Arborimus longicaudus

Included in the genus Phenacomys by some authors (e.g. Carleton and Musser 1984, Repenning and Grady 1988, and Verts and Carraway 1998). Bellinger et al. (2005) noted that recognition of Arborimus as a distinct genus is subject to interpretation of data.

MtDNA data (Bellinger et al. 2005) indicate species-level differences among red tree vole (Arborimus longicaudus or Phenacomys longicaudus), Sonoma tree vole (A. pomo or P. pomo), white-footed vole (A. albipes or P. albipes), and western heather vole (P. intermedius) but no clear difference between the two Oregon subspecies of red tree voles (longicaudus and silvicola). These data further indicate a close relationship between tree voles and A. albipes or P. albipes, validating inclusion of albipes in Arborimus. Bellinger et al. (2005) did not find that P. intermedius clustered with Microtus.

Restricted distribution in western Oregon and extreme northwestern California; prefers old-growth forest habitats that are being eliminated and fragmented by large-scale timber harvesting. Significant and ongoing threats from habitat loss due to forest management practices and increasing frequency and intensity of wildfires.

The red tree vole occurs in coniferous forests in western Oregon and northwest California, USA, from sea level to 1400m (Maser 1966, Manning and Maguire 1999, Huff et al. 2012, Forsman et al. 2016, Durham 2019). In stands containing Douglas-fir, they may occur as high as 1585m (Forsman et al. 2016). In Oregon, the species is known from the Coast Range, Klamath Mountains, and West Cascades ecoregions, but it is most common in the central Cascades and central and southern Coast Range (Forsman et al. 2004, ORBIC 2022). It is absent from the Willamette Valley (Maser 1966). The northeastern-most known populations occur just west of Hood River, in the Columbia River Gorge and the Hood River Basin (Forsman et al. 2009). In California, the species is known from Del Norte County (Murray 1995, Blois and Arbogast 2006), and populations north of the Klamath River in Humboldt County are assumed to be Arborimus longicaudus (Forsman et al. 2016). Populations south of the Klamath River are A. pomo (Blois and Arbogast 2006, Forsman et al. 2016).

There are over 10,000 observation records of red tree voles since 2000 from Oregon alone, but many of these are repeat observations at the same locations and the true number of element occurrences is unknown (ORBIC 2022). USFWS (2019) identified 11 habitat clusters in the range of the North Oregon Coast distinct population segment (DPS) based on connected habitat patches large enough to accommodate >100 individuals.

The primary threats to red tree vole populations include habitat loss and fragmentation due to forest management activities that reduce stand age, canopy interconnectedness, availability of nest structure, and increase stand fragmentation; wildfires; and climate change (Forsman et al. 2016, Huff 2016, Durham 2019, USFWS 2019). These widespread threats are ongoing. Red tree voles are old forest obligates, and younger forest habitats are considered suboptimal though they may provide connectivity between populations (Swingle 2005, Carroll et al. 2010, Price et al. 2015, Huff 2016). Timber harvest removes large trees that support red tree voles. Because they have small home ranges and low dispersal abilities (Forsman et al. 2016, Linnell et al. 2018), timber harvest can destroy habitat for this species and result in fragmented populations (Corn et al. 1988, Forsman et al. 2016, USFWS 2019). Maintenance of forest cover within 200m of red tree vole nests is important for population persistence (Johnston and Moskal 2016). Forest thinning may increase availability of nesting structures over time, but but over the short term reduces inter-tree connectivity and nest structure colonization (Durham 2019). Vegetation management to reduce fuel loads can degrade habitat by reducing canopy interconnectedness (Huff 2016). Historically, wildfire has been the primary cause of habitat loss (Forsman et al. 2016, USFWS 2019). Wildfire is expected to increase under climate change and can further isolate already small populations (USFWS 2019). Catastrophic stand-replacing wildfires are expected to increase throughout the range of this species under climate change, degrading or further fragmenting habitat (Huff 2016, USFWS 2019).

Other threats include disease, predation, and threats associated with small isolated populations (Hanselman 2016, Linnell and Lesmeister 2020). Immune response is affected by age and body condition in this species (Hanselman 2016). Habitat alteration decreases body condition, leaving populations less able to ward off disease (Hanselman 2016). Swiss needle cast affects Douglas-fir; an important habitat component for red tree voles. Although it primarily affects younger trees, it reduces tree growth and prevents red tree vole habitat ingrowth (USFWS 2019). In isolated populations found in younger forests, predation pressure may contribute to long term local population instability (Linnell and Lesmeister 2020). Predation pressure may also be higher in younger forests (Durham 2019).