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.
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.
- 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.
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 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 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
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
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.
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 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
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
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.
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.
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.
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
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
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:
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.
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