Beetles Taking Massive Toll On Signature High-Elevation Trees
By Deborah Richie Oberbillig
Diana Six doesn’t mince words when asked whether whitebark pines have a fighting chance to fend off mountain pine beetles.
“I think the tree will be toast in most places,” pronounces the UM entomology professor. “The whitebark may well become functionally extinct.”
One of the premier experts in bark beetle ecology, Six fields interviews from around the world about an insect the size of a match head that is decimating Western forests. While most news focuses on lodgepole pine mortality, an even bigger story is unfolding.
Whitebark pines will become so few they likely can’t fulfill an ecological role, she says. Her dire prediction is based on the impacts from the beetle flying up mountainsides above mid-elevation lodgepole forests and into new terrain. Like an invading army sweeping through an unarmed populace, the beetle is plundering the already besieged whitebark, a victim of an exotic pathogen called blister rust.
Typically, mountain pine beetles take about seven to 10 years to move through a lodgepole pine forest. Six says a warming climate is allowing beetles to assault the high-elevation whitebarks and kill entire stands in less than half that time.
“The first season you may see one or two red-needled whitebarks, but by the second season a third of the stand is dead, and by the third season close to 100 percent of the mature trees are killed,” Six says. “It’s scary.”
Head to the high country around Yellowstone National Park, the Wind River Range and the Tetons, and swathes of red trees are clear indicators of the rapid demise of a keystone species. Found in scattered locations throughout the high northern mountains of the U.S. and southwest Canada, the five-needled pines once were able to live for 1,000 years or more, providing an annual cone-seed banquet to wildlife.
Grizzly bears in the Yellowstone area seek out the oily nutritious seeds of the whitebark before winter hibernation. The Clark’s nutcracker collects, caches and feeds on the seeds too. In the process, the bird plants seeds that will grow into a new forest. The tree itself serves as a nurse tree for other plants in the fragile alpine ecosystem and helps anchor snow in the high country to prevent early runoff.
What will happen without whitebark? Is there any hope for its survival? While the U.S. Fish and Wildlife Service is determining whether to list the species under the Endangered Species Act, researchers like Six are working as fast as possible to study and learn from a tree species on the brink.
To find out why whitebarks appear to be falling so quickly to beetles, Six and her graduate students focus on how lodgepole and whitebark differ in response to attacking beetles.
A tree in great shape produces plenty of resin for pitching out or drowning beetles, so it would take a horde to overcome this first line of defense. For those that do, they meet up with a deadly trap that’s induced by their presence.
“You know that Christmas tree smell, the one that we all like so much? That’s actually a natural pesticide,” Six says.
Called monoterpenes, these chemicals are noxious to most insects. The beetles are adapted to low levels, but high levels can kill them. That’s just what the tree tries to do next – form a lesion around a beetle that is made from resin chock full of concentrated monoterpenes.
But can whitebark pines employ this tactic? Up in the alpine country, Six and her students are tapping into whitebark and lodgepole pines where they overlap ranges to measure their resin flow and compare defenses.
The results so far seem unequivocal. When beetles have a choice between a lodgepole and a whitebark, they choose the latter, until they have to return to lodgepole for a tougher fight that requires higher numbers of beetles. Rarely do whitebarks put up that second line of defense.
Mountain pine beetles aren’t complete strangers to whitebark pines. In the past the beetles entered the forests in brief epidemics that corresponded to drought, particularly the Dust Bowl era of the 1930s and then another dry spell in the 1970s.
“Historically, the weather would change and effectively shove the beetles back down the mountain,” Six says. “Now, with chronic warming from climate change, the beetles aren’t being shoved off.
“Whitebark pines haven’t evolved the defenses to deal with the beetle,” Six says. “They don’t produce as much resin for starters, but it’s more complicated than that.”
She suspects that whitebarks are stressed from long-term drought, countering the myth that this tree deals well with dry conditions. While the twisted limbs bear the scars of life among blizzards and hurricane-force gales buffeting the high peaks, the hardy trees are surprisingly vulnerable.
A warmer climate is drastically affecting snowpack in the West. In Montana, snow melts out a full three weeks earlier than it has in the past. Meanwhile, warmer temperatures lengthen the growing season. If trees are growing for a longer period of time, they need even more water – a problem for both lodgepoles and whitebarks.
A drought-stricken tree is a stressed tree, Six emphasizes. A stressed tree, in turn, has fewer defenses to resist invading beetles.
Not only does it take fewer beetles to kill a whitebark, Six suspects the tree also may be a better food source. A graduate student in her lab is investigating if more larvae are produced from whitebark than from lodgepole pine.
“Blister rust enters the picture, too,” Six says. “Trees with the greatest severity of blister rust are the most likely to be attacked by beetles. They build up in those trees, and then spread out to the healthier whitebarks around them.”
While whitebark pines resistant to blister rust are important in the restoration efforts for this species, Six points out that these trees have no better defenses against the pine beetle.
“We’re trying to put all the pieces of the story together to find out why the beetle is so much worse for whitebark pines,” Six says.
The broader worldwide story of climate change has taken Six to South Africa, where another beetle is decimating an equally fragile ecosystem, this one dominated by a cactus-like tree called a Euphorbia.
“The systems that are super delicate are the first to go as climate change worsens,” Six says. She believes it’s critical to learn from these ecosystems on the edge to prepare for what may happen in more resilient ecosystems in the future.
“We are going to have to face the reality that as humans we can’t fix everything,” she says. “With climate change we are now facing some really difficult situations.”
With a new National Science Foundation grant, she’s studying how climate change may affect the relationship between mountain pine beetles and the fungi they carry into the trees with them. Without the fungi as a supplemental food source, the beetles are puny and unable to produce offspring. The project focuses on the ability of the fungi to adapt to higher temperatures – ratcheting up the heat in controlled laboratory experiments.
If warming temperatures eventually kill off the fungi, the beetle would succumb, too. That sounds like good news, but Six takes the ecological view. The Western forests evolved with mountain pine beetles. As is the case with fire, they’ve played a pivotal role in maintaining the health of lodgepole forests. The loss of beetles entirely would be as damaging as suppressing all fires.
Then there’s the complex wonder of the pine beetles that Six has come to value. For example, they carefully select only two kinds of life-giving fungi to carry in their mouths to a new tree. They also release pheromones (secreted chemicals) to invite other beetles to attack a tree, and still other pheromones to send a message that the tree is full. They make the pheromones from the tree’s monoterpenes – yes, those same toxic balsam-smelling chemicals that can kill the beetle in high levels.
“They do such amazing stuff,” Six says. “You never get bored working on these beetles. I need about 30 more lifetimes to study them.”
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