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USFS video from the air over California's dead trees

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The Sierra Nevada forests viewed from the air. Video by USFS. This shows the catastrophic devastation and wildfire threat after 4 years of drought and bark beetle infestations. For more information: http://southtahoenow.com/story/06/23/2016/usfs-66-million-dead-trees-sierra-26-million-october

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University of Wyoming Ruckelshaus Institute of Environment and Natural Resources

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UF-IFAS School of Forest Resources & Conservation

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BEETLE - Our Future Forests: Beyond Bark Beetles

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The mountain pine beetle has transformed our western forests in recent years. Meet this diminutive insect and glimpse its life in the pines. Visit http://www.beyondbarkbeetles.org to view the rest of the videos in this ten-part series exploring how bark beetles are changing our forests, and human responses to those changes.

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UF-IFAS School of Forest Resources & Conservation

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Pine Bark Beetle Treatment | Tree Barber Tree Service

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UF-IFAS School of Forest Resources & Conservation

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A looks at how bark beetles have overtaken Western forests.

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UF-IFAS School of Forest Resources & Conservation

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California Drought Causing Infestation Of Tree-Killing Bark Beetles

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California's historic drought is causing an infestation of bark beetles that's destroying trees in the state's forest lands. Conservationists say it's hit epidemic proportions in some areas. (5/5/15)

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(2) spread of blue-stain fungi. Several species of bark beetles may attack in concert, partitioning the tree by elevation. Roundheaded pine beetle, western pine beetle, mountain pine beetle, and several species of Ips may all be found on severely infested trees.

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The conifer forests of the North American west have been under a massive assault over the past decade by bark beetles: one species alone, the mountain pine beetle, has killed more than 70,000 square miles' worth of trees, equivalent to the area of Washington State, and two recent studies have shed some light on how climate change is helping fuel the assault, and what's likely to happen in a world that continues to warm.

The first, published in the journal Ecology, shows how intense drought can bring on a population explosion in the voracious insects - and how this creates a vicious cycle of tree-killing even when drought subsides. The second, published in Proceedings of the National Academy of Sciences (PNAS) reveals that warming lets beetles move to higher elevations, where they're encountering trees that are unusually susceptible to infestations.

Two recent studies shed some light on how climate change is helping fuel the assault of bark beetles, and what's likely to happen in a world that continues to warm.
Credit: Colorado State Forest Service

Given the enormous destruction wrought by bark beetles in recent years, it's natural to think of them as unstoppable eating machines. "Because we notice the big outbreaks," said Ken Raffa, a University of Wisconsin entomologist who co-authored the PNAS study, in an interview, "it's easy to picture forests as one big salad bar for the insects. But most of the time, they only go to the most stressed trees."

That's because trees have evolved defenses against the beetles in the form of natural chemicals that repel or even kill the attackers. In times of severe drought, however, the number of trees under stress in a forest will skyrocket. When that happens, as it did during a 2001-2002 Colorado drought documented in the Ecology study, the dramatic increase in relatively defenseless trees allows beetle populations to skyrocket.

"Once the beetles reach epidemic levels," Teresa Chapman of the University of Colorado, the study's lead author, said in an interview "the epidemic has a life of its own. Now, even if favorable conditions return, there are just too many beetles and [the trees] can't defend themselves."

Favorable conditions will probably return less easily in years to come, however, as with a warming world, scientists expect droughts to be more intense, come more often and last longer.

Meanwhile the beetles, whose numbers would normally be held in check by cold winters that kill their larvae, are surviving in greater numbers from one year to the next as winters in the U.S. continue to get warmer. Not only that: as spring comes earlier and temperatures stay warm for longer, the beetles can fly further than they once did, allowing them to extend their range.

This range extension includes movement to higher altitudes, where beetles are encountering tree species they would normally not run into. That includes the white bark pine, the subject of the PNAS study. "What we find is that unlike the [lower-elevation] lodgepole pine, the white bark pine hasn't evolved any defenses against the beetles," Raffa said.

The combination of increasing stress on trees and decreasing limits on the beetles' range and winter survival, say scientists, makes it almost inevitable that the insects will continue to experience a population boom - at the expense of the great conifer forests of America's mountain west.

Related Content:
In Fear of Firebugs Across the West
Warming climate is helping bark beetles spread in the West
Warming Winters in the U.S.
Spring Comes Early Across the U.S.
Report Details Climate Change, Extreme Weather Links

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Why Bark Beetles are Chewing Through U.S. Forests

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The conifer forests of the North American west have been under a massive assault over the past decade by bark beetles: one species alone, the mountain pine beetle, has killed more than 70,000 square miles' worth of trees, equivalent to the area of Washington State, and two recent studies have shed some light on how climate change is helping fuel the assault, and what's likely to happen in a world that continues to warm.

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Learn About the Park - Yosemite National Park (U.S. National Park Service)

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National Park Service Logo National Park Service

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Park hydrologists collect streamflow data to determine snowmelt run-off levels and stream depths along the forceful Merced River.

Hydrology: Yosemite is famous for its waterfalls, particularly the 2,425-foot Yosemite Falls-North America's tallest. However, water shapes this landscape in myriad other ways. Winter snowpack is a major driver of the park's hydrologic system, so it is carefully measured at 13 different courses with records dating back to 1931. Spring floods reshape the land during a natural house-cleaning that scours river channels and redistributes rock, soil, silt, and sand. Dry summers, especially at the lowest elevations, force plants to adapt to a lack of water and the presence of fire.

Geography & GIS: Geography is both a natural and a cultural science, just as Yosemite is a site of incredible natural beauty as well as a rich cultural history. Mapping reveals the relationship people have with the spaces they inhabit. Complex mapping software, called geographic information systems, allows park scientists to carefully map and analyze Yosemite's 750,000 acres. By understanding where rockfalls occur most often or how fire behaves in these forests, scientists can assess management needs and protect the park and the people within it. Projections of climate change effects, for instance, reveal why wildlife and plants are moving higher in elevation to find suitable habitat.

View national and Yosemite GIS datasets on an NPS Data Store.

Watch a Yosemite Nature Notes 10-minute video on Yosemite maps that portrays the physical and cultural landscape of the park.

Wilderness: Wilderness is a place unchanged by people: a place of solitude, a place of peace, a place of adventure and learning. Here, you will find no cars, no roads, no electricity, and no modern conveniences. Nearly 95 percent of Yosemite is congressionally designated wilderness, which, "in contrast with those areas where man and his own works dominate the landscape, is an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain."

Learn more about Yosemite's Research and Studies, including ecosystem conditions affecting lichen and black oak tree growth, and animals like the Sierra Nevada yellow-legged frog.

Watch a Yosemite Nature Notes video on wilderness.

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Natural Features & Ecosystems - Yosemite National Park (U.S. National Park Service)

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Hydrology: Yosemite is famous for its waterfalls, particularly the 2,425-foot Yosemite Falls-North America's tallest. However, water shapes this landscape in myriad other ways. Winter snowpack is a major driver of the park's hydrologic system, so it is carefully measured at 13 different courses with records dating back to 1931.

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Bark beetles: A natural and dramatic forest disturbance | Rocky Mountain Research Station

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Armies of tiny bark beetles are ravaging drought-weakened pine trees throughout California in a fast spreading epidemic that biologists fear could soon turn catastrophic.

Local, state and federal officials are virtually helpless against the pestilence, which has turned hundreds of thousands of acres of forest brown and left huge fire-prone stands of dead wood.

The trees are being devoured by millions of native beetles, each about the size of a grain of rice. The insects, thriving in the warm weather and lack of freezing temperatures, are overwhelming the defenses of water-starved trees, attacking in waves and multiplying at a frenzied pace, depositing eggs under the bark that hatch into ravenous larval grubs.

"Things are looking really, really bad," said Tom Smith, a forest pest management specialist for the California Department of Forestry and Fire Protection. "Basically we've got native bark beetles that are attacking the pines. They are only successful in attacking the trees when the trees are stressed. Right now all the trees are stressed because of drought."

The infected trees are on private and public lands, in national parks, wilderness areas and managed forests. There seems to be no solution short of removing the dead and dying trees and hoping against hope for rain and cold. The worst of it is in the southern part of the state, but pest management experts say the plague is moving north.

Mount Diablo is one of the hardest hit areas in Northern California, where a forest of Coulter pine, also known as big-cone pine, has been devastated. Bill Miller, an environmental scientist for the California Department of Parks and Recreation, said close to 50 percent of a nearly 1-square-mile stand of Coulters on the north side of the mountain is dead or infested with beetles.
Conditions favor beetles
"It's a combination of these trees being stressed and a little weaker from drought, and the number of beetles that are there right now," said Miller, who first noticed a line of dead trees on Meridian Ridge last spring and has watched helplessly as the marauding beetles have spread. "Conditions right now favor the development of beetles. We have a lot more susceptible trees and the beetles are making use of an opportunity."

Large numbers of dead pines have been reported in the forested hills above Clear Lake and in some areas of the Sierra around Lake Tahoe, but it is south of the Bay Area where the trees are really going into death throes.

