Beetle hysteria has raised its head again, and I am not talking about the Fab four. A prominent article in the New York Times titled “Tiny Beetle Adds New Dynamic to Forest Fire Control Efforts” quotes many foresters and others who suggest that beetle-kill trees across the West will create larger wildfires and by implications are “destroying” our forests.
For instance, Montana’s State Forester Bob Harrington said as much at conference recently, as in the article. While it may seem “intuitively obvious” that dead trees will lead to more fires, there is little scientific evidence to support the contention that beetle-killed trees substantially increases risk of large blazes. In fact, there is evidence to suggest otherwise.
At the heart of this and many other media reports are flawed assumptions about fires, what constitutes a healthy forest, and the options available to humans in face of natural processes that are inconvenient and get in the way of our designs.
For instance, one study in Alaska’s Kenai Peninsula looked at 2,500 years of bark beetle events, and wildfires and could find little correlation between the two. Similarly a study that looked at burn patterns of the 1988 Yellowstone fires found little evidence that recent bark beetle outbreaks had substantially increased fire spread in the Park (though an earlier beetle event did seem to increase fire spread slightly—more on why below).
In fact dead trees don’t automatically lead to more fires since climate/weather events, not fuels, largely controls large blazes. If the climate/weather isn’t conducive for fire spread, it doesn’t much matter how much dead wood you have piled up, you won’t get a large fire. As an extreme example, think of all the dead wood lying around on the ground in old growth west coast rainforests—lots of fuel, but few fires—because it’s too wet to burn. But it’s even more complex than that generalization.
First to understand why beetle-killed trees don’t necessarily lead to large fires, one needs to know more about how bark beetles affect forests. Younger trees are not killed by beetles, and remain in the forest to fill the void created by the death of more mature trees. In effect bark beetles “thin” the forest but typically there are still lots of trees growing on the site--some large mature trees and a lot of smaller ones. So the “forest” is not destroyed, nor does it “disappear” as may be implied from the hysterical statements coming from logging proponents, and others connected to the timber industry.
Furthermore, mature trees are not hapless victims. When a bark beetle attempts to bore into the tree, the tree uses its sap to push out the beetle and any eggs. A strong healthy tree with sufficient resources can often flush beetles out. It is not unlike the ability of a healthy moose to deter a wolf attack. Indeed, wolves are seldom successful in taking down a healthy mature moose unless circumstances give them the upper hand. Trees under stress from drought or damage from other causes are more vulnerable, just as a moose suffering from lower nutrient as a result of drought is more likely to be killed by wolves. So while beetles do kill trees, they aren’t able to “destroy the forest”—many smaller trees and even a significant number of mature trees survive.
Since bark beetles tend to focus on larger trees and not all trees are killed, this has important implications for fire risk. Fine fuels—not large snags—are the prime ingredient for sustained fire. So what you have after a major beetle outbreak is a lot of standing upright big boles. You can’t get big logs to burn unless you have fine fuels beneath them to sustain the heating process. That is why one uses small kindling and other fine fuels to start a campfire and must continuously feed small wood under the bigger logs to keep the fire going. Assuming you have the right conditions for a fire in the first place, a forest fire will spread more rapidly and with greater intensity in a totally green forest than a sea of dead boles, in part because the green forest possesses a lot more fine fuels in the form of resin-filled needles and small branches.
Furthermore, tree flammability is not constant, but varies over time. It is highest immediately after beetles kill the tree, and brown needles and small branches remain on the tree. However, after a winter or two, the needles and smaller branches are knocked from the trees and their flammability goes way down since the remaining upright snags are actually quite resistant to flames. It is generally only after understory trees released by the death of more mature canopy trees grow taller and provide a ladder into the canopy that fire hazard again increases. These ladder fuels, along with any dead snags that have toppled to the ground, can potentially lead to greater fire hazard. But this process takes decades. Thus the immediate threat from bug killed trees is not likely to be great, especially if the climate/weather is wet.
Of course, if you have the right conditions for a big burn, the dead trees will burn, but typically not at any greater potential than a forest of green trees. Green trees, after all, with their flammable resins in needles and branches are highly combustible under extreme drought and high temperatures. Indeed, there is some evidence to suggest that green trees will burn even hotter and with greater intensity than say a dead snag.
