Creature Feature - Sonoma Tree Vole

February, 2018


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A red tree vole (Arborimus longicaudus), cousin of the Sonoma tree vole, gathering needles. Photo: Stephen DeStefano, USGS.

The Sonoma tree vole (Arborimus pomo), also known as the California red tree mouse, is a small rodent that spends nearly its entire life high in the canopy of old-growth forests. is vole inhabits northwestern California—from Sonoma County north to the South Fork of the Smith River in Del Norte County—and occupies the wetter forests between the coast and the western slope of the Coast Range. Near the northern end of its range, it can overlap with its cousin, the red tree vole. e density of voles is highest near the coast. As you leave the fog belt, the density of voles decreases with the loss of moisture.

The Sonoma tree vole exclusively eats conifer needles, and of these, primarily eats those of the Douglas-fir and grand fir. e vole eats a LOT of needles—on average, about 2,400 per day—and only eats a portion of each needle, carefully chewing around the thin resin ducts. e resin ducts, which resemble coarse, straight hairs, are then discarded or used as linings for their nest. Because the vole is difficult to see, researchers often determine their presence through finding wads of discarded resin ducts on the ground beneath an occupied tree.

The average tree vole never moves far from the place it was born. Females will typically stay put on one tree, while male voles will visit several trees. One tree is often home to multiple generations of the same family, with different generations occupying different branches in the tree.

The characteristic pile of discarded resin ducts (which run along the outside edges of Douglas-fir needles) produced by a red tree vole when eating. Photo: Petrelharp, Wikimedia CC.

Sonoma tree voles are presumed to be declining in number and the species is regarded as rare or uncommon. is is largely owing tothevole’suniquelifehistoryand the specialized habitat it requires. e vole has a lengthy gestation and adolescent period, probably because of the low nutritional value of the conifer needles they consume. Litter size is also small, particularly for a rodent, often consisting of only one or two young. e small litter size and slow development period means that the vole has little resiliency to disturbances—like logging—that affect its abundance.

Habitat loss has also taken its toll. The vole is associated with wet, old-growth forests—the large, heavy branches providing a good structure for the vole’s nest—but it can survive in younger, second-growth forests. Because most of California’s old-growth forests have been logged, the vole has lost most of its historic habitat. The remaining habitat is often fragmented, disconnecting populations and reducing the species’ resiliency.

The decline of the vole also could affect other wildlife. e vole’s number one predator is the iconic northern spotted owl. Other predators include other owls (like the barred owl and the saw-whet owl), raccoons, and fishers. Because the vole is an important prey species for spotted owls, and because the vole is associated with old-growth forests, the vole is often used as an “indicator species” of overall ecosystem health.

The vole doesn’t enjoy much protection. Only five percent of known nest sites are located within protected areas (i.e., parks and wilderness areas). Because the vole often coexists with the northern spotted owl, habitat protections for the owl often can benefit the vole. On private land, conservation measures are largely voluntary. Humboldt Redwood Company’s Habitat Conservation Plan requires the company to monitor for the vole and emphasizes the importance of protecting nest trees.

The Sonoma tree vole is identified by the state as a “species of special concern” that the state is monitoring because of population declines or loss of habitat that could result in its listing under the state Endangered Species Act—although this special designation does not significantly affect land management practices.



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