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Beetle Devastates Yellowstone Whitebark Pine Forests

Jesse A. Logan and William W. MacFarlane

articlehighlights

The warming climate has made conditions suitable for a massive outbreak of mountain pine beetles, which are now infesting the whitebark pine forests in the Greater Yellowstone Ecosystem (GYE). The infestation has other ecological consequences such as:

  • devastation of forests in the region
  • loss of a critical food supply for grizzlies and other wildlife
  • negative impacts on water and watersheds
  • deterioration of biodiversity
  • decline in the aesthetic value of an iconic ecosystem

March 2010

InvadedForest.jpg

Pine trees dying from an outbreak of mountain pine beetle infestation in the Greater Yellowstone Ecosystem. To view these forests on Google Maps, enter the degrees of 43.775189 latitude and -110.170233 longitude. Photo: U.S. Forest Service.

The Greater Yellowstone Ecosystem

The mountain pine beetle is killing pine trees in many areas of North America. The beetle’s effects are particularly devastating in the Greater Yellowstone Ecosystem (GYE). Yellowstone National Park looms large in the history of conservation—not only in the history of the United States but of the entire world. As the world’s first national park, its example has inspired the creation of other parks and natural reserves worldwide; and within the United States, it has played an important role in the creation of the National Forest System.

Yellowstone National Park forms the core of the Greater Yellowstone Ecosystem.

The history of the area goes back to the late 1800s:

  • The first National Forest (the Shoshone) was established in 1891, which abuts the eastern boundary of the Park.
  • Subsequently, national forests and protected areas were established in surrounding areas.
  • This collection of national parks, national forests, and wildlife refuges has become collectively known as the Greater Yellowstone Ecosystem.1

The GYE is an informal designation, and the exact boundaries vary with each agency.

The GYE is about the size of South Carolina.
  • Currently accepted boundaries include approximately 8,093,712 ha (20,000,000 ac)—an area roughly the size of South Carolina.
  • A significant part of this (approximately 1/3) is formally protected as either national parks or designated Wilderness areas.
  • Much of the remaining land maintains its wild character.

“A wilderness area can have two dimensions: a place that is mostly biologically intact; and a place that is legally protected so that it remains wild, free of industrial infrastructure, and open to traditional indigenous use, or low impact recreation.”2 Even though the exact boundary is ambiguous, it is recognized generally that the GYE is one of the last enduring, large, and nearly intact ecosystems of the Earth’s northern temperate region.

Historic climates were often too cold for mountain pine beetle outbreaks.

Across the vast GYE, an important component of the ecosystem may be facing catastrophic collapse. The high-elevation whitebark pine (Pinus albicalus) forests are experiencing unprecedented outbreaks of a native insect, the mountain pine beetle (Dendroctonus ponderosae), a type of bark beetle. This devastation was largely unexpected. Under historic climate regimes, these areas were too cold for the beetle to thrive. Although past tree mortality did occasionally occur during periods of unusually warm weather (e.g., the 1930s and 1970s), these outbreaks were short lived and limited in scale. Unfortunately, with the level of anthropogenic (human-caused) climate warming that has already occurred, the harsh conditions needed to protect these forests have become increasingly rare. Therefore, significant tree death caused by mountain pine beetles is taking place year after year, and if these outbreaks continue unabated, the ecological collapse of this important ecosystem appears likely.3

Whitebark pine ecosystems in GYE

Whitebark pines form the highest forests in the GYE.

Whitebark pine is the main pine tree found at high elevations throughout the GYE. Typically, forested landscapes above about 8,500 feet (2,600 meters) are either “climax,” which is where the plant community is dominated by mature whitebark pine trees, or mixed spruce/sub-alpine fir forests with whitebark pine as an important component.4 John Muir was amazed to find a six-inch whitebark pine in the high sierras that was over 450 years old. The oldest documented whitebark pine is well over 1,000 years old.5

Whitebark pine is a slow growing and long-lived tree that has adapted to the harsh environments of the high mountains.6 These forests provide a beautiful open-canopy environment with diverse understory plants and abundant wildlife. Growth forms range from an upright form with a distinctive “bushy” canopy, often with multiple stems growing in clumps of several trees, to a stunted brush-like Krummholz7 form found above timberlines. These ancient forests form the rooftop of the continent and they have evolved to have a special relationship with wildlife.