State forestry officials estimate that from 20 to 40 percent of the trees are dead or dying between Calaveras County and the Kings County area near Fresno, with entire hillside forests completely brown.

In Cambria, the picturesque coastal community between Big Sur and San Luis Obispo County, 80 to 85 percent of the region's native Monterey pine forest is dead or dying. Tree mortality is also bad in the Tehachapi Mountains, in the far southern part of the state.

"In the southern Sierra Nevada it's devastating," Smith said. "It's epidemic and its probably going to get even worse."

Tree deaths are not unusual during sustained hot, dry periods. It happened during previous droughts over the past four decades. In 2003, then-Gov. Gray Davis proclaimed a state of emergency for Los Angeles, Riverside, San Bernardino and San Diego counties because of a bark beetle infestation that at one point threatened more than 6 million acres of California forest.

But experts say this die-off has the potential to be even worse. The beetle population is normally kept in check by the winter cold, but three years of above-average temperatures and lack of snowfall have given the growing bug hordes free rein to search and destroy.

"It's scary in areas - the amount of dead and dying trees I am seeing," said Smith, who has been studying bark beetles in California forests for 15 years. "This is the worst I've seen."
Tree mortality doubles
Sheri Smith, the regional entomologist for the U.S. Forest Service in California, said bark beetle and drought-caused tree mortality more than doubled across forests in California last year and is expected to increase even more this year. The Forest Service mapped tree mortality across 820,000 acres of forested land in 2014 compared with 350,000 acres in 2013, according to the service's annual Aerial Detection Survey Program results. The results of this year's surveys, which are just getting under way, are not expected until later in the year.

Smith and others said the drought is impacting many tree species, including oaks and incense cedars, but pine and fir trees are the most obvious victims. Some 460,000 acres with dead or dying fir trees were observed last year by the Forest Service, she said.

"Many communities are in that lower elevation pine belt, so the trees around them are what they notice dying," she said. "That's where people live, where they drive and what they see."

The bark beetle infestation is not limited to California. The ravenous insects have decimated 45 million acres of forest in the Western United States in recent years, including 15 million acres of Forest Service land. Scientific studies have shown that trees are dying faster than ever in the old-growth forests of California and the mountains of the West. Scientists have linked the phenomenon to rising temperatures, earlier than normal snowmelt, drought, forest fires and insect infestations.

There are 19 different types of pines in California, including sugar, ponderosa and lodgepole pine. Red and white fir are the most common species of fir in California, Smith said. Forest pathologists say bark beetle infestations are the primary difference between pine and fir trees and other species that are also suffering ill effects from the drought.

Of the 220 species of bark beetle, about a dozen feed on California pines, including the Western pine beetle, the pine engraver or Pinyon ips and the red turpentine beetle. Smith said the beetles attack stressed trees, which have trouble producing enough pitch to drive away the insects. The beetles then release pheromones that attract other beetles, which quickly overwhelm the tree's defenses.

The different beetle species specialize in certain types of pines and generally stay in specific regions. The trees on Mount Diablo are being attacked by the California fivespined ips. Western pine beetles attack ponderosas, but have also been known to dive into Coulter pines. The mountain pine beetle is wreaking havoc on the lodgepole and ponderosa forests in the Rocky Mountains and in British Columbia.

The insects bore through the pine's bark where they lay eggs. The larvae then feed on the tree's living tissue, which cuts off the tree's ability to transport nutrients. An infestation can involve several thousand beetles, which can then spread to neighboring trees.
Acres of tinder created
The result is that there are now large pockets of California forest, in some cases stretching hundreds of acres, full of mostly dead tinder.

Experts say frequent fires once controlled the beetle populations in California until flame suppression became a forestry management hallmark. Although fire officials believe the dead wood from beetle outbreaks increases the likelihood of out-of-control fires, a recent University of Colorado study found that bug-infested forests are no more likely to burn than other forests.

But forestry officials and biologists say the issues are bigger than just fire. Dying bug-infested forests disrupt the food web, wildlife and local economies. Falling trees have crushed cars and closed campgrounds. The soil in denuded forests can erode away, taking with it forage material for birds, mammals and insects.

"The amount of dead and dying trees is certainly a very significant fire hazard," said Ken Pimlott, the director of Cal Fire. "It's a multifaceted challenge. Large stands of dead trees create another problem also because, when you get into the Sierras, many of the homes have been built in and around these trees. We don't want to create alarm, but this is a forest health issue that has become a public safety issue."

Pimlott said the situation in California screams for more diligent forest management, including thinning of overcrowded stands of timber, removal of dead trees and clearing of dry brush. Problem is, all of that costs money, which many property owners can't spare and legislators are generally reluctant to part with.

Climate scientists aren't sitting idly by while others do all the hand wringing. As the forests shrink, they say, less carbon dioxide is absorbed. That means more greenhouse gases will enter the atmosphere resulting in the acceleration of global warming, researchers said.
Changes likely permanent
Experts say some of the changes to the ecosystem are likely to be permanent unless the weather pattern in the state gets moist soon.

"Even if we see a wet pattern this year, the damage is already done to these forests and it is going to take a few years to recover," Pimlott said. "We are watching to see what scale it grows to. There is really nothing we can do when it gets to this point to change the outcome."

Meanwhile, the drought persists and, as if things weren't already bad enough, Mother Nature has produced even less rain and snow in California this year than last. It is a situation ripe for hungry beetles, which, in turn, create more fuel in a wildfire-prone state.

Peter Fimrite is a San Francisco Chronicle staff writer. E-mail: pfimrite@sfchronicle.com. Twitter: @pfimrite.

California bark beetles
Individuals are not much larger than a piece of cooked rice.

They survive in trees that are stressed or diseased and cannot secrete enough defensive resin, or sap, to drown the beetles.

The beetles emit pheromones that attract other beetles.

They lay eggs in the moist inner bark of the tree and the larvae feed on the living tissue, cutting off the tree's ability to transport nutrients.

Find more information about bark beetle management at http://www.fs.usda.gov/main/barkbeetle/barkbeetlemanagement
Common California species
Mountain pine beetle: Native to the forests of western North America, it attacks ponderosa, lodgepole, sugar and other types of pine trees, but does not infect Jeffrey pine.

Western pine beetle, right: Native to Western North American forests, it primarily attacks ponderosa and Coulter pines in California.

Douglas fir beetle: Native to the forests of western North America, it only attacks Douglas fir trees, often in areas where there has been wind damage.

Pinyon ips: This pine engraver beetle has a spiny back end typical of all the "ips" species. It attacks pines and green slash pine and often kills from the top down.

Fir engraver beetle: This native California beetle attacks fir trees, but not Douglas fir. Its larvae spend the winter under the bark and emerge from June through August.

Jeffrey pine beetle: Native throughout the range of Jeffrey pines, it has a distinctive reddish hue. These voracious beetles will kill a tree and then move on to neighboring green trees.

Red turpentine beetle: The largest of the pine bark beetles in California, it primarily infests sick or damaged trees and freshly cut stumps. Attacks by this reddish-brown beetle don't always kill trees, but may predispose them to attack by other beetles.

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Bark beetles ravaging drought-stricken forests in California

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Bark beetles ravaging drought-stricken forests in California Armies of tiny bark beetles are ravaging drought-weakened pine trees throughout California in a fast spreading epidemic that biologists fear could soon turn catastrophic. Local, state and federal officials are virtually helpless against the pestilence, which has turned hundreds of thousands of acres of forest brown and left huge fire-prone stands of dead wood.

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Recognizing the signs of beetle infestation in your trees is an important consideration when living within the forest. A tree that appears to be perfectly healthy with green needles may also be infested. Once a tree has been successfully "mass attacked" by beetles, the tree will die. The tree can remain green for 12 months or longer after a successful beetle attack. By summer to fall of the following year after the attack, the tree will start to flag; showing signs of health decline.

Flagging is the term given to the process of the tree's needles turning from green, to yellow, to red. It is best to assess whether or not your trees have been attacked by beetles in the late summer/early fall immediately following beetle flight.

Determining whether the attacks have been successful can be difficult. Some foresters believe that 40 successful attacks per square yard will be fatal to the tree. At times, blind attacks occur, which are almost impossible to observe. It is important to note that when the first few beetles successfully attack a tree, they will emit a pheromone that will attract more beetles into the area, and into that host tree.

For every beetle that mates inside your tree, it is estimated that up to 60 new generation beetles will emerge in the following season. Even lightly attacked trees may succumb to further beetle attack in the following year as these trees can serve as an attractant to beetles due to a stress increase to the tree.

Prevention is the most important part of any beetle management strategy. Early prevention through silvicultural methods as well as direct control/suppression efforts are effective options for controlling bark beetles in your forest.

What Should You Look For in Determining Whether There are Beetles In Your Forest:

Pitch Tubes:

The first signs of attack are whitish globs that look like popcorn on the side of the tree's trunk. These signs are are called pitch tubes. Pitch tubes are a defense mechanism of the tree that serves to expel the beetle from the tree's phloem layer. The tree's production of resin can "pitch out" beetles as they bore into the bark. Large pitch tubes often indicate unsuccessful attacks while very small pitch tubes or no pitch tubes at all may indicate successful attacks. Look carefully within the pith tube and you will likely find a beetle inside of the sappy glob.

