Beetles ride global warming up rockies, into vulnerable pines

Illustrated by Perry Shirley.

By John Upton

The grand pine forests that dominate the Rocky Mountains in the American West morph with the montane altitudes. High peaks are home to whitebark pine, a slow growing species that produces energy-rich, pine cone-encased seeds that help grizzly bears grow plump enough to survive hibernation. At lower altitudes are the faster growing lodgepole pines.

The lodgepole pines have long been plagued by occasional infestations of native pine beetles. These dark beetles burrow into tree bark to lay their eggs, which hatch into larvae that feast on the phloem. (Phloem is a tender organ found just beneath the bark that ferries sugars produced by photosynthesizing leaves to other parts of the tree.)

A full blown infestation of phloem-munching beetle larvae is generally fatal. But lodgepole pines have developed a repertoire of defenses against the herbivorous creepy crawlies. They churn out sap and pour it over the invading beetles. They exhale chemicals that repel and kill the adults, prevent eggs from hatching and wreak general havoc with the beetles’ diminutive ecosystems.

Pine trees covered with snow near the top of Polar Peak lift at Fernie Alpine Resort in the Rocky Mountains, British Columbia / Flickr: DCZwick

Whitebark pines have not developed these defenses, at least not to the same extent as their lower-altitude cousins, because they haven’t needed them. The beetles can’t bear the bitter winters that have long swept over the Rocky Mountains’ higher peaks. But now, as climate change sweeps warmer weather over these towering peaks, the whitebarks are in newfound peril.

During occasional warm periods in the past, the beetles would march up the mountains and find a footing in whitebark forests. Then temperatures would return to normal and the pest populations would die off.

“However,” entomologists and ecologists report in the latest edition of Proceedings of the National Academy of Sciences, “recent continuously warm weather has allowed persistent reproduction in this keystone (beetle) species.”

The warming peaks have ushered in an era of beetle infestations that many of the trees have been unable to withstand. More than 100 million acres of mountain forest has been impacted during the past decade. Great forests that used to soak up carbon now lay dead and rotting, releasing their carbon back into the atmosphere, further accelerating the global warming that contributed to their demise.

The Rocky Mountains on Dec. 19, 2012 / Flickr: NASA Goddard Photo and Video

I asked the study’s lead researcher, Ken Raffa, an entomology professor at the University of Wisconsin, Madison, whether he thought the whitepines would be able to evolve defenses against the pine beetles quickly enough to protect themselves from being wiped out. He said he didn’t know: This is something he’s currently investigating, by studying how various tree genotypes are distributed across the mountain landscapes.

But of particular concern to Raffa is the fact that whitepines grow and reproduce very slowly, not producing viable seeds until they reach their 50s, while the beetles can reproduce every year or two, creating an evolutionary handicap.

In addition to marauding beetles, the whitepines also face tremendous threats from white pine blister rust, a ravaging fungus disease. “To be viable,” Raffa said, “whitebark pine would have to escape both.”