Flora of North America species comparison

Species 18–21 (4 in the flora).

The North American representatives of sect. Rivales are morphologically distinctive in being annuals, biennials, or short-lived perennials (especially in cold regions) with very short, conic-tapered styles. Plants grow primarily in wet habitats, especially where periodically inundated. These distinctions led E. L. Greene (1906c) to adopt Tridophyllum. However, the gross morphological cohesiveness is not matched by molecular monophyly. Instead, chloroplast data scatter Potentilla norvegica, P. rivalis, and P. supina among the core Potentilla, while P. biennis is sister species to the monophyletic Ivesia-Horkelia-Horkeliella clade (C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011). Nuclear markers, on the other hand, place P. norvegica as sister to the Ivesia-Horkelia-Horkeliella clade (Töpel et al.), suggesting a possible hybrid origin and/or multiple origins of the biennial habit.

Existing herbarium annotations of Potentilla biennis, P. norvegica, and P. rivalis are not reliable, but the three species can be readily distinguished by vestiture of proximal petioles and stems. Only P. biennis has prominently septate gland-tipped trichomes; P. norvegica is characterized by relatively stiff, spreading, tubercle-based hairs to 3 mm, resembling those of P. recta. In addition, the tiny, smooth, pale achenes of P. biennis and P. rivalis contrast sharply with the larger, darker, strongly ridged ones of P. norvegica.

Basal leaves usually wither by anthesis, and cauline leaves (that is, those proximal to the first flowering and/or branching node) often wither by mid-anthesis as well. Unless otherwise indicated, leaves include both basal and cauline leaves. If leaves are ephemeral, and/or where stems branch at the proximal nodes, proximal inflorescence bracts can be used as leaf-equivalents in the keys and descriptions.

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

Species ca. 400 (98, including 1 hybrid, in the flora).

Potentilla as presented here has a circumscription significantly reduced from treatments in most recent floras, since the convergence of morphologic (J. Soják 1985[1989], 2008) and molecular (T. Eriksson et al. 1998, 2003; C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011) interpretations has justified the segregation of P. fruticosa, P. glandulosa and relatives, P. palustris, and P. tridentata into Dasiphora, Drymocallis, Comarum, and Sibbaldiopsis, respectively (A. Kurtto and Eriksson 2003; Ertter 2007). The generic assignment of P. anserina is problematic (Soják 2010); the species is retained here within Potentilla. The senior author has chosen to maintain Duchesnea, Horkelia, Horkeliella, and Ivesia as distinct genera. As a result, the generic arrangement used here nearly recapitulates that used by P. A. Rydberg (1898, 1908d), representing the last comprehensive treatment of Potentilleae in North America.

Species circumscriptions and nomenclature are also significantly revised, reflecting not only the first continental assessment of the genus since Rydberg but also the need to synthesize American and Eurasian approaches to the genus. Whereas P. A. Rydberg’s (1908d) revision has served as the basis for subsequent floristic works in North America, subject to significant lumping of species and realignment of genera (for example, D. D. Keck 1938; J. Clausen et al. 1940), Eurasians generally have used as their starting point a monograph by T. Wolf (1908), a contemporary of Rydberg. Rydberg was handicapped by limited access to type material in Europe, resulting in the misapplication of several names.

Even with the reduced generic circumscription and improved global perspective, Potentilla remains challenging. Key contributing factors are facultative pseudogamous agamospermy and hybridization (summarized by J. Clausen et al. 1940; B. Eriksen 1996), which result in everything from ephemeral F1 hybrids to fully stabilized species of hybrid origin, sometimes across sectional boundaries. In the face of such biosystematic complexity, this treatment uses the operational species concept for arctic species outlined by R. Elven et al. (1999), with the exception that variety has been used as the primary infraspecific rank for temperate species.

Several species have been recently described or resurrected from synonymy for use in the current treatment, and anomalous collections examined during this preparation probably represent additional novelties. The isolated mountain ranges and complex geology of the North American Cordillera in particular contains a rich trove of unresolved diversity. Collectors should be alert for sympatric species in these situations, and the possible hybrid origin of aberrant individuals or clones.