Boring Dust:

Small mounds of 'sawdust' at the base of the tree and in the bark crevice. Boring dust is left when the beetles bore into the bark of the tree. This will have the consistency of flour or fine sawdust and will be reddish in color.

Bark:

Remove a section of the bark to reveal any galleries (tunnels) created by the beetles. Galleries indicate a successful attack. At times these galleries will also contain grubs. Use this method carefully as this will injure the tree and leave it more susceptible.

Woodpeckers:

Woodpeckers find the larvae of the beetle particularly appetizing. An abundance of these woodpecker holes can be a good indication of tree infestation.

Blue Stain:

Beetles bring with them a fungus called 'Blue Stain'. A tree with blue stain fungus is a clear sign of beetle attack; but unfortunately this fungus is only visible when the tree is cut.

NOTE: Beetles prefer to bore into a tree where the bark is the thinnest, namely in the cracks and crevasses. This can make it very difficult to determine the number of beetle hits, and whether or not the hits were successful.

Pitch Tubes

Boring Dust May Litter the Ground

Woodpecker Damage

Blue Stain Fungus

Life Cycle of the Mountain Pine Beetle

Mountain Pine Beetles (MPB) normally have a one-year life cycle but in colder climates and in higher elevations they have the potential to over winter and adapt to a two or three year life cycle. In the event that the beetle's life cycle is greater than one year, the beetles will still produce only one brood. This indicates that the beetles can adapt to climate changes in order to survive.

Beetle flight is climate dependent as well as elevation dependent. Specific areas may vary, but it is in late summer (in general) when adult females emerge. This will occur when ambient air temperature exceeds approximately 16ºC or 61ºF. The adults are about about 5 mm (1/4″) long when they bore out through the bark and fly away from the pine tree in which they matured over the previous 12 months.

The emerging beetles fly through the forest seeking an acceptable host in which they will mate and create a future brood. The dynamics of flight and the determination of an acceptable host is a topic in and of itself. When the female pioneer beetle begins burrowing into a suitable host tree, she releases an attractive aggregation pheromone that attracts both males and females. A mass attack weakens the trees making it easier for beetles to overcome the trees natural defense of pitching the beetles out.

The female beetles tunnel into pine trees and lay eggs in vertical galleries under the bark. In the galleries, females mate with males and lay eggs individually along the sides of the gallery. Female MPBs will lay approximately 60 eggs.

After about two weeks, the eggs hatch and the grub-like larvae make horizontal galleries within the phloem layer of the tree, and the larvae over-winter under the protective bark of the tree. The larvae resume feeding in spring and grow up to 7 mm in length. They can resist -35° temperatures multiple days in the winter months. Larvae pupate in the spring and emerge as adults from July to September.

Part of the destructiveness of MPB is the transmission of blue-stain fungi from the beetle to the tree. The spores of these fungi are brought into the tree by adult beetles during colonization. Together with feeding beetles, fungus grows within the tree, weakening and eventually killing the tree. This mutual network of beetle galleries and blue stain fungi impedes the movement of water within the tree and as a result, there is rapid tree decline. The fungi give a blue-grey appearance to the sapwood.

Mountain Pine Beetle Lifecycle

Life Cycle of the Douglas-fir Beetle

Douglas-fir beetles typically take one year to complete their life cycle. Over-wintering takes place beneath the bark of the tree in which they matured for the previous 12 months. Adults emerge from their host tree and infest susceptible trees typically during late April to mid-June when ambient air temperature exceeds approximately 18ºC or 64ºF. A small percentage of beetles may over winter as larvae. Beetles that have survived the winter as larvae complete their development in spring and early summer. They emerge and attack host trees in midsummer. In addition, a few adults who made initial attacks in the spring may re-emerge to make a second attack in mid to late summer. This "second flight" usually accounts for less than ten percent of the yearly total of attacked trees. Often, these later attacks are concentrated within trees which were previously attacked during the initial spring flight.

Attacks are initiated by female beetles when they bore through the bark and release an aggregate pheromone to attract other female and male beetles that are migrating through the forest. Once the female has attracted a male and mated, she constructs a vertical egg gallery beneath the tree's bark, paralleling the grain of the wood. Eggs hatch within approximately one week and larvae begin feeding at right angles to the egg gallery, starting within the inner bark (phloem). As they mature toward mid-to-late August, they feed more deeply into the bark and grow to 6 mm long. At that point, they construct pupal chambers, pupate, and complete their development by late summer or early autumn. Newly matured adults remain beneath the bark until the following spring when they emerge to make new attacks and renew the yearly cycle.

Douglas-fir Beetle Lifecycle

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Recognizing the signs of beetle infestation in your trees is an important consideration when living within the forest. A tree that appears to be perfectly healthy with green needles may also be infested. Once a tree has been successfully "mass attacked" by beetles, the tree will die.

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There is an eerie feel to this grove of lodgepole pines that I can't quite put my finger on as entomologist Diana Six tromps ahead of me, hatchet in hand, scanning the southwestern Montana woods for her target. But as she digs the blade into a towering trunk, it finally hits me: the smell. There's no scent of pine needles, no sharp, minty note wafting through the brisk fall air.

Six hacks away hunks of bark until she reveals an inner layer riddled with wormy passageways. "Hey, looky!" she exclaims, poking at a small black form. "Are you dead? Yeah, you're dead." She extends her hand, holding a tiny oval, maybe a quarter of an inch long. Scientists often compare this insect to a grain of rice, but Six prefers mouse dropping: "Beetle in one hand, mouse turd in another. You can't tell them apart." She turns to the next few trees in search of more traces. Pill-size holes pock their ashen trunks-a sign, along with the missing pine scent, of a forest reeling from an invasion.

Beetles are chewing their way through American forests, sometimes felling as many as 100,000 trees a day.

These tiny winged beetles have long been culling sickly trees in North American forests. But in recent years, they've been working overtime. Prolonged droughts and shorter winters have spurred bark beetles to kill billions of trees in what's likely the largest forest insect outbreak ever recorded, about 10 times the size of past eruptions. "A doubling would have been remarkable," Six says. "Ten times screams that something is really going wrong."

Mountain pine, spruce, piñon ips, and other kinds of bark beetles have chomped 46 million of the country's 850 million acres of forested land, from the Yukon down the spine of the Rocky Mountains all the way to Mexico. Yellowstone's grizzly bears have run out of pinecones to eat because of the beetles. Skiers and backpackers have watched their brushy green playgrounds fade as trees fall down, sometimes at a rate of 100,000 trunks a day. Real estate agents have seen home prices plummet from " viewshed contamination " in areas ransacked by the bugs. And the devastation isn't likely to let up anytime soon. As climate change warms the North American woods, we can expect these bugs to continue to proliferate and thrive in higher elevations-meaning more beetles in the coming century, preying on bigger chunks of the country.

BEETLEMANIA

From 2000 to 2014, bark beetles destroyed large swaths of forests in the American West-and they're not done yet.

In hopes of staving off complete catastrophe, the United States Forest Service, which oversees 80 percent of the country's woodlands, has launched a beetle offensive, chopping down trees to prevent future infestations. The USFS believes this strategy reduces trees' competition for resources, allowing the few that remain to better resist invading bugs. This theory just so happens to also benefit loggers, who are more than willing to help thin the forests. Politicians, too, have jumped on board, often on behalf of the timber industry: More than 50 bills introduced since 2001 in Congress proposed increasing timber harvests in part to help deal with beetle outbreaks.

But Six believes that the blitz on the bugs could backfire in a big way. For starters, she says, cutting trees "quite often removes more trees than the beetles would"-effectively outbeetling the beetles. But more importantly, intriguing evidence suggests that the bugs might be on the forest's side. Six and other scientists are beginning to wonder: What if the insects that have wrought this devastation actually know more than we do about adapting to a changing climate?

A BUG'S LIFE

An adult mountain pine beetle lays her eggs under the bark. On her way, she disperses fungi that turn the trees' tissue into food for her babies, eventually killing the tree.

Though they're often described as pesky invaders, bark beetles have been a key part of conifer ecosystems for ages, ensuring that groves don't get overcrowded. When a female mountain pine beetle locates a frail tree, she emits a chemical signal to her friends, who swarm to her by the hundreds. Together they chew through the bark until they reach the phloem, a cushy resinous layer between the outer bark and the sapwood that carries sugars through the tree. There, they lay their eggs in tunnels, and eventually a new generation of beetles hatches, grows up, and flies away. But before they do, the mature beetles also spread a special fungus in the center of the trunk. And that's where things get really interesting.

Six focuses on the "evolutionary marriage" of beetle and fungi at her four-person lab at the University of Montana, where she is the chair of the department of ecosystems and conservation sciences. Structures in bark beetles' mouths have evolved to carry certain types of fungi that convert the tree's tissue into nutrients for the bug. The fungi have "figured out how to hail the beetle that will get them to the center of the tree," Six says. "It's like getting a taxi." The fungi leave blue-gray streaks in the trees they kill; "blue-stain pine" has become a specialty product, used to make everything from cabins to coffins to iPod cases.