Potential is not the same as absolute. Most lodgepole pine, the primary species attacked by beetles in the Rockies, tend to be found at moister, higher elevations which simply do not dry out enough to burn well in most years. That is why lodgepole pine forests typically have long rotations between burns—on the order of hundreds of years in some places. Thus the presence of dead trees does not necessarily lead to fires. The probability that any particular bug-killed stand will be ignited by lightning or humans during the few years out of a hundred when they are dry enough to carry a large blaze is actually quite small. So even if there is a lot of dead wood on the ground, that doesn’t mean you will have large blazes. Probability is important—and the probability is low.
Even more importantly the news media often neglects to educate the public about the ecological value of bark beetles as “ecosystem engineers”. Beetles are essential to maintaining biodiversity in our forests. One study of bark beetles in Europe found that bark beetles created habitat for a wide array of other insect species, including many pollinating bees and warps, whose numbers increased in the forest gaps created by bark beetles.
But it’s not just insects that increase as a consequence of beetle kill. Dead trees created by bark beetles are used by cavity nesting birds, bats, and many small mammals. When dead trees fall to the ground, they provide hiding cover for insects, mammals, and amphibians from salamanders to frogs. Dead trees that fall into streams create aquatic habitat for fish. Of course dead trees are utilized by fungi, lichens, and as a source of new nutrients for new plant growth. Thus if we grant that an increase in biodiversity is important to the long term forest health, beetles are actually a sign of “forest health”.
Even the way a tree dies radically affects its future decay trajectory. A tree killed by wildfire, for instance, decays much slower than one killed by beetles. Beetles by penetrating the outer bark of a tree permit other organisms including other insects as well as fungi to enter the tree and begin the decay process. While fire killed trees, charred black by fire, are more resistant to decay. Bark beetles in addition to wildfire, wind throw and other natural events all contribute to different future forest dynamics.
The current spate of beetle outbreaks and dead trees in and of themselves is nothing to be alarmed about. However, there is another reason to be concerned. The current beetle “outbreak” may be harbinger of climate change that may radically alter future forest ecosystems. Warmer temperatures due to climate change may be responsible for the expansion of bark beetle outbreaks in the West (though there is historic evidence to suggest that past outbreaks affected as many or even more acres than what we see today). Cold winter temperatures, for instance, tend to kill beetles and keep their numbers under control.
Warmer temperatures not only increase the survivorship of beetles, but permit beetles to attack trees at higher elevation than in the past, and this has led to the death of many whitebark pine which, though occasionally attacked by beetles, usually are not affected to any great degree by bark beetles due to the whitebark pine’s preference for high cold elevations.
Yes bark beetles are killing many trees, but that necessarily won’t lead to large fires. Even if it did, there’s not much humans can do directly to forests to influence fire risk, except to begin reducing human causes of climatic change. Logging the forest will not significantly influence fire spread, and removal of dead trees has many negative impacts on forest ecosystems. Logging itself creates many additional environmental impacts such as greater sedimentation of streams, invasion of weeds, and so on that are far too often ignored by proponents of active forest management.
Nor can humans have much influence on the spread of beetles. To effectively reduce forest susceptibility to bark beetles, 50-80% of the trees have to be removed. Since that is typically as much, or in many cases even more trees than are killed by bark beetles, such let’s cut the trees to save them seems unwarranted. Plus there is no guarantee that the particular stand of trees that are treated with thinning are the same ones that will be attacked by beetles.
As far as community protection is concerned, it is far more cost effective to reduce flammability of homes than to attempt to reduce the flammability of forests. Focus fire risk reduction in and near homes, not out in the backcountry.
The important take home message is that we need a paradigm shift in our response to bark beetles. We cannot significantly influence large scale ecological processes like bark beetles and wildfire. Rather we must adapt ourselves and communities to learn to live with them. If climate change is ultimately the reason for changing tree vulnerability to beetles, than we should deal with reducing human sources of green house gases.
Secondly, beetles are not destroying our forests, rather are creating new ecological opportunities, increasing biodiversity, and creating greater ecosystem health.
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