Morphological evidence for continued species recognition among white pine dwarf mistletoes (viscaceae): Arceuthobium apachecum, A. blumeri, A. californicum, A. cyanocarpum, and A. monticola

Field identification of dwarf mistletoes (Arceuthobium; Viscaceae) often is complicated due to their reduced morphology and sexual dimorphism, requiring the integration of not only morphologic measurements, but also, geographic, host, and/or phenological data. Difficulties in species determination have plagued the taxonomic classification of closely-related Arceuthobium, and, hence, their recognition as distinct species has also been questioned. Troubles in the classification of Arceuthobium are typified by five taxas in section Campylopoda, ser. Campylopoda - A. apachecum, A. blumeri, A. californicum, A. cyanocarpum, and A. monticola - that infect white pines (Pinus; subg. Strobus) as principal hosts. Each of the aforementioned taxa recently were circumscribed in synonymy with or reduced to subspecies of A. campylopodum; we contend that they deserve separate species recognition. To support our position, we conducted morphological analysis for these six taxa using univariate and multivariate statistical approaches. Results demonstrated that Arceuthobium apachecum, A. blumeri, A. californicum, A. cynanocarpum, and A. monticola can be determined readily to species using morphological data without consideration of geographic location or host specificity. Moreover, all but two of 20 morphological characters across female and male plants of the white pine dwarf mistletoes were significantly different when compared to A. campylopodum. Female and male plants of the latter species were also easily segregated from each of the white pine dwarf mistletoes in the combined analysis of only three characters. Thus, the white pine dwarf mistletoes are well-differentiated morphologically, and therefore, should be considered distinct species.