, including on-going logging, grazing and fire suppression, but whether the current forest stand condition is that far from conditions that have occurred infrequently in the past is a matter of increasing debate.  This is especially important because public land management agencies feel the pressure to DO something beyond waiting for nature to do what it always does to “restore” forest conditions.

Timber harvesting is no longer done just to provide timber companies with profits or consumers with wood. Now lumber companies are involved in a much more noble enterprise—they are logging the trees to “restore” the presumed forest “health.”

The scientific basis for “restoration” is based to a large extent on fire scar studies. These studies suggest that the drier forests composed of lower elevation ponderosa pine and Douglas fir burned frequently and thus kept density low with “park-like” open stands of mostly larger trees. Keep in mind the discussion is focused on lower elevation forests since higher elevation forests like lodgepole pine, fir and spruce are characterized by much longer fire intervals, which have not experienced fuel buildups to any significant degree due to fire suppression, grazing, or timber harvesting.

So we often hear how such low elevation dry forests burned regularly at frequent intervals in “light, cool” blazes that removed the litter and killed the small trees, but did little harm to the larger trees.

Like a lot of myths, there is some truth to this generalization, and no doubt in some areas this characterization is accurate. But more recent studies using different methods have started to question this well-established story-line. These studies are finding that the intervals between fires is much longer than previously suspected, and that stand-replacement blazes (where most of the trees are killed) were likely common even in the lower elevation dry forests.

PROBLEMS WITH FIRE SCAR METHODS

The major method for determining the fire history of an area is to find trees with scars created by fires. If the tree is not killed by the blaze, it will develop a scar that can be counted in the tree rings. This record of past fires is then used to determine the “fire rotation” or the time it takes to burn an area equivalent to one’s study area.

There are four major flaws associated with traditional fire scar studies. These methodological flaws contribute to a bias toward shorter fire rotations—in other words, they tend to overstate the effect of fire suppression on forests because it appears that we are seeing more years between successive fires than we did in the past. If the fire rotation were judged to be longer, however, then much of what is being characterized as unhealthy forest may actually be perfectly normal and healthy.

The first flaw is targeted sampling. A researcher walks through the forest looking for areas with an abundance of fire scarred trees. The trees in this area are then sampled and used to determine the fire history for the area.  In the 1930s the bank robber Willy Sutton was asked why he robbed banks. Sutton is reputed to have replied with the self evident “because that is where the money is.” In a sense that is how fire researchers have gathered their data on fires—they sample in places with a lot of fire scars.

The problem with targeted sampling is that it’s non random. It’s like going into a brewery to poll people about whether they like beer. Places with an abundance of fire scars tend to have naturally low fuel loadings and frequent fires. But these sites may not be representative of the surrounding landscape such as north facing slopes or valley bottoms which may be wetter or have higher productivity and, thus, longer intervals between blazes. In fact, the reason non-sampled areas lack significant numbers of fire scarred trees is often because all trees were killed in a stand-replacement fire, but the omission of such areas from fire history studies leads to the false conclusion that stand-replacement blazes are unusual in dry low-elevation forests.

The second flaw is composite fire scars. Most fire studies add up all the fire scars recorded into a “composite” timeline. The problem with this technique is that the more scars you find and count over bigger and bigger areas, the shorter the fire interval becomes and the more risky your assumption that any fire recorded by one tree burned throughout the entire study area, even though some trees didn’t scar in the fire event. Some fire researchers now try to support this assumption by only including fire scars recorded the same year on 3 or more trees, but the trees do not have to be positioned throughout the study area, so even this will not eliminate the upward bias in frequency of fire in a given study area.

In other words, your composite may suggest a fire burned within your study area once every 5 years or whatever, but if most of these blazes burned only a few trees, then it is not accurate to say that the fire burned the entire area.  How frequent are fires that burn most or all of a large study area? These larger blazes may be far less frequent and take 100 years to burn most or all of the study area. Since the critical issue for the forest is the occurrence of the occasional blaze that burns most, if not all, of the entire study area, the fire rotation for those fires in such an area may be closer to 100 years, not the 5 years you get if you include even the tiny 1-acre fires.

