Submitted by Ted Williams on Mon, 03/12/2007 - 07:46.
By George Wuerthner Forest Magazine, Spring 2007 Beetles have killed trees on more than a million acres in Colorado, and Senator Ken Salazar calls these dead and dying forests "the Katrina of the West." A headline in the Denver Post declares that Colorado hillsides are "blighted"; Joe Duda, forest management division supervisor for Colorado State Forest Service, says that more than 7 million trees have been "destroyed" in the past ten years. Trees are dying, not just in Colorado but across the West. An article in Missoula, Montana's daily paper declared that the state's forests are "under attack." A recent ABC news headline declared, "Beetle Epidemic Rends Northwest Forests." And Mark Rey, head of the U.S. Forest Service, testified at congressional hearings that without management to control these attacks, the public is at risk from catastrophic fire. In his testimony, Rey used the threat and fear of fire to justify logging in roadless areas. The cause of this furor is a small insect, Dendroctonus ponderosae, more commonly known as the mountain pine beetle. To a lesser extent the spruce beetle, Dendroctonus rufipennis, is also a factor in the destruction of spruce forests. For the most part, pine and spruce beetle populations are kept in check by parasites, disease, climate factors and the availability of host trees. But recently, beetle populations have increased throughout the West and are sweeping through pine forests from Arizona to British Columbia, leaving millions of dead trees in their path. In the wake of the widespread die-offs, hype and panic have reached levels normally reserved for wildfire or wolves in the West. The negative attributes used to describe the evolutionary processes are so ingrained that it's difficult to talk about them without using perjorative language-language that implies dire consequences. We are told we are facing a mountain pine beetle "epidemic" and that an "infestation" of trees will create "unhealthy" forests. But the fact is, none of these assumptions about the destruction of forests by beetles is supported by ecological research. There is no denying that pine beetle populations and activity have increased throughout the West in recent years, and there's no doubt that the dying trees they leave behind can offend our aesthetic sense of what a forest should look like. But whether the abundance of dead trees is a sign of "unhealthy" forests or whether the uproar is an exaggerated response to a normal ecological process-predation upon prey by a co-evolved species-is not clear. Bill Romme, a professor of ecology at Colorado State University who has studied fire ecology in Yellowstone, southwest Colorado and elsewhere, says that evidence from the past may indicate that insect outbreaks are, to a large extent, "natural" ecological events. "We know that extensive outbreaks occurred in the past: we have written documentation of outbreaks in the early twentieth century, [and] photos from the late nineteenth century show what looks like extensive insect-caused mortality," he says. Tree-ring evidence indicates even earlier outbreaks. Nevertheless, Romme cautions that current outbreaks are occurring synchronously over very large regions, involving several species of trees and insects. The size and scale of these events are larger than we have seen in recent years, perhaps precipitating the heightened alarm from some quarters. Many forest ecologists believe beetle activity may actually have a positive influence upon the ecosystem. "There is now evidence that in many cases forests are healthier after an insect outbreak," says Tim Schowalter, a Louisiana State University professor of entomology. He says we need to rethink our traditional view of insects as destructive. "The fact is, we will never resolve our problems with catastrophic fires or insect epidemics until we restore forest health, and in this battle insects may well be our ally, not our enemy." Beetle-killed trees provide ecological services for decades after their death. For instance, dead trees are important for cavity-dependent species, from flying squirrels to nesting birds. When the trees fall to the ground, they provide food and housing for creatures from small mammals to salamanders to forest snails. If the trees fall into a stream, they become an important component of aquatic habitat, slowing water velocity and decreasing erosive ability, protecting stream banks, and creating natural log structures where fish and aquatic invertebrates find shelter. At the landscape scale, beetle infestations create a mosaic of forest patches of various ages, densities, species composition and successional stages. In a sense, it can be argued that a healthy forest is one with an abundance of dead trees. Beetles naturally thin forest stands, adjusting tree numbers to available water and nutrients-performing for free a task that would cost active (i.e., human) management millions. The insects almost never kill all the trees in an area, and seldom attack younger trees. Upon the death of mature trees, understory trees are released from competition and grow rapidly to take a place in the