With the increased recognition of moss as a secondary forest product has come an increased concern for the ecological ramifications of moss harvest. Three issues of primary concern are (1) characterizing the epiphyte communities impacted by harvest, (2) assessing the host preferences of these epiphytes, and (3) describing the availability of harvestable epiphytes. Harvestable epiphytes are generally large mats of clean, green plant material that can be easily accessed and removed from their substrate by a commercial moss harvester. Harvestable epiphytes on tree trunks and shrub stems were sampled at 10 sites in each of the Cascade and Coast Ranges in northwestern Oregon. Frequency of occurrence and biomass are reported for 50 species found in harvestable epiphyte mats, categorized as "target" (7 species), "nontarget" (37 species), and "incidental" (6 species), according to their commercial utility. Target species were more abundant in stands with relatively high hardwood basal area and densities of harvestable epiphyte hosts. Nontarget and incidental species were more abundant on hosts with relatively large surface areas and in stands with relatively high hardwood basal. All species were less abundant in stands with high conifer basal area. Eleven species of hosts, including seven shrubs and four trees, supported harvestable quantities of epiphytes, although most samples were collected from the clonal shrub vine maple (Acer circinatum). Epiphyte mats on tree trunks and shrub stems differed in epiphyte species composition. Relative frequency of occurence and abundance were used to determine host preference by several epiphytes, including Orthotricum lyellii for A. circinatum and Neckera douglasii for Alnus rubra. Harvestable epiphyte biomass ranged from 24 to 1469 kg/ha (dry weight). Biomass is a function of both site quality and availability of suitable hosts. Epiphyte mat accumulation on Acer circinatum was more rapid, and more variable, in the Coast Range sites than in the Cascade Range sites. A model describing the factors influencing epiphyte mat accumulation is proposed and management implications for the harvestable moss resource are discussed.