The Mountain Pine Beetle: a Friend or Foe of the Whitebark Pine?

By: Kelsey Maxwell, Summer Naturalist, 2020

Everyone who visits the Sawtooths should take some time to enjoy the magic and mystery of a whitebark pine forest. The whitebark pine is easy to identify compared to its conifer counterparts. Its needles grow in bunches of five, and instead of growing straight and tall like the lodgepole pine, the whitebark pine tends to grow massive lower branches and a wide-sweeping canopy.

Whitebark pine trees tend to have thick trunks, massive lower branches, and a broader canopy than many other conifer trees.

In addition to being eccentrically beautiful, the whitebark pine also provides unique ecosystem services. Their broad canopies shade snowy slopes and slows the melting of snowpack. This helps to recharge groundwater and extend growing season for neighboring plants. Furthermore, their seeds are a highly nutritious and an irreplaceable food source for the red squirrel, the Clark’s nutcracker, and the endangered grizzly bear.[1]

The whitebark pine and the Clark’s nutcracker have a mutualistic relationship. The seeds provide a critical food source for the nutcracker, while the bird’s seed caches allow for new whitebark pines to grow.

These mighty trees have dominated subalpine ecosystems for thousands of years. In fact, the oldest living whitebark pine (which is over 1,200 years old) can be found in the White Cloud Wilderness, just southeast of Stanley on Railroad Ridge![2] They have reigned supreme by outcompeting other tough conifers at high elevations and adapting to benefit from minor disturbances like wildfire.[3] But over the past 100 years, human disturbance has wreaked havoc on these ecosystems leading the whitebark pine to the brink of extinction.

The decimation of the whitebark pine is mostly due to rapidly warming temperatures. Climate change is increasing the frequency of high-intensity forest fires which strip the soil of critical nutrients.[4] Furthermore, warmer temperatures are making these trees more vulnerable to diseases and insect pests; the worst offenders are the mountain pine beetle and white pine blister rust.[5]

The mountain pine beetle is native to the area and has been killing whitebark pines for centuries. But as the climate warms, the whitebarks are losing one of their best defenses against these beetles – excruciatingly cold nights that kill the beetles in their larval state.[6] Conversely, the white pine blister rust is a non-native fungus that was introduced from Asia. Because the whitebarks lack defense mechanisms against the killer fungus, it has rapidly swept through forests, killing nearly every mature whitebark in sight.[7] 

Now, scientists are scrambling to find out how we can help the whitebark pines thrive and adapt to our rapidly changing environment.  For years, foresters believed that suppression was the solution – stop the fires, remove the killer beetles, and eradicate the white pine blister rust.[8] But the solution is just not that simple. Like I mentioned before, these ecosystems are highly adapted to disturbance. They have evolved to benefit from wildfire, and maybe even mountain pine beetle, too!

That’s right, these killer bugs may actually help these trees recover and adapt to the white pine blister rust!

A study published in 2010 by Evan Larson and Kurt F. Kipfmueller about whitebark forests in Central Idaho, Montana and Oregon had some truly fascinating results about the impact of disturbance on whitebark repopulation.[10] The study collected data on climate, topography, and plant abundance, and compared it to the presence of whitebark seedlings and saplings. They then were able to identify several statistically significant correlations between certain biophysical characteristics and the abundance of whitebark pine saplings. Let me try and break it down for you. 

  • Whitebark pine regeneration was LOW in areas with:
    • low levels of disturbance and warmer temperatures. In these areas, the whitebark pine was being outcompeted by the subalpine fir.
    • Compounding disturbances in close succession. For example, pine beetle infestation closely followed by several high intensity fires a few years later.
  • Whitebark pine regeneration was HIGH in areas that were:
    • higher, drier, and cooler
    • impacted by a high level of beetle mortality

So what does this all mean?

As scientists look to replant and rehabilitate forests, priority should be given to the ecosystems that will remain dry and cool, even as the climate changes. There may be little we can do to prevent subalpine firs taking over some lower-elevation forests that used to be dominated by the whitebark pine. This is tragic. But understanding where the whitebark will be outcompeted will be critical to predicting regional population declines for the Clark’s nutcracker, the black and grizzly bear, and other species that depend on these trees.