A healthy tree can usually beat back invading beetles by deploying chemical defenses and flooding them out with sticky resin. But just as dehydration makes humans weaker, heat and drought impede a tree's ability to fight back-less water means less resin. In some areas of the Rocky Mountain West, the mid-2000s was the driest, hottest stretch in 800 years. From 2000 to 2012, bark beetles killed enough trees to cover the entire state of Colorado. "Insects reflect their environment," explains renowned entomologist Ken Raffa-they serve as a barometer of vast changes taking place in an ecosystem.

Typically, beetle swells subside when they either run out of trees or when long, cold winters freeze them off (though some larvae typically survive, since they produce antifreeze that can keep them safe down to 30 below). But in warm weather the bugs thrive. In 2008, a team of biologists at the University of Colorado observed pine beetles flying and attacking trees in June, a month earlier than previously recorded. With warmer springs, the beetle flight season had doubled, meaning they could mature and lay eggs-and then their babies could mature and lay eggs-all within one summer.

That's not the only big change. Even as the mountain pine beetles run out of lodgepole pines to devour in the United States, in 2011 the insects made their first jump into a new species of tree, the jack pine, in Alberta. "Those trees don't have evolved defenses," Six says, "and they're not fighting back." The ability to invade a new species means the insects could begin a trek east across Canada's boreal forest, then head south into the jack, red, and white pines of Minnesota and the Great Lakes region, and on to the woods of the East Coast. Similarly, last year, the reddish-black spruce beetle infested five times as many acres in Colorado as it did in 2009. And in the last decade, scientists spotted the southern pine beetle north of the Mason-Dixon Line for the first time on record, in New Jersey and later on Long Island. As investigative journalist Andrew Nikiforuk put it in his 2011 book on the outbreaks, we now belong to the "empire of the beetle."

In a weird way, all of this is exciting news for Six: She is not only one of the world's foremost experts in beetle-fungi symbiosis, but proud to be "one of the few people in Montana that thinks bark beetles are cute." (She's even brewed her own beer from beetle fungi.) As a child, she filled her bedroom in Upland, California, with jars of insects and her fungus collection. But as a teenager, she got into drugs, quit high school, and started living on the streets. Nine years later, she attended night school, where teachers urged her to become the first in her family to go to college. And when she finally did, she couldn't get enough: classes in microbiology and integrated pest management led to a master's degree in veterinary entomology, then a Ph.D. in entomology and mycology and a postdoc in chemical ecology, focused on insect pheromones.

Six, 58, has light-green eyes ringed with saffron, and long silvery-blond hair streaming down shoulders toned from fly-fishing and bodybuilding. As several fellow researchers stress to me, she is the rare scientist who's also a powerful communicator. "I think about what it means to be a tree," she told a rapt audience at a TED x talk about global forest die-offs. "Trees can't walk. Trees can't run. Trees can't hide," she continued, her sonorous voice pausing carefully for emphasis. "And that means, when an enemy like the mountain pine beetle shows up, they have no choice but to stand their ground."

To a tree hugger, that might seem a grim prognosis: Since trees can't escape, they'll all eventually be devoured by insects, until we have no forests left. Especially since, with our current climate projections, we might be headed toward a world in which beetle blooms do not subside easily and instead continue to spread through new terrain.

But Six has a different way of looking at the trees' plight: as a battle for survival, with the army of beetles as a helper. She found compelling evidence of this after stumbling across the work of Forest Service researcher Constance Millar, with whom she had crossed paths at beetle conferences.

Millar was comparing tree core measurements of limber pines, a slight species found in the eastern Sierras of California that can live to be 1,000 years old. After mountain pine beetles ravaged one of her study sites in the late 1980s, certain trees survived. They were all around the same size and age as the surrounding trees that the beetles tore through, so Millar looked closer at tree ring records and began to suspect that, though they looked identical on the outside, the stand in fact had contained two genetically distinct groups of trees. One group had fared well during the 1800s, when the globe was still in the Little Ice Age and average temperatures were cooler. But this group weakened during the warmer 1900s, and grew more slowly as a result. Meanwhile, the second group seemed better suited for the warmer climate, and started to grow faster.

When beetle populations exploded in the 1980s, this second group mounted a much more successful battle against the bugs. After surviving the epidemic, this group of trees "ratcheted forward rapidly," Millar explains. When an outbreak flared up in the mid-2000s, the bugs failed to infiltrate any of the survivor trees in the stand. The beetles had helped pare down the trees that had adapted to the Little Ice Age, leaving behind the ones better suited to hotter weather. Millar found similar patterns in whitebark pines and thinks it's possible that this type of beetle-assisted natural selection is going on in different types of trees all over the country.

When Six read Millar's studies, she was floored. Was it possible, she wondered, that we've been going about beetle management all wrong? "It just hit me," she says. "There is something amazing happening here."

Last year, Six and Eric Biber, a University of California-Berkeley law professor, published a provocative review paper in the journal Forests that challenged the Forest Service's beetle-busting strategies. After scrutinizing every study about beetle control that they could get their hands on, they concluded that "even after millions of dollars and massive efforts, suppression...has never effectively been achieved, and, at best, the rate of mortality of trees was reduced only marginally."

Six points to a stand of lodgepoles in the University of Montana's Lubrecht Experimental Forest. In the early 2000s, school foresters preened the trees, spacing them out at even distances, and hung signs to note how this would prevent beetle outbreaks. This "prethinned" block was "the pride and joy of the experimental forest," Six remembers. But that stand was the first to get hit by encroaching pine beetles, which took out every last tree. She approached the university forest managers. "I said, 'Boy, you need to document that,'" Six says. "They didn't. They just cut it down. Now there's just a field of stumps."

For the timber industry and its friends, beetle invasions have been a handy excuse to open wild areas for logging.

Six and Biber's paper came as a direct affront to some Forest Service researchers, one of whom told me that he believes changing forest structure through thinning is the only long-term solution to the beetle problem. Politicians tend to agree-and beetle suppression sometimes serves as a convenient excuse: "It is perhaps no accident that the beetle treatments most aggressively pushed for in the political landscape allow for logging activities that provide revenue and jobs for the commercial timber industry," Six and Biber wrote in the Forests review.

Take the Restoring Healthy Forests for Healthy Communities Act, proposed in 2013 by then-Rep. Doc Hastings (R-Wash.) and championed by then-Rep. Steve Daines (R-Mont.). The bill sought to designate "Revenue Areas" in every national forest where, to help address insect infestations, loggers would be required to clear a certain number of trees every year. Loggers could gain access to roadless areas, wilderness study areas, and other conservation sites, and once designated, their acreage could never be reduced. The zones would also be excluded from the standard environmental-review process.

Six and other scientists vehemently opposed these massive timber harvests-as did environmental advocates like the Sierra Club and Defenders of Wildlife, the latter warning that the harvests would take logging to " unprecedented and unsustainable levels." The bill passed the House but died in the Senate last year. But Daines, now a senator and one of 2014's top 10 recipients of timber money, vows to renew the effort so as to "revitalize Montana's timber industry" and "protect the environment for future generations."

This summer, Six plans to start examining the genes of "supertrees"-those that survive beetle onslaughts-in stands of whitebarks in Montana's Big Hole Valley. Her findings could help inform a new kind of forest management guided by a deeper understanding of tree genes-one that beetles have had for millennia.

If we pay close enough attention, someday we may be able to learn how to think like they do. University of California-Davis plant sciences professor David Neale champions a new discipline called "landscape genomics." At his lab in Davis, Neale operates a machine that grinds up a tree's needles and spits out its DNA code. This technology is already being used for fruit tree breeding and planting, but Neale says it could one day be used in wild forests. "As a person, you can take your DNA and have it analyzed, and they can tell you your relative risk to some disease," Neale says. "I'm proposing to do the same thing with a tree: I can estimate the relative risk to a change in temperature, change in moisture, introduction to a pathogen."

Right now, foresters prune woodlands based on the size of trees' trunks and density of their stands. If we knew more about trees' genetic differences, Neale says, "maybe we would thin the ones that have the highest relative risks." This application is still years off, but Neale has already assembled a group of Forest Service officials who want to learn more about landscape genomics.

Six, meanwhile, places her faith in the beetles. Whereas traditional foresters worry that failing to step in now could destroy America's forests, Six points to nature's resilience. Asked at TEDx how she wants to change the world, she responded, "I don't want to change the world. We have changed the world to a point that it is barely recognizable. I think it's time to stop thinking change and try to hold on to what beauty and function remains."
This story was supported by a Middlebury College Fellowship in Environmental Journalism.

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Bark beetles are decimating our forests. That might actually be a good thing.

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There is an eerie feel to this grove of lodgepole pines that I can't quite put my finger on as entomologist Diana Six tromps ahead of me, hatchet in hand, scanning the southwestern Montana woods for her target. But as she digs the blade into a towering trunk, it finally hits me: the smell.

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Abstract

Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.