The whitebark pine ecosystem depends on the Clark’s nutcracker.
NutcrackerBird.jpg

The Clark’s Nutcracker has a special relationship with pine trees. Photo: United States Fish and Wildlife Service.

The entire whitebark pine ecosystem depends on a reciprocal relationship between the pine tree and a bird in the crow family, the Clark’s Nutcracker (Nucifraga columbiana).8 Unlike the cones of many pines, whitebark cones remain tightly closed until they are torn apart—typically by an animal seeking the large, highly nutritious seeds. Clark’s Nutcrackers harvest and bury thousands of whitebark pine seeds in caches from which they retrieve seeds to feed on throughout the year. The reproduction of whitebark pine is almost wholly dependent on surplus or neglected seeds buried in nutcracker caches. Whitebark pine reforestation occurs when these neglected seeds subsequently germinate. In effect, the bird harvests and plants seeds for future generations of whitebark pine.9,10

The trees also feed the endangered grizzlies.

Clark’s Nutcrackers are not the only wildlife that utilize whitebark pine seeds. During late summer and fall, red squirrels (Tamiasciurus hudsonicus) harvest and store large numbers of pinecones in middens (small piles) located on the ground beneath the trees. Grizzly bears (Ursus arctos), in turn, raid these middens for the high quality food provided by the nutritionally rich (high-fat content) seeds. This abundant food resource is available at little risk to the bears as red squirrels pose no threat to the great bears.11

Whitebark pine nuts are especially important for impregnated female bears during the time prior to hibernation, the period of hyperphagia, during which the intake of highly caloric foods is essential for the adequate accumulation of fats required to sustain the bear throughout the winter.12 Female reproductive success is related directly to their nutritional condition entering hibernation. A loss of this important food resource results in not only higher direct overwinter mortality, but it also reduces the reproductive potential of the bear population drastically.

Perhaps even more importantly, when there is a shortage of pine nuts in the remote whitebark pine habitat, ravenous bears are more likely to focus on alternative foods, like garbage or hunter “gut piles,” which brings them into conflict with humans more often. While human/bear conflicts may be dangerous for people in the short term, the conflicts are usually disastrous for the bear in the long term. Nowhere are whitebark pine seeds more important to grizzlies than in the GYE, where alternative food such as berries or spawning salmon is scarce. Here, as nowhere else, whitebark pines not only feed the bears but also keep them out of trouble to some extent.13

Whitebark pine forests are critical to other animals.
mountainelk.jpg

Elk grazing in the Rocky Mountains. Photo: Oksana Hlodan.

In addition to the bird-squirrel-bear story, whitebark pine forests are critical to other wildlife. At some time during the year, the fauna that benefit from the forests include:

  • elk (Cervus canadensis)
  • mule deer (Odocoileus hemionus)
  • big horn sheep (Ovis canadensis), and
  • pronghorn antelope (Antilocapra americana)

For example, elk typically calve in high country forests during spring or early summer. In these environments, whitebark pine often provides their only cover for thermal regulation and protection from predators.

Whitebark pines regulate water resources.

Beyond the whitebark pine’s importance to wildlife, the ecosystem’s protective canopy also provides other important ecological amenities such as the regulation of alpine hydrology. The source of water for most western rivers is accumulated winter and spring snowfall. Whitebark pine forests are responsible for both the distribution of winter snowfall, by providing wind breaks that shelters snow from the incessant wind, and prolonging snowmelt in the spring because of the shade provided by the whitebark pine canopy. Without this protective shading, peak stream-flow would occur earlier and be shorter.