Sections that have been established for North American Potentilla [for example, by A. Nelson (in J. M. Coulter and Nelson 1909), O. A. Stevens 1959, B. C. Johnston 1985] are mostly based on a subset of validly published but unranked groups by P. A. Rydberg (1898, 1908). Species numbers for primarily Eurasian sections have been provided by J. Soják (pers. comm.). The sectional arrangement used here is both pragmatic and tentative; it also represents an implicit phylogenetic hypothesis. The arrangement reflects molecular analyses (C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011), in which the first four sections correspond to well-supported clades that diverge basal to the ivesioid clade and the remaining sections. The majority of species beyond the first six sections are mingled in a largely unresolved crown clade, informally termed the Potentilla core group by Dobeš and Paule, and the Argentea clade by Töpel et al. Their results also suggest an Asian origin for Potentilla in the Oligocene, multiple colonizations in North America, and rapid radiation of the core group beginning in the Pliocene.

Detailed morphology and anatomy in Potentilla are provided by P. A. Rydberg (1898) and T. Wolf (1908). With some exceptions, floral morphology is of less taxonomic value than are leaf division and leaflet incision, particularly of basal leaves, which renders specimens without them often impossible to identify correctly. Habit and inflorescence architecture are also important features, such that specimens with fully developed inflorescences are typically more confidently identifiable than those in early anthesis.

Vestiture, especially of petioles and abaxial leaflet surfaces, is of critical diagnostic importance. Four primary hair groups are recognized by B. Eriksen and B. A. Jurtzev (1999): straight (or curved), crispate (short, twisted hairs, called crisped in the current treatment), floccose (corresponding to cottony in FNA terminology), and glandular (mostly short-stipitate). We further divide straight hairs into short hairs 0.2 mm, which sometimes form an even underlayer to other hair types, and long hairs for all other straight to curved hairs. Because these five groups can occur in any combination, all are addressed separately in descriptions. Hair density is categorized as sparse, common (or moderately), abundant, or dense if obscuring the surface; rigidity of long hairs is categorized as soft, weak, or stiff.

In keys and descriptions, stem length includes inflorescence, hairs are eglandular, and leaves are basal unless otherwise indicated. Rosetted plants have a basal rosette of leaves from a largely unbranched caudex; tufted plants have multiple stems from a short-branched caudex; and matted plants have dense, spreading, multi-branched caudices. Stolons and stoloniferous are used for plants that sprawl on the ground with apparently solitary flowers arising from nodes of flagelliform stems, even though these may be more homologous to prostrate cymose inflorescences than true stolons. Flagelliform stems form short shoots at the nodes but usually are not otherwise branched. In plants with stems arising centrally from ephemeral basal rosettes, new basal rosettes for the following year often appear in fall and might be confused with persistent rosettes. The diagnostic character of marcescent whole leaves (in sects. Niveae and Rubricaules) is in addition to sheathing stipular leaf bases and petioles, which are also commonly present on plants lacking marcescent whole leaves. Transitions between the extremes of strictly palmate and strongly pinnate leaves are roughly quantified as a fraction of the leaf axis (petiole plus rachis) bearing leaflets. Cauline leaves consist of foliar and bractlike structures on the stem proximal to the proximalmost branch. Leaflet dimensions and teeth number are of the larger leaflets (central in palmate leaves, terminal or distal in pinnate leaves). Leaflet tooth length is measured on the distal edge of the largest and medium-sized teeth. The central portion of the leaflet between opposite tooth sinuses is called the undivided medial blade. Hairs projecting beyond the tips of the leaflet teeth are referred to as apical tufts. Pedicel length is measured from the flower to the subtending bract, even if this appears to be a mid-pedicel bract. Because pedicels at the proximal nodes are often significantly longer than elsewhere, pedicel length is primarily for the nonproximal nodes, with the proximalmost extremes noted separately. Curved pedicels are those that commonly become sigmoidally recurved in fruit; this is characteristic of sections with predominately prostrate inflorescences. Measurements of pedicels, epicalyx bractlets, hypanthia, and sepals are taken at flowering and fruiting but not in bud. Hypanthium diameter is an external measurement of pressed flowers. Although not otherwise indicated, petals of most species commonly have a basal patch of more intense color, which often coincides with ultraviolet nectar guides (N. Naruhashi and H. Ikeda 1999). Style features are based primarily on dried mature styles that are tardily deciduous. Rugose achenes are often smooth until mature.

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