canopy. According to ecologists at the David Suzuki Foundation in British Columbia, beetles perform many beneficial services to the forest. The foundation's website states, "Beetle outbreaks create diversity in forest structure, tree ages and species composition at stand and landscape scales, which are important for forest ecosystem health, diversity, and productivity." BUGS AS WOLVES To put the recent beetle activity into perspective, some understanding of the beetle's life history and its relationship to forests is needed. Mountain pine beetles are found in low numbers in lower-elevation pine forests, particularly among ponderosa pine and lodgepole pine forests. These tree species appear to have co-evolved with the beetle. The relationship between beetles and trees is somewhat analogous to the linkage found among wolves and their large prey, such as moose and elk. Much as wolves concentrate their attacks upon large herbivores that return the greatest calories per investment in the chase, the female mountain pine beetle seeks out large-diameter trees; they have the largest inner phloem layer, the living material that transports nutrients and water up and down the tree. The phloem layer is where she lays her eggs, and large trees have a greater ability to support a large population of beetle larvae. As a result, mountain pine beetles tend to attack forests with mature trees. However, like elk or moose, trees are not just passive victims. They respond to attack by extruding defensive allelochemicals-pitch and resins-that seek to "throw" the beetles out of the tree's bark. Trees with large reserves of moisture and carbohydrates can easily fend off minor beetle attacks. Even if the tree fails to keep a female from laying eggs, a strong tree can still flood the egg chambers, killing the larvae. Beetles are most successful when they seek out trees that are vulnerable because they are stressed, injured or otherwise less fit due to overpopulation or other factors. Nevertheless, the female beetle can increase her chances for reproductive success if she can overwhelm the tree's defenses. As she bores into the trees, the female beetle releases pheromones that attract many other beetles. Under certain favorable conditions, beetle populations grow rapidly and, like wolves, they can successfully attack and kill healthy individuals by overpowering tree defenses through mass attack. Beetles can exist at low populations for decades, held in check by other insect predators, birds and climatic conditions. Only under particular circumstances that occur infrequently can they increase in abundance to the point where they cause high mortality in trees. The first condition is forest age; there must be a sufficient number of large host pines to support a hefty population of pine beetles. Second, there must be enough time for the adult beetles to emerge in the spring to lay eggs so the larvae have sufficient time to grow to maturity, but not emerge so early as to be killed by spring frosts. Extremely cold winter temperatures appear to regulate the beetle by killing larvae. This is one reason why until recently, mountain pine beetles were rare among northern populations of lodgepole pine. Climate appears to be playing an important role in current beetle outbreaks. Mild winter temperatures that many attribute to climate change have reduced winter die-off of the beetles, while extended drought has increased the vulnerability of trees. Global warming appears to be exacerbating things further, allowing the beetles to invade stands farther north and at higher elevations than commonly seen in the past. For instance, beetles are now invading some high-elevation whitebark pine stands in the Rockies-something that has occurred infrequently in the past. In addition, large-scale fires and insect outbreaks in the last century have created extensive areas of trees that are now reaching mature sizes that make them ripe for beetle predation. Forest practices such as clear-cutting have regenerated large stands of vulnerable same-age, single-species trees. Fire suppression, along with the effects of livestock grazing, particularly in ponderosa pine forests, has created dense stands of trees that are weakened by the extended periods of drought that have been occurring throughout the West. LOG, THIN OR WAIT? In an effort to stem these large-scale insect outbreaks, many agency personnel, politicians and citizens support aggressive timber cutting to check the spread of beetles. Romme agrees that good forest management-including judicious timber harvest-can prevent or greatly reduce the likelihood of an outbreak being initiated at a stand level, by maintaining vigorous trees capable of resisting the insects. But he hastens to add that such prescriptions usually don't work once an infestation is in progress. University of Colorado ecologist Tania Schoennagel echoes this point. "Trying to manage forests to stop a beetle outbreak is like trying to outrun a freight train -- you simply can't run fast enough." By the time populations have reached significant levels, as they have in Colorado, it is almost impossible to stop the spread through logging. She says that