Grizzly bears raid the whitebark pine seed caches in the ground created by Clark’s nutcrackers, these bears are probably just climbing this tree for fun.

Moreover, the pine beetle may be a disturbance that positively impacts the future of whitebark pine forests. Like wildfire, the pine beetle kills large swaths of mature trees that provide seedlings and saplings with the sunlight they need to grow.[10] Whitebark pines have survived other major periods of warming (during the Holocene), and the Clark’s nutcracker has been found to cache seeds in beetle kill regions.[10] Larson and Kipfmueller used this evidence to indicate that the whitebarks may already be adapted to severe pine beetle disturbance and thus have the defense mechanisms to weather this storm.[10]

Additionally, beetle kill could defend against the white pine blister rust.  This study found that even though blister rust infections were not very common in the areas surveyed, the rust-resistant seedlings and saplings were outcompeting their counterparts that lacked the rust-resistant genotype. [10] This finding is consistent with a growing body of evidence that has found that the whitebark pine is adapting rapidly to white pine blister rust.[9] So ultimately, Larson and Kipfmueller’s findings led them to hypothesize that beetle kill could act as a catalyst for repopulating whitebark pine forests with rust-resistant trees.

The death of these ancient and massive trees is truly devastating. My heart hurts when realizing that we have managed to decimate a species that, for thousands of years, has been resilient enough to survive in some of the harshest environments on earth. But learning about the whitebark pine forests has also taught me that a beautiful forest, an old growth forest, or an undisturbed forest, is not always a healthy forest. 

So, as we continue to search for solutions to the problems facing whitebark pine ecosystems, we cannot forget about the historical power and prevalence of disturbance in these systems. Fire brings new life and killer beetles help the species evolve. However, we do need to eradicate the only disease the whitebark pine is not equipped to manage – human-caused climate change.  

[1] Forcella, Frank; Weaver T. 1980. Food production in the Pinus albicaulis–Vaccinium scoparium association. Proceedings of the Montana Academy of Science.

[2] Perkins, Dana L.; Swetnam, Thomas W. 1996. A dendroecological assessment of whitebark pine in the Sawtooth–Salmon River region, Idaho. Canadian Journal of Forest Research. 26: 2123-2133.

[3] Murray, Michael P.; Bunting, Stephen C.; Morgan, Penny. 2000. Landscape trends (1753-1993) of whitebark pine (Pinus albicaulis) forests in the West Big Hole Range of Idaho/Montana, U.S.A. Arctic, Antarctic, and Alpine Research. 

[4] Keane, Robert E. 2001. Successional dynamics: modeling an anthropogenic threat. In: Tomback, Diana F.; Arno, Stephen F.; Keane, Robert E., eds. Whitebark pine communities: Ecology and restoration. Washington, DC: Island Press: 159-192.

[5] Fryer, Janet L. 2002. Pinus albicaulis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station

[6] Six, Diana L. 2002. Interactions between mountain pine beetle and white pine blister rust in whitebark pine.

[7] Hoff, Raymond J.; Ferguson, Dennis E.; McDonald, Geral I.; Keane, Robert E. 2001. Strategies for managing whitebark pine in the presence of white pine blister rust. In: Tomback, Diana F.; Arno, Stephen F.; Keane, Robert E., eds. Whitebark pine communities: Ecology and restoration. Washington, DC: Island Press: 346-366. 

[8] Agee, James K. 1996. Fire in the Blue Mountains: A history, ecology, and research agenda. In: Jaindl, R. G.; Quigley, T. M., eds. Search for a solution: Sustaining the land, people and economy of the Blue Mountains. Washington, DC: American Forests: 119-145.

[9] Hoff, Raymond J.; Ferguson, Dennis E.; McDonald, Geral I.; Keane, Robert E. 2001. Strategies for managing whitebark pine in the presence of white pine blister rust. In: Tomback, Diana F.; Arno, Stephen F.; Keane, Robert E., eds. Whitebark pine communities: Ecology and restoration. Washington, DC: Island Press: 346-366.

[10] Larson, Evan & Kipfmueller, Kurt. (2010). Patterns in whitebark pine regeneration and their relationships to biophysical site characteristics in southwest Montana, central Idaho, and Oregon, USA. Canadian Journal of Forest Research. 40. 476-487.