The mountain pine beetle has co-evolved with some pines. A native insect it 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.

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.

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.

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.

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.

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.

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. Winters are becoming mild enough that even adult beetles, a freeze-intolerant stage, are surviving.

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. 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.

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.

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.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.