Late last fall, about 20 federal scientists toured southeastern Utah, prodding sickly and dead juniper trees, peeling back bark, snapping off branches and digging the dirt around root collars in search of clues to what could be killing the West’s most hardy tree species.

Trip leader Liz Hebertson, a plant pathologist with the U.S. Forest Service’s Forest Health Protection program, buried her face in a dying juniper’s foliage, which had turned a telltale shade of deep yellow, dabbing at the trunk with a small hatchet to get a look at the nutrient-moving phloem beneath the bark.

“Look very carefully and sometimes you’ll see fine little threads," said Hebertson, who describes her work as “CSI: Nature."

"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,” said Hebertson, her hair dotted with the yellow juniper needles falling from the branches. “Frass is just fundamentally a mixture of insects’ poop and boring dust.”

Hebertson and her colleagues could see the galleries and dust trails left by beetles, but was the damage enough to kill these trees on Alkali Ridge?

Most likely not, according to a preliminary report. Several months after the scientists’ two-day field trip, the mystery persists although most signs indicate last year’s severe drought, the worst on record for the Four Corners region, may be pushing many junipers over the edge.

However, the report continued, "pinyon pine, a species less tolerant to drought, had not exhibited symptoms of drought-induced stress last spring. This observation suggested that perhaps other abiotic factors, damaging insects, or diseases might be contributing to, or were primarily responsible for, the juniper decline."

Trees under attack

The die-off was documented last year by Kay Shumway, a retired science educator and botanist from Blanding who first noticed the junipers turning yellow on the southern end of Cedar Mesa. Thanks to his tireless efforts to document the deaths of the region’s signature tree, the Forest Service and other federal agencies began investigating last fall and academic scientists are setting up studies to figure out why an organism so well equipped for survival is now dying in droves in Utah’s San Juan County.

Although juniper is sometimes treated as a trash tree to be ripped out of the ground in the name of habitat restoration, it is a vital part of southern Utah’s ecosystem, stitching together fragile desert landscapes. Widespread juniper mortality would deliver an ecological blow similar to what Utah has experienced where bark beetles have run amok in national forests.

But explanations for the juniper deaths are not nearly as clear cut as they are for the Uinta Mountains’ lodgepole pines and Wasatch Plateau’s Engelmann spruce.

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. The afflicted junipers, by contrast, show only modest levels of infestation.

“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,” the report said.

The scientists searched for signs of fungal infections but found little.

"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 recommends continued monitoring and asked the Forest Service to complete an aerial survey this summer to "assess the extent and severity of the juniper decline and crown dieback" across the Four Corners region.

Twice the Forest Service scheduled such surveys, and both times they were canceled due to inclement weather, according to John Guyon of the Forest Health Protection program based in Ogden.

Mapping the juniper mortality is crucial for understanding the extent of the problem and detecting patterns that could bring the causes into sharper focus. It would also provide a baseline against which to measure the spread of mortality.

Rains returned

The region’s drought reversed shortly after the scientists’ visit when precipitation returned to San Juan County in record amounts. Southeastern Utah enjoyed a snowpack containing more than double the amount of moisture it receives in a typical winter.

Will that put the brakes on the juniper die-off? It’s hard to say without the baseline data that aerial surveys could provide, 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 said. “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 said. “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.”

His research will couple these measurements with 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.”

Currently, the juniper mortality is far from uniform. Some parts of San Juan County appear unaffected, such as the middle of Cedar Mesa, while junipers are dead and dying on the mesa’s southern and eastern margins, said Shumway, who acted as a guide on the scientists’ field trip.

"The concern is what is going to happen next year if the beetle flies off and lays eggs in some more trees," said Shumway, while surveying the dying trees around Alkali Ridge.

This area east of Blanding appears to be a hot spot where about half the junipers are afflicted, with the smaller trees showing the greatest severity.

In recent dry years, junipers across the border in Colorado turned bronze but then recovered when rains returned. Utah’s yellowed junipers, on the other hand, are goners.

Forest Service scientists gathered beetles from trees they inspected last fall and cut down a few dead junipers to remove cross sections of the trunk for further study in a lab, where they coaxed out more clues.

“We’ll seal off the ends with wax. We’ll put it in an enclosed box that’s totally black on the inside, and we seal off all seams in the box,” Hebertson said. “There’s one little window of light that attracts the insects when they emerge. They head toward the light. They get into a trap and they fall down into a cup.”

The goal was to identify the beetles residing in the tree, although Hebertson said she was not aware of any wood-boring species that would be considered a primary killer of juniper.

The types of insects later identified were those 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.”

Whatever the cause, the juniper die-off adds to a litany of woes facing Western forests that will likely complicate land management for years and keep the scientific community busy looking for answers.