Tree voles are endemic (native) to the humid coniferous forests west of the crest of the Cascade Mountains in Oregon and northwestern California. The red tree vole occurs in western Oregon from the Cascade crest to the Pacific coast, with a geographic range covering approximately 16.3 million acres across multiple land ownerships. Voles, however, were found to be uncommon or absent in much of the North Coast Range and North Cascades of Oregon. The range of the dusky tree vole is not clearly delimited within this area, but it is generally attributed to the northern Oregon Coast Range. Historically, tree voles collected north of Eugene and west of the Willamette Valley were typically classified as the dusky tree vole, while those collected north of Eugene and east of the Willamette Valley were almost all identified as red tree voles.
There is not yet complete resolution in the scientific community as to whether or not the dusky tree vole is a valid subspecies of the red tree vole (Arborimus longicaudus). Information provided below specific to the red tree vole also applies to the dusky tree vole unless otherwise noted. Tree voles are small rodents less than 8 inches long, and weighing up to 2 ounces. A long, fur-covered tail accounts for about 50 percent of the vole's total length. Their thick coats range in color from reddish-brown to orange-red. The darker coat color is characteristic of the dusky tree vole.