During the 21st century, mean annual global temperature is expected to increase between 1.8 and 4.0 degrees Celsius (°C) as a result of growing atmospheric greenhouse gas concentrations created by human activities. Across North America, the rise in temperatures is projected to exceed global mean increases, particularly at high latitudes and elevations, and more frequent extreme weather events are expected ( IPCC 2007). Associated changes in precipitation patterns may result in earlier and longer dry seasons across the western United States, with a greater frequency and duration of droughts ( Seager et al. 2007). These changes in climatic conditions over the next century will significantly affect the condition, composition, distribution, and productivity of multiple ecosystems ( Easterling et al. 2000).

Coniferous forests, which provide essential ecosystem services and host a vast array of plant and animal species, are expected to be significantly affected by shifts in temperature, precipitation, and atmospheric greenhouse gas concentrations ( McNulty and Aber 2001). By the end of the century, about 48% of the western US landscape is predicted to experience climate profiles with no contemporary analog to the current coniferous vegetation ( Rehfeldt et al. 2006). Population irruptions of phytophagous insects-disturbance events important to forest ecosystem functioning-are also directly sensitive to climate change components ( Dale et al. 2001). Although there are many possible avenues for atmospheric changes to influence phytophagous insect outbreaks, because of the direct link between insect population success and seasonal temperature ( Danks 1987), outbreaks are predicted to be affected dramatically by global warming ( Bale et al. 2002). Rapid genetic adaptation of insects to seasonal changes in temperature has already been documented ( Balanyá et al. 2006, Bradshaw and Holzapfel 2006), and range expansion has occurred in many cases as species move into new niches created by increasing temperature ( Battistia et al. 2006, Nealis and Peter 2009).

Native bark beetles (Coleoptera: Curculionidae, Scolytinae), which evolved within the coniferous forest ecosystems of western North America, are key agents of change in these systems. Beetle outbreaks raise tree mortality rates and can result in subsequent replacement by other tree species and plant associations ( Veblen et al. 1991). In recent decades, billions of coniferous trees across millions of hectares have been killed by native bark beetles in forests ranging from Mexico to Alaska, and several of the current outbreaks are among the largest and most severe in recorded history ( Bentz et al. 2009). Although gaps exist in our understanding of the processes, it is clear that mechanisms contributing to widespread bark beetle outbreaks are complex and include density-independent factors, in addition to spatial and temporal dependencies at multiple scales ( Aukema et al. 2008, Raffa et al. 2008). Large areas of suitable host trees of susceptible vigor, age, and density are required for an outbreak to develop ( Fettig et al. 2007). Because bark beetle population survival and growth are highly sensitive to thermal conditions, and water stress can influence host-tree vigor, outbreaks have been correlated with shifts in temperature ( Powell and Logan 2005) and precipitation ( Berg et al. 2006). However, a comprehensive synthesis of the direct and indirect effects of climate change on the population dynamics of bark beetles is lacking.

In this article we assess and synthesize the state of knowledge regarding effects of climate change on bark beetles that cause extensive conifer mortality in the western United States and Canada (table 1). We discuss potential direct and indirect impacts of climate change on multiple aspects of host trees, and bark beetle community ecology and population dynamics. We present two case studies to illustrate the potential impacts of global warming on the population outbreak dynamics of eruptive bark beetles.

Direct effects of climate change on bark beetles

Of the hundreds of native bark beetle species in the western United States and Canada, few species (< 1%) attack and reproduce in live trees. Frequently referred to as aggressive bark beetles, these species can kill healthy trees and have the capacity to cause landscape-scale tree mortality ( table 1). Host selection and colonization behavior by bark beetles are complex processes that involve both long- and short-range behavioral components ( Graves et al. 2008) with multiple thresholds and rapid feedback ( Raffa et al. 2008). Bark beetle adults have sophisticated chemoreceptors and behaviors that allow them to recognize host tree species in addition to the potential defensive capacity of the host. Once a host is selected, colonization requires overcoming constitutive and inducible tree defenses, which include anatomical, physical, and chemical components ( Franceschi et al. 2005). These defenses are overcome only when a critical minimum number of beetles are recruited to the host tree. This number varies with changes in host vigor, and therefore will be affected indirectly as host trees respond to a changing climate (see below). In most cases, recruitment is facilitated by aggregation pheromones that attract other colonizing adults ( Raffa 2001). Following aggregation and subsequent mating, adults lay eggs in the phloem, and larvae excavate feeding tunnels in this tissue or in the outer bark, depending on the beetle species-a process that eventually results in the mortality of the host tree. Mature adult beetles of the next generation tunnel outward through the bark and initiate flight in search of a new host tree.

Synchronous adult emergence and life-cycle timing, critical strategies used by many bark beetle species to kill trees, are in large part dictated by several temperature-dependent physiological processes. Direct effects of climate change on bark beetle population dynamics will therefore occur predominantly through the influence of temperature on life- history strategies that (a) maintain adaptive developmental timing leading to synchronized population emergence and life-cycle timing, and (b) facilitate cold tolerance and avoidance of low-temperature-induced mortality.

Developmental timing. The time required to complete a generation varies among bark beetle genera, species within a genus, populations within a species, and individuals within a population. Some species, such as the western pine beetle, Dendroctonus brevicomis LeConte, and the piñon ips, Ips confusus LeConte, produce more than one generation per year. Others, including the spruce beetle, Dendroctonus rufipennis [Kirby], and mountain pine beetle, Dendroctonus ponderosae Hopkins, require one, two, or even three years to produce a single generation, depending on the temperature profile at a particular locale within their large geographic ranges. Moreover, as a result of the evolution of narrow thermal windows that minimize excess physiological costs ( Pörtner and Farrell 2008), the adaptation of life-cycle timing to a local climate can result in a genetically predetermined variability in temperature response across the range of a single bark beetle species ( Bentz et al. 2001). Although little is known about specific temperature-dependent developmental processes of many bark beetle species, research suggests that at least two predominant strategies, diapause and direct temperature control, have evolved to maintain appropriate life-cycle timing. Each strategy may be differentially affected by climate change.

Diapause is a dynamic, endocrine-mediated and environmentally driven dormancy that occurs at a specific life stage. Diapause not only offers a mechanism to keep insects synchronized with their environment and food availability but also provides tolerance to environmental extremes ( Tauber et al. 1986). Although not all species have been investigated, diapause has been demonstrated or suggested to exist in five bark beetle species indigenous to western North America: (1) fir engraver, Scolytus ventralis LeConte ( Scott and Berryman 1972); (2) spruce beetle ( Hansen et al. 2001); (3) Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins ( Ryan 1959); (4) eastern larch beetle, Dendroctonus simplex LeConte ( Langor and Raske 1987); and (5) pine engraver, Ips pini (Say) ( Birch 1974). The effect of climate change on these species will depend on the life stage in which diapause occurs. For example, high summer temperatures prevent facultative prepupal diapause of the spruce beetle, resulting in beetles that complete their life cycles in a single year, compared with two years when the diapause is invoked, potentially leading to exponential population growth ( Hansen and Bentz 2003). Conversely, an obligatory adult diapause initiated by low temperatures, as in spruce beetles and Douglas-fir beetles, could be disrupted by higher minimum temperatures.

Direct temperature control also maintains appropriate seasonality in bark beetles ( sensu Danks 1987). In the mountain pine beetle, for example, life stage-specific developmental thresholds aid in synchronizing adult emergence at appropriate times of the year ( Powell and Logan 2005). Later life stages (e.g., fourth instar larvae) have higher temperature threshold requirements for development than earlier life stages. The higher temperature thresholds serve to synchronize individuals during autumn, as temperatures decrease, and also prevent development to the cold-intolerant pupal life stage ( Logan and Bentz 1999). Temperature pattern throughout a life cycle is therefore critical to appropriate seasonality, and predicted rises in temperature could affect both developmental thresholds and rates in multiple life stages. Additional fitness parameters potentially affected include higher adult longevity and prolonged adult emergence and flight. Warming trends have been associated with shifts in generation duration for populations of spruce beetle in Alaska, Utah, and Colorado ( Hansen et al. 2001, Werner et al. 2006), and mountain pine beetle in high-elevation forests ( Bentz and Schen-Langenheim 2007).

Little is known about developmental strategies of the many bark beetle species indigenous to the southwestern United States and Mexico. These species are significant because of their potential to move northward with climate change by following range expansion of current hosts or by adapting to novel hosts. The roundheaded pine beetle, Dendroctonus adjunctus Blandford, for example, colonizes pines from southern Utah and Colorado south into Guatemala, and its life-cycle timing differs depending on geographical location ( Chansler 1967). Although it appears that the northern extent of this bark beetle species, and others restricted to the southwestern United States and Mexico, is currently limited by climate and not host-tree availability ( Salinas-Moreno et al. 2004), information is lacking on temperature-dependent physiological aspects of its life history that may be range limiting. The polyphagy exhibited by the roundheaded pine beetle and other bark beetles currently found in Mexico suggests they may do well in pine species they would encounter in a northward range expansion, potentially invading niches vacated by beetle species whose population success is disrupted by climate change. Novel species assemblages could be created, as exemplified by recent documentation of the Mexican pine beetle, Dendroctonus mexicanus Hopkins, formerly thought to be limited to northern Mexico, and the southern pine beetle, Dendroctonus frontalis Zimmermann, attacking the same individual pine trees in the Chiricahua Mountains of Arizona. This incidence is potentially a result of an increase in climate suitability and the number of beetle generations per year ( Waring et al. 2009).