thinning to increase tree vigor could be successful at low population levels, but once populations are high, beetles will successfully attack even smaller trees. Limited logging and thinning of trees may have a place as surgical tools on a small, healthy area to prevent the loss of trees, perhaps around a community. However, even if widespread thinning of living trees as a preemptive strike against beetles were a successful mechanism, the presumed benefits must be weighed against the environmental impacts that are associated with logging. Timber cutting often requires roads, which act as vectors for tree disease and weed invasion, and fragment forest ecosystems. For instance, aggressive logging to suppress a beetle invasion on Idaho's Targhee National Forest during the 1970s and 1980s dramatically changed elk migration in the area, and new roads made them especially vulnerable to hunters. As a consequence, the elk hunting season had to be reduced from two months to a week. Other species, including wolverines and grizzly bears, appear to be sensitive to human intrusions. Increased soil erosion and soil compaction often accompany logging operations, posing problems for aquatic ecosystems and water infiltration. Logging also removes future snags and downed woody debris and may have long-term impacts on nutrient cycling. Finally, the trees that foresters select to cut are not necessarily the same ones that would die as a consequence of beetle infestation. No one really knows what genetic consequences widespread thinning or logging might have on future forest vulnerability. Perhaps the very trees that foresters seek to remove will be the ones most resistant to global warming influences, or that have greater resistance to future exotic diseases or insect infestations. Forests have coexisted with beetles and wildfire for centuries, but the environmental impacts of human logging and thinning are a new evolutionary force whose long-term consequences we do not fully understand. Timber industry proponents also advocate logging of beetle-killed trees in the name of forest recovery. However, Schoennagel says that in almost all cases, logging inhibits recovery rather than helping forests recover from fire or insect outbreaks. "Following an insect outbreak, there are often smaller trees already growing in the understory that are released from competition and have a jump start on forest recovery," she says. "Logging has the potential to set the process of recovery back by damaging or killing already-established trees that have begun the process of forest regeneration." Schoennagel suggests logging can also potentially increase erosion on post-insect outbreak sites that, unlike post-fire landscapes, have an intact understory that helps curb erosion. "There may be economic reasons for salvage logging," Schoennagel says, "but rarely is there a strong ecological justification for salvage logging." Today, salvage logging beetle-killed trees is also justified by a fear of increased fire hazard. There is, however, limited evidence that dead trees contribute to higher fire risk in the long term. It's true that for a few years after a tree dies, its brown needles and fine fuels may increase flammability, but this period is short-lived. Once tree branches are broken off by wind and winter storms, the dead boles are fairly resistant to fire. Anyone who has tried to start a campfire knows that you don't pile on large logs-no matter how dead and dry-and try to ignite them. You need fine fuels to pre-heat the larger pieces of wood. But fuels aren't what drive the fires in many of the ecosystems affected by beetles. Dominik Kulakowski, a research scientist at the University of Colorado, says fires in spruce-fir forests that have been attacked by this bark beetle are no more frequent, extensive or severe than in other forests. "The historical occurrence of large and severe fires over the past centuries has been strongly associated with severe droughts," he says. "Fires in these forests are more contingent on climatic conditions and less contingent on the relative abundance of live and dead fuels." Simply put, if conditions are dry enough, large and severe fires in these forests are likely, and if conditions are not dry enough such fires are not likely, irrespective of the history of insect outbreaks. Kulakowski says forests themselves are not ultimately threatened by outbreaks of native beetles. "Lodgepole pine has evolved in the presence of mountain pine beetle, and Engelmann spruce has evolved in the presence of spruce beetle," he says. There are numerous examples of stands having been attacked-even very severely-and recovering. "This recovery can take time, but it does happen," he says. Perhaps the two greatest limitations we have when dealing with heightened beetle activity is lack of proper perspective with which to appreciate natural episodic events like fires and beetle outbreaks, and the patience to let Nature's evolutionary processes operate. In the end the greatest threat to our forests may come not from beetles or wildfires, but from overly aggressive management in the name of forest health and safety. ? Forest Magazine %7C FSEEE