Furthermore, altered hydrology not only affects water resources for humans, it has important implications for coldwater fisheries. Early and spiked spring flow translates into a greater likelihood of dangerously reduced flow and lethal high temperatures later in the summer. During the summer of 2007, the record fishing restrictions on the Yellowstone River resulted from just such a flow scenario and such restrictions will become increasingly likely as high elevation forests are lost to bark beetles.

In short, whitebark pine provides the foundation for some of the most magnificent intact landscapes remaining on the North American continent. In the words of Ron Lanner, “Working in concert, the Clark’s Nutcracker and the whitebark pine build ecosystems.”14 These forests immeasurably increase the diversity and richness of the GYE, and their loss would result in irreparable consequences for future generations.

The mountain pine beetle

The mountain pine beetle has co-evolved with some pines.

The mountain pine beetle is a native insect that most likely moved north along with its host pines as the trees colonized the Rocky Mountains. The mountain pine beetle is unique; it belongs to the relatively small group of “aggressive” bark beetles that must kill their host to reproduce successfully. To do so, the bark beetles make a hole in the tree bark, all the way to the phloem layer (the tissue that carries nutrients to the various parts of the tree)—the part they like to eat. They also lay their eggs in this location. Special chemicals are released by the first beetle invaders, and these attract more and more beetles. The trees try to block the entry of beetles by increasing their own production of resin; however, the beetles are resilient and introduce blue-stain fungi that helps to overcome tree defenses and also provides an important food for developing larvae. Eventually, the phloem layer is too injured to deliver food and water to the tree, so the trees die from the inside out. Mountain pine beetles may also seek to inhabit already damaged trees.15

beetlegraph.jpg

Figure 1

Left: A diagram of the life cycle of a mountain pine beetle (by Alberta Sustainable Resource Development). Right: A cross section of a pine tree (by Inside Education).

Episodic outbreaks are common in the principle host, lodgepole pine (Pinus contorta), and they can be truly spectacular events. Even though large-scale outbreaks are common in lodgepole pine, they do not typically pose a threat to continuity of the ecosystem as a whole.16

Effects of global warming

Climate and weather play a key role in mountain pine beetle numbers.

The role of climate as a regulating mechanism for the distribution of mountain pine beetles in northern and high-elevation areas was recognized by scientists early on.17,18 Their early findings indicated that seasonal weather of the high mountains imposed two key constraints that served to protect whitebark pine from mountain pine beetles:

  • First, historical winter temperatures in whitebark pine habitats were frequently cold enough to kill all mountain pine beetle life stages everywhere but in the most protected sites, i.e., in the tree bole (trunk) beneath the insolating snow cover.
  • Second, summer temperatures typically did not provide enough hotter temperatures to complete an entire life cycle in one year. Mountain pine beetles are univoltine, that is, they have one brood a year.

This combination of cold temperature, winter mortality, and cool summer temperatures served to keep mountain pine beetle populations in check. The simultaneous occurrence of conditions necessary to change this historical pattern occurred only infrequently in high-elevation whitebark pine forests.3 With the advent of a warming climate, however, winter temperatures have become mild enough to allow substantial overwinter survival of all bark beetle life stages; and there is sufficient summer thermal energy to complete an entire life cycle in one year.

Global warming allows the beetle to expand north and into higher elevations.

With the onset of anthropocentric global warming in recent decades, the ecological relationship between mountain pine beetle and whitebark pine has undergone a fundamental shift. The potential for climate change to intensify mountain pine beetle activity in whitebark pine was first recognized in theoretical modeling studies.19,20 These studies suggested that increased mountain pine beetle activity in whitebark pine would be a good “canary in the coal mine” indicator for the ecological impacts of climate change. Unfortunately, subsequent events have played out much along the lines of these early model predictions, with two important differences: the anticipated impacts occurred earlier than predicted; and the spatial extent and intensity of mortality was much greater than anyone could have projected. Although the models accurately predicted the qualitative impacts of a warming climate, the failure to predict the speed and intensity of mortality has resulted from the vulnerability of whitebark pine to mountain pine beetle attack. Apparently, the vulnerability of this species of pine to attacking beetles is much greater than that of the lodgepole pine.