Indirect effects of climate change on bark beetles

Bark beetle population success will be influenced indirectly by the effects of climate on community associates, host-tree vigor, and host abundance.

Community associates. Upon colonizing a tree, bark beetles introduce an array of fungi, bacteria, nematodes, and mites that can significantly influence their fitness ( Hofstetter et al. 2006, Cardoza et al. 2008). The relationship between bark beetle species and their associates is often described as symbiotic, as many bark beetles have evolved morphological adaptations to assist in the transport of specific associates, derive nutritional and defensive benefits from them, or both ( Klepzig and Six 2004). For example, spruce beetles have specialized body structures to carry associated nematodes ( Cardoza et al. 2008), and mountain pine beetles and western pine beetles have structures to transport symbiotic fungi ( Klepzig and Six 2004). Developing mountain pine beetle larvae acquire vital nutrients not found in plant tissue by feeding on two fungi, Grosmannia clavigera and Ophiostoma montium, the hyphae of which spread throughout the phloem and sapwood following inoculation into the tree by attacking beetles. Although both fungi are important, evidence suggests that one species ( G. clavigera) supports faster brood development, larger body size, and higher brood production than does the other ( Bleiker and Six 2007). Each fungus possesses different thermal ranges for optimal growth ( Rice et al. 2008), and seasonal temperature dictates which fungal species is ultimately vectored by dispersing beetles ( Six and Bentz 2007). Because benefits to the mountain pine beetle are not the same for each fungal species, shifts in temperature and precipitation associated with climate change could indirectly affect mountain pine beetle population success through direct effects on fungal symbionts.

Other community associates and trophic interactions, including avian predators and insect parasitoids and predators ( McCambridge and Knight 1972, Boone et al. 2008), undoubtedly will also be influenced by abiotic factors associated with climate change. The ecological roles and temperature dependencies of the majority of bark beetle community associates are not well understood, which hampers full comprehension of the consequences of climate change on bark beetle population dynamics.

Host-tree physiology. Climate change will influence bark beetle-host interactions in complex or nonlinear ways. Although plants tend to thrive in carbon-enriched atmospheres, mature wildland conifer species are not necessarily carbon limited and therefore may not express large growth increases in response to increased carbon dioxide (CO2; Millard et al. 2007). However, when grown in the presence of elevated CO2, plants accumulate carbon and the carbon-to-nitrogen ratio increases ( Zvereva and Kozlov 2006), resulting in reduced nutrition (low nitrogen content) for insect herbivores ( Mattson 1980). To compensate, insects consume more but grow more slowly, a trade-off with the potential to disrupt phenological synchrony important to bark beetle survival, in addition to prolonging exposure to mortality agents. The indirect negative effects of enhanced CO2 on bark beetle growth and survival are, at least in part, outweighed by other climate-change-induced effects on host trees, including reduced defenses.

An important consequence of climate change is higher frequency and severity of droughts ( Seager et al. 2007). In addition to directly affecting tree death through carbon starvation and cavitation of water columns within the xylem, climatic water stress can have profound effects on tree susceptibility to bark beetle attack. To avoid drought-induced hydraulic failure, plant stomates close to restrict transpiration. However, stomatal closing also limits carbon assimilation, which can result in carbon starvation ( McDowell et al. 2008). Changes in carbon assimilation will also alter within-plant allocation of carbohydrates available for growth, defense, and tissue repair, affecting the production of constitutive or inducible chemical defenses ( Herms and Mattson 1992) and thus a tree's ability to respond to bark beetle invasions. Hydraulic failure may be further amplified when water transport is interrupted by symbiotic fungi inoculated into trees during the bark beetle attack process ( McDowell et al. 2008). Drought-induced alterations to tree defensive capacity ultimately reduce the threshold number of beetles necessary for a successful mass attack, thereby relaxing the constraints on critical thresholds that must be surpassed for bark beetle outbreaks to occur.

Although the mechanisms connecting drought stress to bark beetle outbreaks are not well understood in western North American ecosystems, it is clear that effects will vary regionally and by bark beetle-host species complex because of differences in critical feedbacks driving beetle population dynamics, as well as physiological differences among tree species. For example, the regional-scale piñon ips epidemic associated with severe drought in the southwestern United States ( Breshears et al. 2005) ended as the supply of drought-stressed trees was exhausted. In contrast, although drought stress facilitated progression from an incipient to epidemic mountain pine beetle population in British Columbia, a significant correlation with precipitation was no longer found after the beetle population became self-amplifying ( Raffa et al. 2008). In both cases, elevated temperatures, which directly influence bark beetle population success in ways described above, were associated with drought conditions that affected tree stress.

In a rapidly changing environment, coniferous tree species will persist through migration or adaptation to new conditions, or they will go locally extinct. The fate of any individual species will depend on multiple factors, including phenotypic variation, fecundity, and biotic interactions ( Aitken et al. 2008). On the basis of the best existing data for 130 tree species in North America and associated climate information, and assuming no limitations to individual tree growth, McKenney and colleagues (2007) predicted that the average range for a given tree species will decrease in size by 12% and will shift northward by 700 kilometers (km) during this century. Under a scenario in which survival occurs only in areas where anticipated climatic conditions overlap with current climatic conditions, niches for tree survival will decrease in size by 58% and shift north by 330 km. More specific to this synthesis, range predictions for several hosts of notable bark beetle species provide striking comparisons. Relative to contemporary distributions, by 2060 the range of Engelmann spruce, Picea engelmanni Parry ex Engelm., a principal host for spruce beetle, is projected to decrease by 47% within the contiguous western United States. During the same period, the areas inhabited by ponderosa pine, Pinus ponderosa Laws., and Douglas-fir, Pseudotsuga menziesii Franco, hosts of several bark beetle species, are projected to increase by 11% and 7%, respectively ( Rehfeldt et al. 2006). If we assume that current tree distributions are driven by nonclimatic biotic factors such as bark beetles in addition to climate, models used to forecast specific tree distributions may already include an inherent measure of the influence of bark beetles on tree species distributions.

Case studies

Despite uncertainties, changes in temperature predicted by general circulation models can be an important basis for estimating biological response to changing conditions (Millar et al. 2007). When used in conjunction with quantitative models that are based on a mechanistic understanding of biological responses to temperature, results can provide insight into ecosystem responses to climate change. We explore the potential effects of changing climate on bark beetle outbreak dynamics using two case studies: (1) spruce beetle and (2) mountain pine beetle. We chose these species for analysis because mechanistic models for predicting temperature effects on population success have been developed. Because models incorporating the direct effects of climatic changes on conifers and their subsequent response to bark beetle attacks are not currently available, our quantitative assessments are based solely on bark beetle population success. Our assessment assumes no change in current tree distributions, and that thermal conditions conducive to bark beetle population success result in increased levels of tree mortality, although we do not explicitly model the impacts to forests.

The spruce beetle and mountain pine beetle models (described below) are driven by hourly temperature (interpolated between the minimum and maximum temperatures on successive days) and were integrated with weather and topography using BioSIM (http://cfs.nrcan.gc.ca/factsheets/biosim). We ran models for 25,000 simulation points across North America, north of Mexico, with an emphasis on mountainous and forested areas. Elevations were obtained from digital elevation models (DEM) at 30-as resolution (http://eros.usgs.gov/products/elevation/gtopo30/gtopo30.html). Because weather inputs are stochastic and responses are nonlinear, we replicated each model run 30 times, each with a stochastically different one-year temperature time series for each simulation point and normals period. We averaged model output over replicates. From these averaged outputs, maps were generated by universal kriging, with elevation provided by the input DEM as external drift variable ( Hudson and Wackernagel 1994). Probability values were linearized by logistic transformation before interpolation and before the maps were back transformed. Maps depict a continuous measure of bark beetle population success (as defined by each insect model) for the three climate periods. Model output was masked using polygons that estimate the 20th-century locations of spruce (for spruce beetle) and pine (for mountain pine beetle) habitat in the United States and Canada ( Little 1971).

Spruce beetle model results. Model predictions suggest that during the historical period 1961-1990, the majority of spruce forests in Alaska, and those at high elevations in the contiguous western United States and northern latitudes of Canada, would have a moderate to low probability of spruce beetle populations developing in a single year ( figure 1a, 1d). In the period 2001-2030 and again from 2071 to 2100, we would expect substantial increases in spruce forest area with high probability of spruce beetle offspring produced annually rather than semiannually ( figure 1b, 1c, 1e, 1f). By the end of the century, the change in temperatures across the boreal forests of central Canada may cause markedly higher probability of spruce beetle outbreak potential, based on developmental timing alone. A model for predicting the cold tolerance of this insect is not available. In addition to favorable weather, large expanses of mature spruce forest are required for a widespread outbreak. Therefore, although spruce beetle outbreak potential will be enhanced by higher temperatures throughout the century, reductions in the range of Engelmann spruce in the western United States, also as a result of climate change ( Rehfeldt et al. 2006), could cause an overall reduction in long-term spruce beetle impacts in that region.

