Seeing (less) red: Bark beetles and global warming

This post contributed by Jesse A. Logan, retired research entomologist living in Emigrant, Montana.

The Greater Yellowstone Ecosystem (GYE) is an ecological reserve of regional, national and international significance. This collection of National Parks, National Forests, wildlife reserves and tribal lands is generally recognized as one of the last remaining large, nearly intact, ecosystems of the Earth’s northern temperate region. Climax whitebark pine (Pinus albicalus Engelman) forests comprise the majority of forested habitat above 2,750 meters and extend to the highest elevation as a crooked krumholtz growth form. By functioning as both a foundation and a keystone species, whitebark pine is an important ecological component of the GYE.

Unfortunately, the foundation whitebark forests of the GYE are facing catastrophic collapse due to a combination of an introduced pathogen, unprecedented attack by a native bark beetle and climate change. Whitepine blister rust is a pathogen introduced near the turn of the past century, and its effect is to first compromise the reproductive capacity of the tree, eventually (requiring an average of twenty years in the GYE) leading to the tree’s death. On the other hand, attack by the native mountain pine beetle either immediately leads to the  tree’s deaths, or the tree successfully defends itself and repulses the attacking beetles. The seriousness of these threats to the integrity of high-elevation forests is indicated by the recent finding by the US Fish & Wildlife Service that whitebark meets the criteria for a threatened or endangered species; in addition, despite their risk of extinction, the FWS did not add whitebark to the endangered species list due to lack of sufficient funding.

Under historic climate regimes, these high elevation forests were simply too cold for the mountain pine beetle (MPB) (Dendroctonus ponderosae) to thrive. Although, past MPB-caused whitebark pine mortality did occur during periods of unusually warm weather—for example, in the 1930s—these outbreaks were short-lived and limited in scale. With the publication of the first Interngovernmental Panel on Climate Change report in 1990, research on the potential for increased MPB activity in whitebark pine began to occur. Model predictions of high intensity MPB outbreaks began to be realized across the southern range of whitebark pine by the early 2000s. By 2005, USDA Forest Service Aerial Detection Survey (ADS) data showed significant MPB caused mortality across large areas of GYE whitebark pine. This mortality is first evident by large numbers of red trees (symptomatic of trees killed the previous summer), subsequently followed by vast areas of gray trees — the residual ghost forest — is shown in the photos above. In those photos of Hoyt Peak from Avalanche Peak near Sylvan Pass, Yellowstone National Park, the dramatic decline in whitebark forests that is common all across the GYE is evident.

Although the red-tree signature of MPB mortality has been an all-too-obvious aspect of Yellowstone high mountains for the past 10 years, expect to see fewer red trees this summer.  This avalanche of tree-killing bark beetles has resulted from rising temperatures that allowed them to consistently occupy the high mountains where weather was once too cold. The combination of warm summers and mild winters required by the beetles was historically uncommon in the high mountain habitats of whitebark pine. With climate change, this combination, necessary for beetle success, has become the new norm.

Global warming, however, is just that, global in scale. Climate change is brushed in broad strokes; ecology happens locally. There can and will be large variations in local weather as the alarming rate of global warming inexorably continues. One such local anomaly occurred in early October 2009. Temperatures across the entire GYE plummeted to record lows (see table to the right). During early October, temperatures in the -20 °F (-29 °C) range were common. Given the opportunity to acclimate to winter conditions, the mountain pine beetle can easily withstand such temperatures, but this cold snap occurred before the beetles had a chance to adjust. Additionally, it occurred before an insulating layer of accumulated snow could serve to protect the beetles that live just under the bark.

Do fewer red trees mean the outbreak in whitebark pine is over? Let’s hope so, but so long as the current temperature trend continues, it seems unlikely. While it is true that current beetle populations may be low, they have not been eliminated. As weather once again becomes favorable for the beetle, these residual populations are capable of explosive population growth. So long as there are available whitebark to serve as food (generally trees greater than 6 inches in diameter), a warming climate means deep trouble for GYE whitebark pine forests.

During the summer of 2009, a comprehensive aerial survey of whitebark mortality was conducted for the entire Greater Yellowstone Ecosystem. Results of this survey indicated that there are many areas in the Ecosystem where there are not enough whitebark to support outbreak beetle populations. In effect, they have eaten themselves out of house and home. However, in other parts of the Ecosystem, there are more than enough remaining whitebark for the beetles to thrive.

Jesse A. Logan is a retired research entomologist for the Interior West Bark Beetle Project (RMRS RWU-4501). He retired in 2006, and moved to Emigrant, Montana in order to continue his research and pursue life’s true quest: backcountry skiing and fly fishing.

Photo Credit (picture): Jesse Logan and Jeff Hicke
Photo Credit (table): Jesse Logan
Photo Credit (topographic map): Jesse Logan via National Geographic TOPO! software