BiioSIMSChart.jpg

Figure 2

Annual minimum temperature and modeled mountain pine beetle survival for the Togwotee Pass, WY. Data was taken at a site close to the massive mortality occurring in the Teton Wilderness, GYE. Temperatures were interpolated and survival was predicted using the model described by Régnière and Bentz.23 Source: Logan and MacFarlane.

Beetles are surviving winter because of seasonal warmer temperatures .

The combination of chronic warm weather and vulnerability to attacking beetles has produced a worst-case scenario. We have consistently observed large numbers of successfully attacked trees in late spring/early summer.21 Apparently, re-emerging beetle parent adults from the previous summer, perhaps augmented by an early phase emergence of newly emerging adults, are responsible for this mortality.22 Winters are becoming mild enough that even adult beetles, a freeze-intolerant stage, are surviving.23 These surviving beetles, at even relatively low densities, have been able to attack new whitebark pine trees successfully. We have observed that strip attacks, in which only a portion of the tree’s phloem tissue is killed, are more commonly observed than in lodgepole pines.

Broods produced by re-emerged adults may experience enough thermal energy to complete the life cycle within the same year of attack.24 Even if this early brood does not reach the adult stage, all life stages—even those previously susceptible to winter mortality—are surviving. The result is a bipeak emergence of early, re-emerged beetles, and a later traditionally timed emergence. The combination of a warming climate and vulnerability to attacking beetles has resulted in a shift from non-overlapping, semivoltine (life cycle requiring two years to complete) generations to overlapping, univoltine generations that also have a greater potential to reproduce.

The future of whitebark pine ecosystems

Whitebark pine is a keystone species, crucial to the entire ecosystem.

Since whitebark pine is a foundation and a keystone species (a species that plays a critical role), its loss would seriously impact the ecological integrity of the entire GYE, with repercussions reverberating from the highest mountains to the river valleys below. Undoubtedly, a major disturbance has already taken place and shows no indication of abatement. Ecological disturbances including “catastrophic” disturbances such as the large 1988 fires, however, are an integral component of this ecosystem.25 The relevant question becomes, how likely is the loss of whitebark pine forests and the collapse of the ecological services they provide?

A disturbance of this magnitude in whitebark pine is unprecedented in the ecological history of the GYE, and several aspects of whitebark pine ecology indicate they may not readily adapt to such large-scale disturbances.16

Whitebark pine is more vulnerable to large-scale attacks than other pines.
  • The first indicator is the nature of the relationship between whitebark and the Clark’s Nutcracker. The tree is more dependent on the bird than the bird is on the tree; consequently, there is a threshold of cone/seed density where the opportunistic nutcracker will seek alternative food sources.26 If cone-bearing overstory trees (larger, taller trees) are removed by mountain pine beetles in large areas, then recovery from even small disturbances will be problematic.6

  • The second indicator is that due to the high nutritional value of whitebark pine seeds, seeds that are not protected in Clark’s Nutcracker caches are utilized by other wildlife—such as squirrels or bears. As a result, the copious seed bank typical of forests resilient to large-scale disturbance (i.e., lodgepole pine) is not present in whitebark pine forests.

  • Finally, the patchy spatial structure of whitebark pine forests, which has served them well for protection from disturbances such as fire, is no deterrent to the mountain pine beetle. Since rock does not burn, it is an impediment to fire; however, these bare areas pose no barrier to dispersing beetles. In effect, the mountain pine beetle is the fire that does not go out with winter snows; their attacks continue year after year, as long there are sufficient host trees and seasonal weather remains favorable.27

The bright side

Although it may be difficult to see any positive elements in the current situation, we must not forget the inherent strength and hardiness in whitebark pine, which is:

Whitebark pine has some characteristics that may be the answer to its survival.
DwarfPine.jpg

Windswept dwarf whitebark pine on Electric Peak. Photo: RG Johnsson, Yellowstone National Park Service.