Mountain pine beetle. The mountain pine beetle ranges throughout southern British Columbia, portions of eastern Alberta, and most of the western United States. The geographic distribution of the beetle generally reflects the range of its primary hosts ( table 1), although the range of lodgepole pine extends further to the north and ponderosa and other pine species further to the south than where mountain pine beetles are currently found. In the past decade, widespread mountain pine beetle outbreaks in British Columbia and the northern and central US Rocky Mountains have been severe and long lasting, affecting more than 25 million ha (USDA Forest Service, Forest Health Protection, and Natural Resources Canada, Canadian Forest Service). Population outbreaks are also occurring in areas outside the recorded historical range, including lodgepole pine forests in central British Columbia and lodgepole pine and jack pine hybrids, P. banksiana Lamb., in western Alberta ( Nealis and Peter 2009). Significant tree mortality caused by mountain pine beetles, relative to historical records, has also recently occurred in high-elevation pine forests across the western United States ( Gibson et al. 2008). We analyzed the influence of future temperature patterns on mountain pine beetle population success within its current range in the western United States and Canada using models describing the insect's seasonality and tolerance to cold. Because pine hosts extend beyond the current mountain pine beetle range, and because the beetle is a known polyphage, we also provide projections of the potential for expansion into pine forests of northern, central, and eastern Canada and the eastern United States.

The seasonality model used here was described by Logan and Bentz (1999), analyzed by Powell and Logan (2005), and used by Hicke and colleagues (2006). In addition to predicting the probability of beetles developing in a single year, as described above for the spruce beetle, a constraint on the timing of adult emergence is included in the model to further describe the adaptive nature of a particular temperature regime to the mass attack process and subsequent population survival. In the model, if annual adult emergence occurs before 1 June or after 30 September for three or more consecutive years, that temperature time series is considered maladaptive. A higher number of years (out of the 30-model-run replicates for each stochastically different one-year temperature time series) that were not part of a maladaptive series translates to a higher probability of mountain pine beetle success, and hence a higher risk of associated tree mortality. We also used the cold tolerance model developed by Régnière and Bentz (2007) to predict the probability of annual survival given a one-year temperature regime. This model describes the dynamic temperature-dependent process of polyhydric alcohol accumulation and loss influencing mountain pine beetle supercooling capacity. We ran the model on the same series of 30-year weather data for each simulation point and set of climatic normals, and we calculated the location average probability of survival. A higher probability of low-temperature survival translates to a higher probability of mountain pine beetle population success.

Mountain pine beetle model results. Temperature data from the historical period 1961-1990 show that the majority of the area within the current range of the mountain pine beetle had low predicted probability of adaptive seasonality, although scattered areas throughout the area had moderately high probability ( figures 2a, 3a). During this same period, low temperature survival would have been high in coastal regions and other low-elevation forests across the current range, although quite low in high-elevation areas of the United States and Canadian Rocky Mountains ( figures 2d, 3d). As temperatures rise throughout this century, the area suitable for both adaptive seasonality and low-temperature survival is predicted to grow, although results are highly spatially variable ( figures 2b, 2e, 3b, 3e). Notably, both models predict greater probability of population success in portions of the current range that have experienced significant increases in tree mortality caused by mountain pine beetles during the past decade, including high-elevation forests of the western United States and Canadian Rocky Mountains, and lodgepole pine forests in central British Columbia, Colorado, and central Idaho. We note that this insect's flexibility in life-history strategies appears greater than previously anticipated ( Bentz and Schen-Langenheim 2007), and our working definition of adaptive seasonality and associated rules that drive the seasonality model may be too restrictive. Moreover, observed genetic variability in response to temperature ( Bentz et al. 2001) is not currently incorporated, and the development model used for this analysis was derived using data from populations in northern Utah and central Idaho. Predictions for mountain pine beetle in the southwestern United States, in particular, may differ as new developmental parameters are incorporated. An updated modeling framework that addresses these concerns is being developed.

Our modeling results provide some insight into concerns expressed about the potential for mountain pine beetle range expansion across the boreal pine forests of central Canada and into pine forests of central and eastern United States. The potential for adaptive seasonality in central Canada decreases dramatically from the historical period to the end of this century, with high probability of population success restricted to northern provinces ( figure 3a, 3b, 3c). Given assumptions of the seasonality model, these results suggest that substantial warming would disrupt the emergence timing and generation duration required for population success. Across pine habitats in the central and eastern United States, adaptive seasonality remains low throughout the century. An increase in low-temperature survival is predicted for spatially isolated areas in Canada, including west-central Alberta, where mountain pine beetle has recently been found attacking lodgepole/jack pine hybrids ( Nealis and Peter 2009). By the end of the century, cold-survival probability substantially increases across Canada, although in areas key to mountain pine beetle migration in central Canada, the probability for low-temperature survival remains low. Our model results suggest that without adaptation to increasing temperature, the probability of mountain pine beetle range expansion across jack pine forests and into eastern US pine forests will remain low to moderate throughout this century ( figure 3c, 3f).

Conclusions

Bark beetle response to climate change can be characterized by a high degree of complexity and uncertainty, as populations are influenced directly by shifts in temperature and indirectly through climatic effects on community associates and host trees. Because changes in climate will not be uniformly distributed across years, and not all temperature-dependent processes will be equally affected, a mechanistic understanding is imperative for making predictions of direct effects of climate change on future population trends. On the basis of temperature projections from the CRCM and mechanistic models developed for spruce beetle and mountain pine beetle, we expect positive changes in thermal regimes conducive to population success of both species throughout this century. Significant temporal and spatial variability in thermal suitability is predicted, however, emphasizing the complexity in both the thermal habitat and temperature-based physiological processes of these insects. Temperature profiles that promote cold-temperature survival may not also result in appropriate developmental timing, and vice versa. Although detailed information on temperature-dependent physiological processes is not available for the majority of bark beetle species in the southwestern United States and Mexico, many of these species are currently limited by climate rather than host availability, suggesting a high potential for range expansion northward. Developmental life-history strategies have evolved to maintain appropriate seasonality, and higher temperatures may not always translate into population growth or range expansion without adaptation to rapidly changing environmental conditions, a phenomenon documented in several insect species (Bradshaw and Holzapfel 2006). Because the extreme difference in generation times between bark beetles and their coniferous hosts dictates a faster relative rate of adaptation for beetles, this is a critical missing component in our predictions. Models that incorporate genetic variability in bark beetle temperature-dependent parameters are also needed.

Despite uncertainty in forecasts of future climate parameters that have been downscaled to a forest landscape, predictions of trends in bark beetle population success as a function of the direct effects of temperature will be instrumental in development and application of strategies for management of future forests. There is clearly a need, however, for a better understanding and more refined models that integrate indirect effects of climate change on host trees with bark beetle population success, as well as interactions among bark beetle outbreaks and other forest disturbances. For example, drought and other processes can homogenize host-tree species age, structure, and vigor, thereby indirectly contributing to landscape-wide, bark-beetle-caused tree mortality ( McDowell et al. 2008). More frequent extreme weather events will also likely provide abundant resources for some bark beetle species creating the potential to trigger localized outbreaks ( Gandhi et al. 2007). Fire, an important forest disturbance that is directly influenced by climate change ( Westerling et al. 2006), can reduce the resistance of surviving trees to bark beetle attack. Furthermore, climate-change-induced shifts in bark beetle outbreak frequency and intensity may indirectly affect patterns and severity of wildfire, although the relationships are poorly understood, highly complex, and temporally and spatially dynamic ( Jenkins et al. 2008).

Bark beetles are inextricably linked to their host trees, and will undoubtedly influence the formation of new western North American coniferous forests as predicted broad-scale tree migrations occur this century. At the retreating and expanding margins of tree distributions, bark beetles may play a significant role in colonizing and killing stressed individuals as trees and their progeny strive to adapt to a changing environment. Current tree distributions may have been significantly influenced by bark beetles preferentially colonizing trees in environmentally compromised positions at the range margins, and future tree distributions will most likely be affected similarly by these agents of mortality.

Rapid and broad-scale tree mortality events, such as those that have recently occurred across western North America, can have long-term impacts on ecosystem structure and community dynamics, with feedbacks that further influence climate and land use ( Kurz et al. 2008, McDowell et al. 2008). Bark beetle outbreaks driven by climate change may also result in trajectories beyond the historical resilience boundaries of some forest ecosystems, causing irreversible ecosystem regime shifts. The recent loss of entire stands of long-lived, high-elevation whitebark pine, Pinus albicaulis Engelm., as a result of the mountain pine beetle ( figure 4) underscores the need for greater understanding of climate change effects on complex interactions important to ecosystem resiliency and stability. Characterizing thresholds for systems beyond which changes are irreversible will be an important component of forest management in a changing climate.

