Flora of North America species comparison

Species 25–30 (15 in the flora).

The convergence of morphologic (J. Soják 1985[1989]) and molecular (T. Eriksson et al. 1998; M. Lundberg et al. 2009; C. Dobeš and J. Paule 2010) evidence shows that the species formerly placed in Potentilla sect. Rupestres or subg. Closterostyles belong in Fragariinae, separate from Potentilla in the strict sense. Drymocallis has accordingly been resurrected for this complex of species, which has centers of radiation in western North America, southeastern Europe, and central Asia (A. Kurtto and Eriksson 2003; B. Ertter 2007; Soják 2006, 2011). Molecular studies by Lundberg et al. suggest that North American and Eurasian species form separate clades and hint at a possible hybrid origin of the genus.

In addition to anthers with a broad connective ringed by a single horseshoe shaped theca (shared with other Fragariinae), Drymocallis is characterized by fusiform styles attached near the bases of the achenes. Leaves are odd-pinnate with a distinct terminal leaflet, in contrast to some superficially similar species of Horkelia. Vestiture consists of various proportions of septate glands to 2 mm, subsessile peglike glands, short spreading eglandular hairs, and, less frequently, rigid bristles to 1.5 mm.

The North American members of the genus, as Potentilla glandulosa and allies, were studied by J. Clausen et al. (1940) in their seminal biosystematic experiments. They determined that the complex consists of a wide diversity of often highly localized ecotypes differing from one another ecologically, physiologically, and morphologically. They also demonstrated that hybrids between those ecotypes are readily generated, aided by a uniform diploid chromosome number of 2n = 14, and that intergradation zones where ecotypes intersect are the norm. Faced with this biosystematic complexity, D. D. Keck (in Clausen et al.) recognized P. arguta, P. fissa, and P. glandulosa, with additional ecotypic variation treated as subspecies of P. arguta and P. glandulosa.

During preparation of the present treatment, it became apparent that the outline by D. D. Keck did not adequately accommodate current evidence of variation in North American Drymocallis, especially beyond California. Subspecies accepted by Keck appeared no more closely related within a species than between species, and extremes of variation occurred beyond those addressed by Keck. The treatment here is an unabashedly provisional alternate framework, which may provide an improved foundation and incentive for much-needed further research on a relatively neglected genus.

The potentially most controversial aspect of this approach is the recognition of more of the variation within Drymocallis at the species level, with full acknowledgment that any attempt to recognize formal taxa in North American Drymocallis will be compromised by wide zones of intergradation and populations that defy placement. The alternative is to accept broadly defined taxa in which the extensive variation is disregarded, including variation of conservation significance and/or comparable to species currently recognized in Eurasia (A. Kurtto and T. Eriksson 2003; J. Soják 2011). Here, species are used for relatively cohesive core populations sharing multiple characteristics in a definable ecogeographic setting, even in the absence of sharp morphologic boundaries. Varieties are used where differences are less distinct and/or the intergradations more complex.

Among the more outstanding unresolved questions are plants from the Pacific Northwest that have been called Drymocallis valida (Greene) Piper [Potentilla valida Greene], which combine features of D. convallaria and D. glabrata. Variation in the Pacific Northwest is generally in critical need of careful analysis, as is that in other major zones of intergradation (for example, northern Great Basin, New Mexico, southern Utah). More collections with basal leaves, fully developed inflorescences, and careful note of petal orientation and color (which becomes unreliable upon drying) will help significantly in identification and future analyses.

In the descriptions, stems refer to flowering stems, with stem length including the inflorescence. Cauline leaves are on the unbranched portion of the stem. Counts of leaflet teeth include all primary teeth and, when double, significant secondary teeth. The inflorescence comprises the branched portion of the stem, including the proximalmost ramification; the fraction of total stem comprised of inflorescence is indicated in each species description. Leafy inflorescences are those in which bracts at the proximal nodes are well developed, that is, generally more than half as long as the subtended internode. Shorthand terms describing inflorescence architecture (for example, congested, narrow) are followed by more precise numeric counterparts. The diagnostic value of pedicel vestiture derives from the relative proportion of short (0.2 mm) simple eglandular hairs and longer septate glands. Epicalyx bractlets are usually simple but sometimes doubled, bilobed, or otherwise toothed; shape and measurements are based on the simple form. Sepal and petal orientation (erect, spreading-reflexed) are at flowering; all tend to be erect in fruit. Numeric ranges given here (for example, stem and flower number), derived from wild-collected herbarium specimens, are sometimes greatly exceeded by the ranges reported by J. Clausen et al. (1940) for cultivated plants.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Drymocallis rhomboidea is centered in the Siskiyou Mountains. It differs from D. campanulata in its smaller, cream-white petals and globular flowers; in addition, the vestiture is usually predominantly short-hairy, except in scattered California populations. Plants from Mount Ashland, the type locality, tend to have more congested, leafier inflorescences than plants found elsewhere in the range.

(Discussion copyrighted by Flora of North America; reprinted with permission.)