The third flaw is an emphasis on the AVERAGE fire interval rather than the DISTRIBUTION of fire intervals. If you read fire studies carefully they will usually note the longest interval without any recorded fire. Often this is a significant period of many decades. Why is this important? Because the average person hears that there were fires, ON AVERAGE, every 5 years and assumes that fires operate like clocks on a regular schedule. In reality, fires burn in episodic groups usually dictated by periodic droughts that are controlled by shifts in offshore currents like the Pacific Decadal Oscillation, thus tend to be grouped together in certain drought prone decades.  The DISTRIBUTION of a fire interval shows clearly that there are always relatively long periods with little fire, even though the AVERAGE fire return interval might be 5 years.

Why is it important that we consider historic distributions of fire intervals rather than average fire intervals? Because the common assumption is that if the fire interval averages 5 years, fires would keep tree density low and reduce fuel build up. However, if it is also typical that there are also extremely long fire intervals of 80 years or more associated with the DISTRIBUTION of fire intervals, then there may not be an “abnormal” build up of fuel or increase in tree density, and nothing is out of the ordinary at all .

Finally the fourth major flaw is that traditional fire-scar studies have not been map based.  Why is a map of the distribution of trees with scars important?  It is only through such mapping that one can determine whether a scarred tree was in the middle of an extensive low-severity fire or at the edge of a high-severity patch.  One must look at the age distribution of the surrounding trees to gain real insight into the kind of fire that the scarred tree recorded.  This is what the more progressive fire ecologists are beginning to do, and they are finding that many fires in dry forest types are severe fires that burn relatively small areas within a larger fire perimeter, just like ALL fire we see burning in the same forest locations today.

Due to these flaws and errors in interpretation, many fire scar histories (but not all) misrepresent the fire regime associated with an area. If the period between fires was occasionally very long, then our forests may not be far out of their historic variability and may be well within that range of variation. If so, they do not require “restoration” because they are not out of balance.

The other major justification for logging is to reduce the chance that a community might be threatened but, as embarrassing as the facts are, there is no evidence that logging conducted miles from a town has anything at all to do with the probability that the nearest houses burn.

The fact that we are seeing more and larger fires fits perfectly with the pattern that is expected under current climatic conditions. In other words, if you have drier weather conditions, with high temperatures, low humidity and high winds, you will get more fires. You will get larger fires.

The prevailing climatic conditions are driving most of the apparent change in fire frequency and severity. For instance, the Southwest is in the grips of a drought that hasn’t been seen in five hundred years. Not surprisingly, there are fires now burning across the region bigger and more intense than any seen in the past.  However, Paleo fire studies confirm that such large fires may not be abnormal when compared to the fires that burned similar severe droughts occurred in the past centuries.

MANAGE FOR ECOLOGICAL PROCESS NOT SOME HISTORIC STAND STRUCTURE

Finally there is too much emphasis on “restoring” stand structure (in other words the presumed appearance) of forests rather than allowing natural ecological processes to occur on the landscape. It is more critical to accept and promote natural processes like beetle outbreaks, wildfires (including stand replacement blazes), and other natural ecological agents than to try to create some presumed historic forest structure that never existed in a steady state (and at taxpayer expense)! If natural ecological processes are allowed to occur on our public lands, then the forest will sort out the kind of appearance and structure that is appropriate for current climatic conditions. Critics will claim that a do-nothing approach outside the WUI will only lead to conversions of our forests to some other vegetation type, but the evidence for widespread type conversions is entirely absent.  Severe fires will restore forest conditions just fine.

All this is not to suggest that all historical reconstructions from fire scar studies are wrong—but it does suggest that most outside the pure ponderosa pine forests of the Southwest (and that’s most conifer forests in the West) are probably biased to some degree. Many of the logging proposals in the West are likely based on flawed assumptions about fire ecology and historic conditions.  And before any “restoration” logging is accepted as necessarily, the underlying assumptions should be carefully evaluated to make sure they are not skewed towards fire rotations that do not characterize the area accurately.

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