Misconceptions about disruption of fire cycles

One of the justifications for logging by the Forest Service around the West is the idea that fire suppression has led to fewer fires, and thus greater fuel build up than historically occurred. Therefore, mechanically reducing fuels—i.e. logging—is reasonable because otherwise we will see large wildfires.

 

However, both assumptions—namely there is fuel build up in all forest types due to fire suppression and secondly that large fires are undesirable—are increasingly being challenged.

 

Many agency people believe that fire suppression has led to fuel build ups that are outside of the historic condition. This is largely based on the research in ponderosa pine forests known as the Southwest Pine model which posits that frequent (1-20 years) low intensity surface fires reduced fuels, precluding large stand replacement fires.

 

So the story goes that fire suppression across the West has created forests with denser trees than would otherwise occur if fires had been permitted to burn unimpeded.

 

While this may be true for some ponderosa pine forests—and even this assertion is being challenged these days– it is much harder to make such a case for most other western forest and plant communities (like sagebrush/grasslands, chaparral, etc.).

 

I do not think there is solid evidence to suggest forest types like lodgepole pine or higher elevation spruce/fir, etc. have been significantly influenced by fire suppression efforts. And the bulk of the acreage burned around the West, particularly in the Rockies, occurs in these forest types.

 

Most researchers that specialize in the fire histories of these lodgepole/fir/spruce forests do not believe such forests are outside of normal historic conditions. Even the idea that there are larger stand replacement fires or beetle-kills than historically occurred is questioned.

 

I would like to point out a typical bias that I see and hear all the time. The Forest Service often promotes the idea that lodgepole pine stands are “overstocked”. Actually lodgepole and many other species always grow that way after a disturbance. There is frequently a significant amount of natural regeneration that is gradually whittled down by natural thinning agents like beetles or fire.

 

It baffles me that the FS tries to prevent beetles from “thinning” the forest when indeed, they believe they are “overstocked”. Beetles, disease and fire will thin them naturally, and these agents are much better at selecting which trees should live and die than any forester.

 

For example, some lodgepole pine due to differences in genetics have more resin ducts than others. Those with more resin ducts are better able to resist beetles because they can extrude the beetle eggs with their sap. So lodgepole pine surviving beetle attacks are more likely to have greater resin ducts. And this genetic ability may confer other benefits as well like resistance to drought.

 

Beyond the issue of “overstocking” many researchers question the notion that lodgepole pine have been significantly affected by fire suppression. Here’s why.

 

There are three principle ideas that you have to simultaneously keep in mind. And it takes a while to get your mind around all these at the same time.

 

The first is that just because you have fuel, doesn’t mean fires ignited and spread. The conditions suitable for fire spread are drought, low humidity, high temps and high winds. Without those ingredients you may have an ignition but it won’t necessarily spread well. Since having all these ingredients in the same place at the same time is relatively rare, most fires will self-extinguish whether they are “suppressed” or not.

 

The second point is that most of the burned acreage (thus natural fuel reductions) occurs in very infrequent but large fires. The occasional small blaze that burns a few acres here and there really has no significant ecological footprint. You can have a lot of these small fires, but they still don’t add up to much acreage.

 

The third point is that wildfires are not like clocks. We tend to think about “average fire intervals” and neglect to consider that often there were/are large gaps of time without any fires. These gaps are often as long as or longer than any effective fire suppression.

 

A study in Yellowstone’s lodgepole/fir forests supports this idea. Between 1972 and 1987 there were some 235 fires in the backcountry that the Park just monitored but did not suppress. Every single one of them went out on their own, and most did not burn more than a few acres. Only 15 out of the 235 burned more than a hundred acres and the total acreage burned during those years was about 33,000 acres. In the context of Yellowstone, none of these fires had any ecological impact or meaning because collectively they do not amount to a hill of beans in terms of reducing fuels across the several millions of acres of forest in the park.

 

If for instance, this had been on national forest lands, under normal FS suppression policies, all of these fires would have been suppressed, but would it have really made much difference? Not really. They were destined to go out and collectively did not burn that much acreage—only slightly more than 1 percent of the forest in Yellowstone during that 16 year interval. Averaged over a hundred years, if burning continued at this rate, little more than 5-7% of the park’s forest would have burned.

 

Of course, fires are not steady state, and such averages mislead what happens. And what happens is that there are few large fires, with occasional punctuated disturbance like the 1988 fires that blazed across 1.6 million acres in and outside of the park.

 

http://www.nps.gov/yell/learn/nature/wildlandfire.htm

 

So the idea that fire suppression has led to much greater fuel build ups may be a myth in most ecosystems because the vast majority of fires are exactly like those in Yellowstone. They really do not affect that much of the forest except during the very rare conditions when fires can spread explosively and rapidly across the landscape.

 

By contrast, it is the infrequent, but large fires (think 1988 in Yellowstone) that significantly affects the landscape. These are the fires that do the natural thinning, fuel reduction, etc. as well as reset the ecological clock.

 

But they are infrequent–often hundreds of years apart–and when they occur, like in Yellowstone in 88 we are essentially helpless to prevent or stop them–because the conditions for fire spread are so good.

 

Since lodgepole pine does not burn except for the most part under severe fire weather. The conditions that are favorable for such burns do not occur very often. Though there is variability in the timing of fire rotations, most studies show that lodgepole across the West tends to burn at longer rotations from 100-400 years.

 

So two things are going on here. First, fire suppression is largely only affecting the fires that are never going to be large in the first place, and thus have little ecological footprint–they are not going to significantly reduce fuel build up.

 

The fires that do affect fuel buildup–the big fires–occur under the extreme fire conditions that we can’t stop, but for better or worse, are infrequent.

 

The third point is that fires are not clock work.

 

If you have a fire rotation of 100-400 years, and up to 400 years can pass without a significant fire, than even if fire suppression were effective at stopping these infrequent large blazes–a questionable assumption as previously explained–these forests are not really out of their historic fire condition.

 

You could have 200 or 300 years pass without a fire–all the while fuels naturally are building–waiting for those right conditions of high temps, low humidity and high winds with an ignition source to set the entire blaze going.

 

You cannot think of these fires like mowing your lawn or something. If the fire rotation is 400 years, you don’t burn a quarter of the forest at 100 years, another quarter at 200 years and so forth. You likely burn some small percentage, but you might also go for 400 years without any blaze. So just because a forest hasn’t burned in 100 years, doesn’t mean there is greater fuel build up than what would have occurred naturally over the course of several hundred years.