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SALT LAKE CITY — A small, invasive insect is setting up shop within Utah's long-stressed conifer forests and wreaking havoc on some portions of the state's subalpine fir population.
First introduced in the Pacific Northwest around a century ago, the balsam woolly adelgid — commonly referred to as BWA — comes from central Europe and can only travel via wind or by latching onto birds and other animals.
It was first detected in the Beehive State in 2017 and has been spreading around the Wasatch Mountains, visibly affecting many of the popular recreation canyons outside Salt Lake City.
New research from the University of Utah and the U.S. Forest Service has documented the current extent of the BWA infestation and created a model for predicting its severity around the Uinta-Wasatch-Cache National Forest.
"We spend so much time in the Wasatch. Almost everybody I know who lives in Salt Lake does some kind of outdoor recreation, and the thought of being in the Cottonwood canyons ... and looking to your right and looking to your left and seeing vast swathes of dead, subalpine fir trees — it's a pretty depressing notion," said Mickey Campbell, lead author of the study and research assistant professor in the Department of Geography at the University of Utah.
Now that these invasive pests have found their way to Utah, what does it mean for the state's forests?
Climate change and infestation
William Anderegg, director of the Wilkes Center for Climate Science and Policy at the U. and co-author of the study, said that temperature is strongly related to the spread and severity of the insect.
"That tells us, at the very least, as temperatures go up, we should be concerned about more spread and higher severity infestation," he said.
As far as where these insects are located, Campbell said, at this point, they're mainly in the Wasatch Mountains. "I would say central to northern Wasatch, pretty much everywhere north of Provo in the Wasatch has seen some level of infestation," he said.
He added that the Uinta Mountains, by and large, are free of balsam woolly adelgids, except for the Uinta's western flanks. Hot spots for infestation include Farmington Canyon and the mountains outside of Ogden.
"The prevailing trend is that warmer areas are more susceptible to BWA damage, and that's really what's driving the difference between infestation in the Wasatch and infestation in the Uintas," Campbell said. "Subalpine fir trees in the Wasatch are ... at the lower elevation, warmer end of the spectrum, whereas much of the Unitas is higher elevation (and) cooler. So BWA's effects are able to be much stronger, much more severe in the Wasatch than in the Uintas."
With this being said, the exact correlation between temperature and BWA infestation is still unknown. Though the scientific consensus is that the warming climate plays a big role in insect infestations in Western forests, it's not clear how.
"Maybe the warm temperatures are actually making the tree more susceptible; so, maybe it's not necessarily that the BWA prefers these climates," Campbell said. "Maybe it's just that the BWA does more damage in these climates because the trees are already stressed from warmer temperatures."
"We don't know exactly what's driving the degradation that occurs in the warmer areas," he continued, "but we do know that warmer areas are those that are seeing the greatest damage."
'Different than bark beetles'
Campbell said that aside from the way BWA move from forest to forest, there are other unique aspects of the insects that make them "different than bark beetles, for sure."
Bark beetles, which can fly and disperse through a forest stand over several weeks, attack a tree by gnawing through its inner bark; the much smaller and flightless BWA quite literally suck the life out of trees, leaving behind toxic saliva in the process.
"It inserts its stylet, which is like a feeding tube, between the cells of the bark and eventually finds parenchyma cells, and it feeds on the fluids and starches that are contained within those parenchyma cells," said study co-author Justin Williams, an entomologist with the Forest Service's Forest Health Protection program.
While feeding, the insect excretes a substance that weakens the tree's natural defenses and causes damage that restricts the flow of nutrients between the root and crown of the tree.
This saliva promotes abnormal cell growth that results in the tell-tale signs of "gouting," or the appearance of swollen branch nodes, Williams added.
"As the infestation grows, and the population of BWA grows, the damage becomes more severe, and those translocation effects become more severe, which eventually causes the tree to die," Williams said.
The process of the tree dying can take as little as three to five years, though some trees could survive for much longer.
Managing forests going forward
The research concluded that 41% of the study area's subalpine fir biomass is climatically exposed to some level of damage. And by 2100, under even moderate climate projections, 79% will be exposed, with 37% predicted to feature relatively high severity.
These statistics — combined with the fact that there isn't a concrete, direct method for combatting BWA infestations and other climate-driven factors — means that Utah's subalpine fir are more stressed than they have been "for quite some time," Campbell said.
"The solution is really just, on one hand, understanding that we may lose a lot of the subalpine fir. It's almost like an accepting inevitability, to a degree," he said.
Having this information now ... arms us with the best available information to be able to make whatever management decisions are necessary to avoid a situation in which the Wasatch becomes a forest of dead trees.
– Mickey Campbell, lead study author
On the other hand, there are proactive measures that can be taken to offset the negative effects of losing subalpine fir, like planting and promoting species that coexist with subalpine fir.
"Our goal is, in part, to pass this information off to the forest managers so that they can be armed with this information. This, kind of, robust, quantitative information that can help them target whatever management strategies they deem to be most suitable," Campbell said. "Having this information now and being able to, sort of, proactively understand where it's likely to get worse, I think, arms us with the best available information to be able to make whatever management decisions are necessary to avoid a situation in which the Wasatch becomes a forest of dead trees."