  • Robust: Whitebark pine is a tough species that has evolved to withstand some of the harshest environmental conditions on the continent. Some of these evolved attributes confer a resiliency that may help bridge even a prolonged climate crises.

  • Slow growing: Whitebark is extremely slow growing; it takes a minimum of 50 years to reach cone-bearing age (a more typical figure in the GYE would be 75-100 years). Slow growth/slow maturation goes hand in hand. Of course, the growth rate heavily depends on site-specific conditions, but it is safe to say that current whitebark pine seedlings will be immune from mountain pine beetle attacks for decades at least.

  • Krummholz: Whitebark pine in the typical high elevation climax forest is the upright, dominant tree over much of the GYE; however, it also exists as a Krummholz growth form above the timberlines. Krummholz-growth trees are usually bushy, dwarf conifers. Importantly, the dwarf Whitebark pine forest is immune from beetle attack.28 As the climate warms, Krummholz forms are also capable of shifting to upright growth, and therefore, may provide a genetic sanctuary for a limited time.

These attributes may well hold the key to the future of whitebark pine within the iconic Greater Yellowstone Ecosystem.

Jesse A. Logan was a research entomologist for the Interior West Bark Beetle Project of the U.S. Forest Service, Utah. He retired in 2006, and he moved to Emigrant, Montana in order to continue his research and participate in advocacy for high-elevation, Rocky Mountain ecosystems. Before joining the U.S. Forest Service, he held faculty positions at Colorado State University and Virginia Polytechnic Institute & State University. He has published extensively on the influence of weather and climate on insect population dynamics. In recent years, this work has increasingly focused on climate change impacts; and, in particular, the altered ecological role of mountain pine beetle in response to this rapidly changing thermal environment. Logan completed his PhD at Washington State University in 1977.
http://www.usu.edu/beetle/jesse.htm

William W. Macfarlane master’s thesis research used remote sensing and GIS to investigate vegetation change related to land use in a rangeland ecosystem over a 150-year period. The completion of this thesis and masters degree in 1999 lead to his current position as a GIS and Remote Sensing Specialist at GEO/Graphics, Inc. in Logan, Utah. Macfarlane’s professional experience involves using GIS and remote sensing to assess land-use and environmental issues world-wide. His work experience includes the development and implementation of GIS databases for vegetation change detection mapping, resource management planning, and wildlife habitat restoration.

Beetle Devastates Yellowstone Whitebark Pine Forests

BioScience Articles

  • » “Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions.”
    Kenneth F. Raffa et al. (June 2008) show that recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Free to read.
    http://caliber.ucpress.net/doi/full/10.1641/B580607
  • » “An Epidemic with Global Consequences.”
    Timothy Beardsley, Editor-in-Chief, BioScience (June 2008), explains how mountain pine beetle attacks affect forest structure for decades. Free to read.
    http://caliber.ucpress.net/doi/full/10.1641/B580601

Beetle Types

All about mountain pine beetles

On this site, you’ll find information on how to identify mountain pine beetles, beetle ecology, susceptible trees, diagnosing beetle infestations, removal of dead trees, and bark beetle management options.
http://www.beetles.mt.gov/

GYCC Whitebark Pine Publications, Reports and Articles

Take a look at the list of publications about whitebark pine gathered by the Greater Yellowstone Coordinating Committee (GYCC).
http://fedgycc.org/WhitebarkPinePublicationsandArticles.htm

Mountain Pine Beetle ReLeaf

Tree Canada’s Mountain Pine Beetle ReLeaf program is aimed at replacing trees lost to the advance of this invasive insect which destroys native pine trees. Learn more or even donate.
http://www.treecanada.ca/site/?page=programs_releaf_mountainpine&lang=en

Top 10 Pine Beetle Prevention Techniques

Mountain Pine Beetle Consultants LLC offer 10 ways you can help prevent infestation, as well as treat current infestations.
http://stopthebeetlenow.com/stop4_020.html

Whitebark Pine Ecosystem Foundation

Support the foundation’s work to save whitebark pine forests.
http://www.whitebarkfound.org/

Impact of Climate Change on the Mountain Pine Beetle and Western Forests

Lesson designed for middle school about pine forest ecology.
http://oceanservice.noaa.gov/education/pd/climate/activities/casestudies/westmountforeststudent.pdf

What is a mountain pine beetle?