Acknowledgments

This effort was facilitated by and coordinated through the US Department of Agriculture (USDA) Forest Service Research and Development Western Bark Beetle Research Group, and was funded in part by the USDA Forest Service Western Wildland Environmental Threat Assessment Center, Prineville, Oregon. The CRCM data was generated and supplied by Ouranos. We thank Jim Vandygriff, Remi St-Amant, and Pierre Duval for assistance with map creation. Discussions with Robert Progar and John Lundquist and comments from Craig Allen and several anonymous reviewers greatly improved this article.

References cited

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Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects | BioScience | Oxford Academic

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Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species.

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Bark beetles. They are not a new phenomenon for California's pine forests, however our ongoing drought, now in its fourth or fifth year depending on who you ask, has stressed the trees to the point that they're no longer producing much sap. This means that the trees' natural defense against the beetles — the sap which drips out and envelops them after they've eaten some bark — isn't as plentiful. This means that the beetles are succeeding in killing more trees, and as a result they're also reproducing in greater numbers and spreading across forests.

The upsetting but beautiful drone footage above, via Dane Christensen, shows how many trees in this one forest have been affected — you can see by the brownness of their needles.

Says Christensen, "While outside Yosemite in Oakhurst, I saw the brown swaths of forest while driving. I threw up my drone and the aerial perspective revealed a haunting view of dying forest as far as the eye (or drone) could see. Tony, who lives in the forest, told me about the bark beetle epidemic and is the narrator in what I call a 'drone-umentary.'"
Related: Uneven El Niño Skewed North, But Water Restrictions May Ease In Parts Of CA

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Video: Drone Shows Devastation Of Bark Beetles On Drought-Stricken Yosemite Forest

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Bark beetles. They are not a new phenomenon for California's pine forests, however our ongoing drought, now in its fourth or fifth year depending on who you ask, has stressed the trees to the point that they're no longer producing much sap.

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Global warming is causing major shifts in the range of small mammals in Yosemite National Park, one of the nation's treasures that was set aside as a public trust 144 years ago, according to a new study by University of California, Berkeley, biologists.

The study, published in the Oct. 10 issue of Science, compared small mammal populations in the park today versus 90 years ago and found that mammals like shrews, mice and ground squirrels have moved to higher elevations or reduced their ranges in response to warmer temperatures, essentially shuffling the species living together in any one spot.

"We didn't set out to study the effects of climate change, but to see what has changed and why" since the last full-scale survey in Yosemite in 1918, said study leader Craig Moritz, UC Berkeley professor of integrative biology and director of the campus's Museum of Vertebrate Zoology. "But the most dramatic finding in the Yosemite transect was the upward elevational shift of species. When we asked ourselves, "What changed?" it hit us between the eyes: the climate."

The central Sierra Nevada has seen a general warming, Moritz noted, evidenced not only by a 3 degree Celsius increase in nighttime low temperatures, but also the receding of glaciers - Lyell Glacier is half the size it was 100 years ago - and the increase in precipitation as rain instead of snow.

While the population movements have not altered biodiversity in the park, UC Berkeley biologists say rapid changes in less than a century could be a problem. Although half the species shifted their ranges, the other half did not, which means communities have been altered and the species interacting with one another have changed.

"These kinds of changes in community composition have been going on forever," said coauthor James Patton, a UC Berkeley professor emeritus of integrative biology who led the field work in Yosemite. "The only thing that makes this different is that it has probably happened in our lifetime. It is the speed with which these changes are taking place that gives one pause."

(Emily Rubidge photo)

According to Patton, if species movement is slow, there is enough time for the community structure to equilibrate with each "turnover event." However, "if change happens too fast," he said, "elements of the system may start to collapse because a keystone element of that system gets pulled out too quickly. That is something we just don't know. From my standpoint, if I had the opportunity, I would want to minimize the rate of change."

This finding highlights the need for protected areas, such as national parks, in which species can move in response to changes in the environment.

"Yosemite has been very well managed so that species have been allowed to move," said Moritz, who is working with state officials to develop conservation strategies targeted to species that are sensitive to climate change. "We need to continue to protect large swatches of public land from land-use changes."

The foundation for the current study is a landmark survey of Sierra Nevada birds, mammals, reptiles and amphibians initiated early last century by Joseph Grinnell, founding director of the museum and a zoology professor at UC Berkeley. With a large number of colleagues and students, Grinnell traipsed through the Sierra Nevada - from the Modoc Plateau in the north to the San Jacinto range in the south - collecting specimens and recording for posterity the variety of life then under threat from gold mining and overgrazing.

Moritz proposed resurveying all the sites visited by Grinnell and his colleagues, starting with the Yosemite transect originally surveyed from 1914 to 1920, which stretched from the San Joaquin Valley through the park and over the Sierra crest to Mono Lake. With funding from the National Science Foundation, the Yosemite Foundation and the National Park Service, the Grinnell Resurvey Project commenced in 2003. This week's Science paper is the first result, appearing on the 100th anniversary of the founding of the museum in 1908.

Patton led the resurvey of the Yosemite transect from 2003 through 2005 and, at the behest of park rangers, surveyed the rest of the park's small mammals in 2006 and 2007 as a baseline for future surveys. The current study involves data from the transect only, obtained at 40 of Grinnell's collection sites and far from the crowded campsites and hotels of Yosemite Valley and Tuolumne Meadows. The field notes by Grinnell and his crew were invaluable, Patton said.

"The impressive thing about these guys - not just Grinnell, but people like Charles Camp, who was an undergraduate at the time - is what incredibly observant natural historians they were, and the detail with which they recorded everything," said Patton, who found passages in the notebooks about the then-novel concept of the "ecological niche" several years before Grinnell published his first papers on it. "In pouring over these notebooks, I keep pulling out tidbits that are amazingly perceptive. These guys were out there thinking about what they were seeing and what it meant in a larger context long before most other people were doing that."

Thanks to these detailed field notes recording not only when, where and what they saw and trapped but also what they failed to observe, the UC Berkeley biologists were able to perform a statistical analysis that makes the study results very solid, said coauthor and conservation biologist Steve Beissinger, UC Berkeley professor of environmental science, policy and management.

"One of the biggest problems we have when comparing the distribution of species now and in the past is false absences. If they didn't see something back then, is it because it wasn't there, or because it just wasn't detected?" he said. Employing occupancy models developed in the past few years, he added, "the Grinnell group's data allows us to go back and, night by night, reconstruct their trapping success for small mammals and develop a probability for detecting each species for Grinnell and for us. This is one of the first studies to use the model to look at climate change and historic changes in range."

The researchers found that, of the 28 small mammals observed in the transect, half had expanded their range upward by more than 500 meters, or 1,600 feet. While low-elevation species such as the California vole ( Microtus californicus) and the California pocket mouse ( Chaetodipus californicus) increased their range by moving upslope, high-elevation species such as the bushy-tailed woodrat ( Neotoma cinerea) and Allen's chipmunk ( Tamias senex) generally decreased their range. Short-lived, fast-breeding species were more likely to expand their ranges than were their long-lived, less fecund relatives, Moritz said.

For the alpine chipmunk ( Tamias alpinus), upward movement has caused its range to shrink and possibly put the chipmunk, one of California's few endemic mammals, at risk of extinction. Although 90 years ago the chipmunk was common on granite slabs in lodgepole pine forests below 7,800 feet, Patton was unable to find any below 9,600 to 9,800 feet, an upward movement of 2,000 feet.

"It used to be very common at Tuolumne Meadows; now you have to get up to treeline to find it," Patton said.

Similarly, on the eastern slopes of the Sierra, the one-ounce pinyon mouse ( Peromyscus truei), has moved entirely out of the pinyon pine/juniper belt into a very different habitat, the montane forest dominated by whitebark pine. "Its preferences were more catholic than we suspected," Moritz said.

(Carol Patton photo)

While higher elevation shifts were driven primarily by climate, Patton noted that lower-elevation changes probably had something to do with land use changes. A third mammal impacted significantly in the past 90 years is the Trowbridge shrew ( Sorex trowbridgii), which used to be the only shrew in Yosemite Valley. Today, with the departure of Indians who regularly burned the valley, the valley is wetter, and the Trowbridge shrew has been totally replaced by the montane shrew ( Sorex monticolus), which moved downslope to recolonize the valley.

The UC Berkeley team, with the assistance of colleagues around the state, is continuing its resurvey of Grinnell's transects and will eventually resurvey coastal mountains as well. This work will provide information about northward or southward movement of animals in addition to elevational movement in response to climate and land use change. One of the key missing pieces, however, is how fast this change has taken place, Patton said.

"We have two surveys 90 years apart, which doesn't give you a rate. This change could have happened decades ago, or last year. We don't know," Patton said.

Other coauthors were Chris J. Conroy and Juan L. Parra of the Museum of Vertebrate Zoology and Gary C. White of the Department of Fish, Wildlife, and Conservation Biology at Colorado State University in Fort Collins.

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10.09.2008 - Yosemite resurvey shows small mammals moving up in world

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Global warming is causing major shifts in the range of small mammals in Yosemite National Park, one of the nation's treasures that was set aside as a public trust 144 years ago, according to a new study by University of California, Berkeley, biologists. The study, published in the Oct.