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Maybe We Should Call Them Oak Voles ...

Oak trees are disappearing from North American forests, and the pesky pine vole might be to blame.

The pine vole, a small rodent that lives underground, prefers oak roots to any other commonly grown seedling. And now a study from Purdue University researcher Robert Swihart in the Canadian Journal of Zoology has identified the rodent as a possible factor in high oak mortality rates that are threatening ecosystems and the hardwood industry.

Oaks already face a gauntlet of enemies, including the rapidly evolving sudden-oak-death mold and resurgent oak borers, but the voles present a puzzling example of death from below.

"You see a lot of mature oaks, but you don't see a lot of oaks in the understory beneath the canopy. If you don't see them there, you won't see mature oaks in 20 to 30 years," Swihart was quoted in a release announcing his study. "We are facing a period in our history that could lead to a great crash in oak availability."

In the laboratory, when offered a variety of tree roots to eat, the vast majority of pine voles chose oak. "Either the oak roots were much more nutritious and had higher energy content, or they contained fewer toxins, or some combination of those factors. Those are the main reasons an animal will choose one food item over another," Swihart said.

Most of the previous studies on why oaks have a hard time regenerating have concerned competition from other seedlings and above-ground animals: Acorns serve as food for small animals and oak seedlings are eaten by deer. But Swihart was inspired by a Purdue colleague, Ron Rathfon, who in 2006 reported that animals (pine voles, it was later realized) had been responsible for killing at least 19 percent of the dead oak seedlings observed in his study area.

While there is little overall loss of oak forest to date, the researchers write that depletion could soon begin in earnest unless foresters, the timber industry and landowners being better managing long-term oak growth and development. If not, Swihart warns, there could be devastating consequences.

"Oak mortality could reduce the capacity of hardwood forests to support wildlife populations that rely on oaks for food - everything from deer and turkey down to mice and songbirds," Swihart said.

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