Scientists study SE Utah juniper die-offs

Is drought-induced stress the primary reason why juniper trees are suffering across southeastern Utah? This tree in Professor Valley shows signs of stress and death. Photo by Sena Hauer

The Colorado Plateau’s juniper trees are some of the hardiest species in the desert’s plant world. They have been known to endure tough conditions such as drought, and proportion their sizes to their sustaining elements, especially water.

But many juniper forests appear to be sick or dead, puzzling scientists. Last fall, a number of officials took a research trip to southeastern Utah to look for reasons behind the die-offs, according to a story in the Salt Lake Tribune.

Liz Hebertson, a plant pathologist with the U.S. Forest Service’s Forest Health Protection program, led the tour. She told the researchers, “Look very carefully and sometimes you’ll see fine little threads. Those threads could be produced by defoliating insects. They could be produced by mites. We’re looking for webbing, fine threads. We’re looking in all of the crevices for frass that’s either been kicked out of the inner bark tissues or out of the bark … Frass is just fundamentally a mixture of insects’ poop and boring dust.”

Blanding botanist Kay Shumway shared his observations with the group. He recovered beetle larvae from dying junipers in San Juan County, and was perhaps the first to document impacts to junipers. The retired science educator is now helping federal scientists determine what is killing the junipers, according to the Tribune.

Shumway became alarmed at junipers turning yellow on the southern end of Cedar Mesa, and his observations prompted Forest Service and other federal officials to take notice. Academic scientists are now more furtively studying why the trees are dying at apparently high rates.

Other trees that are suffering across the West, such as lodgepole pine in the Uinta Mountains, and Engelman spruce on the Wasatch Plateau, have shown heavy beetle infestations. “Those trees look like they were eaten alive, their bark dripping with pitch produced by the trees in a failed effort to repel the attackers,” said the Tribune story. “The afflicted junipers, by contrast, show only modest levels of infestation.”

Officials say the oft-maligned and sometimes overlooked junipers are a vital part of the ecosystem in the Desert Southwest. “Widespread juniper mortality would deliver an ecological blow similar to what Utah has experienced where bark beetles have run amok in national forests,” the Tribune story proffered.

Researchers, having studied the junipers, observed in their report: “In all the large-diameter trees we examined, the total number of flat-headed wood-boring beetle galleries in the inner bark tissues of trunks and large branches was not sufficient to have completely interrupted vascular transport [girdle] within the tree…Declining and dead trees had evidence of secondary insect attack. Although some juniper had died, many symptomatic trees had healthy, green sprigs of foliage growing from their lowermost branches,” the report said. “We did not find evidence of insects or diseases in the root systems of trees we examined.”

The report recommended further monitoring and an aerial survey of the Four Corners region, to look for damage and to create a baseline for this date in time. According to John Guyon of the Forest Health Protection program based in Ogden, surveys have been planned but postponed by the Forest Service due to inclement weather, the Tribune noted.

The 2018-19 winter’s heavy precipitation appears to have halted a lengthy drought, but it’s too early to tell whether it will give some relief to suffering junipers. Baseline data is important to log, said William Anderegg, a University of Utah biology professor who studies the impact of climate change on forests. “It’s crucial to have that part,” Anderegg told the Tribune. “We would like to know regionally how many trees are dying, and you can only know from a plane or satellite.”

Anderegg’s lab has been approved for a Forest Service grant to study the juniper mortality, and it has already set up a monitoring instrument known as an eddy covariance tower in a spot with dying junipers.

“It measures total carbon take-up and water lost in a patch of forest, a good metric of the overall health of the trees. A healthy forest will be taking up a lot of carbon,” Anderegg told the Tribune. “It puts a sensor above the trees sensing the eddies of air and recording the carbon dioxide concentrations going up and going down. By measuring wind and carbon levels, you can determine how much carbon is being taken up.”

Officials will observe his research in addition to data collected from the trees’ tissues. “We are trying to figure out if drought is killing these trees,” he said, “and what are the effects on an ecosystem scale.”

Shumway pointed out that some areas of the Four Corners are more heavily affected than others. The middle of Cedar Mesa, for example, appears rather unscathed. But junipers are dead and dying on the mesa’s southern and eastern margins. “The concern is what is going to happen next year if the beetle flies off and lays eggs in some more trees,” said Shumway.

An area east of Blanding appears to be heavily affected, and about half the junipers, especially smaller trees, are suffering. But another area that has shown impacts just across the border in western Colorado turned bronze but then recovered when rains returned. Utah’s yellowed junipers, on the other hand, are dead.

Officials have identified the types of beetles that typically infest trees weakened by harsh weather, poor site conditions and other stressors, according to the report. “Abiotic factors such as air pollution, smoke, or temperature extremes might explain the scale of symptoms we observed,” the report said, “but drought-induced stress remains the most plausible explanation,” the Tribune story noted.