A primer for teachers with links to other resources.
http://www.pc.gc.ca/apprendre-learn/prof/sub/mpb-ddp/page3_e.asp

Field Trip Earth

Activities for grades 9–12, where students develop an integrated project through the comprehensive study of a species, a region, or both.
http://www.fieldtripearth.org/strategy.xml?id=37

  1. The term “Greater Yellowstone Ecosystem” was coined by pioneer grizzly bear researcher Frank Craighead to describe the area used by grizzly bears in the Yellowstone Park area. Craighead, F. 1979. Track of the Grizzly. San Francisco: Sierra Club Books.
  2. The WILD Foundation. 2009. What is a wilderness area? http://www.wild.org/main/about/what-is-a-wilderness-area/ (accessed February 8, 2010).
  3. Logan, J. A., W. W. Macfarlane, and L. Willcox. 2010. Whitebark pine vulnerability to climate change induced mountain pine beetle disturbance in the Greater Yellowstone Ecosystem. Ecological Applications 19: (in press).
  4. Arno, S. F. 2001. Community types and natural disturbance processes. In D.F. Tomback, S.F. Arno, and R.F. Keane (eds). Whitebark Pine Communities: Ecology and Restoration, pp. 74–88. Washington: Island Press.
  5. Petit, C. 2007. In the Rockies, Pines die and bears feel it. New York Times, 30 January, page 1, Science section. http://www.nytimes.com/2007/01/30/science/30bear.html?ei=5090&en=dd41933c05b4992b&ex=1327813200&partner=rssuserland&emc=rss&pagewanted=all (accessed March 22, 2010).
  6. Perkins, D. L., and T. W. Swetnam. 1996. A dendroecological assessment of whitebark pine in the Sawtooth-Salmon River region Idaho. Canadian Journal Forest Research 26: 2123–2133.
  7. Krummholz is a term for the dwarf, shrub-like growth form of many conifers, including whitebark pine, occurring above the timberline in environments that are too harsh (short growing season, freezing winds, etc.) for upright growth. This stunted growth form is environmentally, not genetically, determined. As conditions moderate at higher elevation (longer growing season) these dwarf trees can assume an upright growth form, resulting in an upward shift in timberline.
  8. Lorenz, T. J., C. Aubry, and R. Shoal. 2008. A review of the literature on seed fate and life history traits of Clark’s nutcracker and pine squirrels. USDA Forest Service, PNW-GTR-742.
  9. Tomback, D. F. 1982. Dispersal of whitebark pine seeds by Clark’s nutcracker: a mutualism hypothesis._ Journal of Animal Ecology_ 51: 451–467.
  10. Tomback, D. F. 2001. Clark’s nutcracker: Agent of regeneration. In: Tomback, D. F., S.F. Arno, and R.E. Keane (eds)._ Whitebark Pine Communities: Ecology and Restoration_, pp. 89–104. Washington, DC: Island Press.
  11. Mattson, D.J., K.C. Kendall, and D.P. Reinhart. 2001. Whitebark pine, grizzly bears and red squirrels, In D. F. Tomback, S.F. Arno, and R. F. Keane (eds.). Whitebark Pine Communities: Ecology and Restoration, pp. 121–136. Washington: Island Press.
  12. “Hyperphagia” is the period prior to hibernation when bears eat as many high-fat foods as possible, as much as 20,000 calories/day, to gain several pounds per day. This is a critical time to expand the fat reserves necessary to carry the bears through winter.
  13. D. J. Mattson, B. M. Blanchard, and R. R. Knight. 1992. Yellowstone grizzly bear mortality, human habituation, and whitebark pine seed crops. Journal of Wildlife Management 56: 432–442.
