A morphological comparison of Arceuthobium abietinum and A. Campylopodum (Viscaceae) and nomenclatural changes for A. Abietinum

Fir dwarf mistletoe (Arceuthobium abietinum, Viscaceae) parasitizes true firs (Abies spp.) in the Pacific Northwest through California, the Southwest, and into northern Mexico. At present, fir dwarf mistletoe consists of two special forms (formae speciales, f. sp.) and one subspecies that are morphologically similar, but exhibit a high degree of host specificity. However, A. abietinum has been treated as a subspecies of A. campylopodum or conspecific with it in some taxonomic treatments. Therefore, we undertook this study to compare the morphologies of these dwarf mistletoes; collecting morphological data from nearly 100 A. abietinum and 60 A. campylopodum populations collected across most of their geographic ranges and analyzing these data using univariate and multivariate statistical tests. Our results demonstrated that the special forms and subspecies of A. abietinum are morphologically distinct from A. campylopodum, thereby, supporting the continued recognition of A. abietinum as a separate species. Furthermore, our analysis found significant differences for several of the characters we examined between the special forms of A. abietinum. Therefore, we recombined the special forms as subspecies: A. abietinum subsp. abietinum (formerly f. sp. concoloris) and A. abietinum subsp. magnificae (formerly f. sp. magnificae). The previously described A. abietinum subsp. wiensii was morphologically distinct from subsp. abietinum and subsp. magnificae. The distinctiveness of these taxa was supported by their host affinities as well.