Long thought to produce only one generation of tree-killing offspring annually, some populations of mountain pine beetles now produce two generations per year, dramatically increasing the potential for the bugs.
Because of the extra annual generation of beetles, there could be up to 60 times as many beetles attacking trees in any given year, their study found. And in response to warmer temperatures at high elevations, pine beetles also are better able to survive and attack trees that haven’t previously developed defenses.
That’s from the University of Colorado, Boulder news release for a new study in in The American Naturalist.
We’ve known that climate change favors invasive species, but the mountain pine beetle infestation is far worse than anyone had imagined even a decade ago. This this new study, “Mountain Pine Beetle Develops an Unprecedented Summer Generation in Response to Climate Warming,” spells out the grim facts:
The current MPB epidemic is the largest in history, extending from the Yukon Territory, Canada, to southern California and New Mexico…. To date, more than 13 million ha [hectares] of trees have been killed in British Columbia. The MPB-killed trees in British Columbia alone will release 990 million tons of CO2 into the atmosphere, an amount equal to five times the annual emissions from all forms of transportation in the country. Forests affected by bark beetles also have altered hydrology and biogeochemical cycles. Thus, extensive beetle kill is altering forest ecology and tipping conifer forests from regional carbon sinks to carbon sources, thereby creating positive feedback for climate-change factors.
For more on the amplifying feedback, see “Nature: Beetle tree kill releases more carbon than fires.”
It turns out that there has been an “exponential increase in the beetle population.” Why has infestation been nonlinear? The study’s abstract explains:
The mountain pine beetle (MPB; Dendroctonus ponderosae) is native to western North America, attacks most trees of the genus Pinus, and periodically erupts in epidemics. The current epidemic of the MPB is an order of magnitude larger than any previously recorded, reaching trees at higher elevation and latitude than ever before. Here we show that after 2 decades of air-temperature increases in the Colorado Front Range, the MPB flight season begins more than 1 month earlier than and is approximately twice as long as the historically reported season. We also report, for the first time, that the life cycle in some broods has increased from one to two generations per year. Because MPBs do not diapause and their development is controlled by temperature, they are responding to climate change through faster development. The expansion of the MPB into previously inhospitable environments, combined with the measured ability to increase reproductive output in such locations, indicates that the MPB is tracking climate change, exacerbating the current epidemic.
[Read about diapause here. I welcome a simpler explanation from any biologist reading this.]
For more background on the study, here is an extended excerpt from the news release:
This exponential increase in the beetle population might help to explain the scope of the current beetle epidemic, which is the largest in history and extends from the Sangre de Cristo Mountains in New Mexico to the Yukon Territory near Alaska.
“This thing is immense,” Mitton said. The duo’s research, conducted in 2009 and 2010 at CU’s Mountain Research Station, located about 25 miles west of Boulder, helps explain why.
“We followed them through the summer, and we saw something that had never been seen before,” Mitton said. “Adults that were newly laid eggs two months before were going out and attacking trees” — in the same year. Normally, mountain pine beetles spend a winter as larvae in trees before emerging as adults the following summer.
These effects may be particularly pronounced at higher elevations, where warmer temperatures have facilitated beetle attacks. In the last two decades at the Mountain Research Station, mean annual temperatures were 2.7 degrees Fahrenheit warmer than they were in the previous two decades.
Warmer temperatures gave the beetle larvae more spring days to grow to adulthood. The number of spring days above freezing temperatures increased by 15.1 in the last two decades, Mitton and Ferrenberg report. Also, the number of days that were warm enough for the beetles to grow increased by 44 percent since 1970.
The Mountain Research Station site is about 10,000 feet in elevation, 1,000 feet higher than the beetles have historically thrived. In their study, Mitton and Ferrenberg emphasize this anomaly.
“While our study is limited in area, it was completed in a site that was characterized as climatically unsuitable for (mountain pine beetle) development by the U.S. Forest Service only three decades ago,” they write.
But in 25 years, the beetles have expanded their range 2,000 feet higher in elevation and 240 miles north in latitude in Canada, Mitton said.
Ferrenberg had the idea to monitor the beetles at higher elevations partly because trees at lower elevations have been attacked by beetles for centuries and have developed some defenses.
Lodgepole pines at higher elevations tended to have a lower density of resin ducts, which transport resin, the sole defense against beetles. The number of resin ducts in a tree can be a “marker” for whether a tree has a higher or lower resistance to a beetle attack, Ferrenberg said.
The trees at higher elevations had not faced the same intensity of beetle attacks as those at lower elevations until temperatures warmed, and they have not faced pressures of natural selection exerted by attacking beetles. “The trees in that area are somewhat naïve in their response,” Ferrenberg said.
These data help explain why westbound motorists emerging from the Eisenhower Tunnel on I-70 can look up, from 11,000 feet in elevation, and see beetle-killed trees. “We think we see some of the reason for the fact that this epidemic is so widespread,” Mitton said.
And so the bark beetle is a major unexpected, nonlinear impact of manmade global warming with devastating economic and environmental consequences — that is also an unexpected amplifying feedback of a warming.
And we’ve only warmed about a degree Fahrenheit in the last few decades. Imagine the unexpected nonlinear impacts and feedbacks we face when we warm 10 times that this century, as we are likely to do if we keep listening to the do nothing or do little crowd.
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