  14. Lanner, R. M. 1996. Made for Each Other: A Symbiosis of Birds and Pines. New York: Oxford University Press.
  15. Halloin, L. 2003. Major bark beetles of the intermountain west. Washington Dept. of Natural Resources: Olympia, WA. http://www.dnr.wa.gov/Publications/rp_fh_wadnrbarkbeetle.pdf (accessed March 29, 2010).
  16. For a readable account of the “aggressive” tree killing bark beetles, and the general role of bark beetles in the forests of western North America, refer to, Bentz, B. (ed.). 2005. Bark Beetle Outbreaks in Western North America: Causes and Consequences. Bark Beetle Symposium, Snowbird, Utah, p.42. Salt Lake City: University of Utah Press. http://www.usu.edu/beetle/symposium/SymposiumCover.pdf
  17. Amman, G. D. 1973. Population changes of the mountain pine beetle in relation to elevation. Environ. Entomol. 2: 541–547.
  18. Safranyik, L. 1978. Effects of climate and weather on mountain pine beetle populations. pp 77-84. In A. A Berryman, G. D. Amman, and R. W. Stark (eds). Proceedings, Symposium: Theory and Practice of Mountain Pine Beetle Management in Lodgepole Pine Forests, 25–27 April 1978, Moscow, ID. Moscow, ID: University of Idaho Forest, Wildlife and Range Experiment Station.
  19. Logan, J. A., and B. J. Bentz. 1999. Model analysis of mountain pine beetle seasonality. Environ. Entomol. 28: 924–934.
  20. Logan, J. A., and J. A. Powell. 2001. Ghost forests, global warming, and the mountain pine beetle. American Entomologist 47: 160–173.
  21. The observation of early emergence has also been noted by other mountain pine beetle researchers working in the GYE, for example, see: Bentz, B. J., and G. Schen-Langenheim. 2007. The mountain pine beetle and the whitebark pine waltz: has the music changed? In Proc. Conference Whitebark Pine: A pacific cost perspective. USDA Forest Service R6-NR-FHP-2007-01, pp 43–50. Portland, OR: Pacific Northwest Region, Forest Service, U.S. Department of Agriculture.
  22. Powell, J. A., and J. A. Logan. 2005. Insect seasonality: circle map analysis of temperature-driven life cycles. Theor. Popul. Biol. 67: 161–179.
  23. Regniere, J, and B. J. Bentz. 2007. Modeling cold tolerance in the mountain pine beetle, Dendroctonus ponderosae. Journal of Insect Physiology 53: 559–572.
  24. JAL—author’s unpublished computer simulations.
  25. Barker, R. 2005. _Scorched Earth: How the Fires of Yellowstone Changed America. Washington: Island Press.
  26. McKinney, S. T., C. E. Fiedler, and D. F. Tomback 2009. Invasive pathogen threatens bird-pine mutualism: implications for sustaining a high-elevation ecosystem. Ecological Applications 19: 597–607.
  27. Gibson, K., K. Skov, S. Kegley, C. Jorgensen, S. Smith, and J. Witcosky. 2008. Mountain Pine Beetle Impacts in High-Elevation Five-Needle Pines: Current Trends and Challenges. USDA Forest Service, Forest Health Protection R1-08-020.
  28. Small diameter pine trees, in general, are “immune” from beetle attack, simply because there is not enough phloem tissue in these small trees to support beetle populations. For whitebark pine, trees need to be between 7 and 8 inches DBH (diameter measured at 4.5 feet) before being attacked by the beetles. Coincidently, for the slow growing and maturing whitebark pine, trees reach this size at approximately the same age they begin to bear cones.

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