News and Opinion from Sisters, Oregon

Dwarf mistletoe a problem in forests

Dwarf mistletoe continues to pose a threat to Sisters Country forests.

Dwarf mistletoe falls into the broad definition of a pathogen and is a parasite that infects coniferous trees such as the firs and pines in our local forests. Although mistletoe does have some chlorophyll capable of producing nutrients, that capability is a mere fraction of what typical plants produce. As a result, mistletoe gets the vast majority of its water and sustenance from a host tree; and it’s voraciously parasitic in doing so — literally sucking the life from its host by sending root-like tendrils into the tree itself.

While a host tree can tolerate some mistletoe, the drain of nutrients and water from a significant infestation takes a severe toll. Over time, the tree’s growth will be stunted, perhaps deformed, and the tree weakened. It make take several years for the tree to actually die from an infestation; but, in the meantime, the weakened tree also becomes more susceptible to other diseases and attacks from insects such as pine beetles. Infected trees have a far higher mortality rate than healthy trees.

Local interest in the disease recently emerged for two reasons: First, a significant infestation west of Sisters, in and around Suttle Lake, has sparked a major logging operation to remove thousands of infected trees. Second, the disease has become noticeable at the Whychus Creek Scenic Overlook Trail.

Infestations have been seen on young ponderosa pines right at eye level along the Overlook Trail — which makes the Overlook an excellent place to view the parasite up close.

The Suttle Lake project is ongoing, and logging operations have caused some temporary trail and road closures in recent weeks. Jean Nelson-Dean, Forest Service Public Affairs Officer, said that 249 acres are being treated, with 2,500 trees being felled or pruned. It is estimated that 900,000 board feet of timber will be sold. Trees in campground areas will not be worked on until fall.

The project is expected to continue into early next year.

“They have removed approximately 250,000 board feet to date,” Nelson-Dean said.

She said that translates to about 60 log truck loads from the first 50 acres of the project area.

The temporary trail closures have mostly ended. The Blue Bay Trail was reopened last month, and the Dark Lake closure is expected to end Wednesday. However, the 600 Road east of Dark Lake will be closed for the next two weeks.

The Suttle Lake area has been the subject of mistletoe control efforts for many years.

Brent Oblinger, Forest Pathologist for the Forest Service, said, “There was some Douglas fir pruning in the Suttle Lake area in the fall of 1999. Since then, the trees started putting on healthy tops.”

Oblinger added, “There are at least seven different species of dwarf mistletoe in the Sisters Ranger District that are species-specific to host trees.”

He explained that where infected trees are removed, replanting will be undertaken with tree species that are not susceptible to attack from the local mistletoe infestation.

In a prepared statement, he said, “No larch dwarf mistletoe was observed on any larch in the project area. Ponderosa pine and western larch are planned to be planted because they are tolerant or relatively resistant to Armillaria root disease and other root and stem decays present in the project area as well. No other dwarf mistletoes are present in the project area that can infect western white pine, ponderosa pine and western larch….”

As noted along the Three Creek Lake Road, however, ponderosa pine has its own mistletoe nemesis.

“Western dwarf mistletoe is very common on ponderosa pine in the Sisters Ranger District,” Oblinger said. “The Melvin Butte Project involved pruning and thinning from 2016 to 2019.”

That project was farther south on the Three Creek Lake Road, and much of the pruning was accomplished by the Youth Conservation Corps.

Under certain conditions, dwarf mistletoe can also affect forest fire behavior. Heavy concentrations of the parasite often form on the low hanging branches of trees, with the heavy clusters forming growths called “witches brooms.” These clusters, with their dense structure also tend to snag dead, falling pine needles and other forest debris. This collection of combustible material exacerbates the already-flammable nature of these clusters and creates ladder fuels that can cause a fire to spread upward into the tree crown.

Studies have shown that trees heavily infested with mistletoe are more than twice as likely as healthy trees to die in a fire. Further, burning witches brooms can act like fire bombs when they break free and fall. On steep slopes, they have even been known to roll downhill like flaming tumbleweeds, further spreading a fire.

“Dwarf mistletoe has been around for 35 million years, evolving along with its conifer hosts,” Oblinger noted.

As a result, control of the parasite will undoubtedly continue to be a focus of forest management and planning in the Sisters area for many more years to come.


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