Earlier this month, a small group of interested citizens gathered at the Evergreen Library in Evergreen, Colo., to attend a Tuesday night program called “Beyond the Headlines: The Pine Beetle Infestation.” For some, it may seem a strange time to hold such a program, as the weather has been pretty wet and there hasn’t been much pine beetle activity in their neck of Colorado’s Front Range.
But a potential disaster looms just over the next ridge, figuratively and literally.
“The community does know that it is a problem that is going to be upon us itself in two or three years,” said Janice Tang, a librarian who attended the event. “They’re coming over the continental divide at this point. They’re up in Vail. They’re wreaking havoc in the pine forests in Vail.”
Several species of bark beetles – such as mountain pine beetle (Dendroctonus ponderosae), piñon ips beetle (Ips confusus), and spruce beetle (Dendroctonus rufipennis) – are attacking and devastating the predominantly conifer forests of western North America from British Columbia to New Mexico. Tens of millions of acres of western forests have been affected by die-offs of infected trees the past few years, causing more than $1 billion in damage annually in the United States alone.
The problem will likely worsen, unless steps are taken to the reduce greenhouse gas emissions that lead to a warmer climate, according to a recently released government report, Global Climate Change Impacts in the United States.
How global warming is affecting the delicate biological cycles of a single family of insects offers a stark warning on unforeseen ecosystem disruptions that climate change will usher in if left unchecked.
Bark beetles are a natural part of the forest ecosystem. They help increase the diversity of forest stands, killing older or weaker trees and creating patches of forest that differ in species composition, stem density, age, and successional stage. The beetles, by helping break down dead wood, also contributing to the recycling of minerals and nutrients. But, when outbreaks are widespread, they can cause massive die-offs and increase the likelihood of massive, devastating fires that can undo their normally beneficial effects.
Climate change influences the frequency, intensity, and distribution of bark beetle outbreaks by affecting both the beetles as well as the trees themselves. Kenneth F. Raffa, a professor in the departments of entomology and of forest and wildlife ecology at the University of Wisconsin and lead author of 2008 paper on the dynamics of bark beetle outbreaks in the journal BioScience, said that two climatic factors are important: temperature and drought.
“Elevated temperature has beneficial effects on bark beetles in two ways,” Raffa said. “One, it reduces the amount of mortality they experience in the wintertime. That’s obviously particularly important at the higher latitudes and the higher elevations. The other thing that elevated temperature does is that it can reduce the time needed for them to complete a generation. … You put those two elements together, their shorter life cycle and their higher survival, and you have more beetles, and more beetles allow them to attack healthier trees.”
All life stages of bark beetles can be killed by low temperatures, but the degree of coldness is not as important as its timing. Barbara J. Bentz, a research entomologist at the USDA Forest Service’s Rocky Mountain Research Station, explained why.
“They’re not like us. When we get cold our tissues freeze and, boom, we’re dead. They metabolize these – basically we call them anti-freeze compounds. Glycerol is the one mountain pine beetle uses. What glycerol does is allow their tissues to not freeze” said Bentz. “We’ve recorded them down to minus 40 C when they have the maximum amount of glycerol in their bodies.”
Anti-freeze compounds such as glycerol take a lot of energy to make, Bentz said, so bark beetles rely on temperature trends to signal when to begin and end production of the compounds. Cold temperatures are more effective at killing the beetles when they are caught unprepared for the cold – particularly in the fall, when they are beginning production of the anti-freeze compounds, and spring, when they begin cutting back production of the compounds.
Since the beetles’ body temperature – thus their metabolic rate – is controlled by the environment, it should come as no surprise that warmer temperatures, therefore a faster metabolism, can shorten the time it takes for them to develop to their adult stage.
For organisms that can increase their numbers at potentially exponential rates, the shortening of the generation time can lead to explosive population growth – and devastating outbreaks in affected forests. This is more of a problem in higher latitudes and higher elevations, where warmer temperatures allow bark beetles that normally complete their life cycle in two years to do so in one.
In addition to boosting the growth side of the population equation, the shortening of the generation time may also decrease some types of mortality.
“If it takes one year instead of two years, they’re probably exposed to less predator-caused mortality,” Bentz said. “If it takes two years, they have to go through two winters, thus two periods of potential mortality from cold.”
While changing temperatures primarily affect the beetles, drought primarily affects the trees themselves. Drought stress taxes trees’ metabolism, making it more difficult for them to make the chemicals they use to defend themselves against a variety of pests and pathogens. Bark beetles can sense the concentration of the chemicals in individual trees and are attracted to those that are weakened by drought.
“Those chemicals that the beetle uses as an attractant are for the most part the same chemicals that the tree uses to defend itself, so concentration becomes very important,” Raffa said. “It’s often if the tree produces small amounts of those chemicals, the beetle can detect that. If the tree produces large amounts of those chemicals, then the beetle knows that that might be just too toxic a dose for it to contend with.”
Warming temperatures in the West allows bark beetles to have a greater effect on forests at higher latitudes and higher elevations – places where cooler conditions once kept them in check.
“As it has gotten warmer, it’s allowed mountain pine beetle, for example, to be at higher densities at further latitudes than in the past,” Raffa said. “It’s probably been in those higher latitudes at some kind of marginal basis for a long time, but never really had a big outbreak like now. These conditions have allowed it to change.
“The same with the higher altitudes. I think mountain pine beetle has always gotten into the high-elevation whitebark pine stands. It would get warm, and the beetle would go up there and kill some trees, then it would get cold, and the beetle would die off.
"The difference is now we’re seeing one year after another warmer than what it was historically and lots of tree mortality up in those high stands.”