Flora of North America species comparison

Species 8 (2 in the flora).

J. Holub (1972b) argued that the correct name for this group, when segregated from Epilobium, should be Chamerion (Rafinesque) Rafinesque ex Holub. However, A. N. Sennikov (2011), using the more recent and clarified ICBN (Melbourne Code), correctly argued that the name of the segregated genus should be Chamaenerion, and that opinion is followed here.

All species of Chamaenerion tested to date have been self-compatible, although the strong protandry can result in low fertilization rates if there is a shortage of pollinators. Flowering is diurnal, with each flower generally remaining open for three to five or more days.

All eight species (nine taxa) of Chamaenerion are restricted to the northern hemisphere, and six of eight species occur only in Eurasia; Circaea is the only other genus of Onagraceae in which most species occur outside of the western hemisphere (D. E. Boufford 1982b). Chamaenerion is generally divided into two sections, each with four species (T. Tacik 1959; and, as Chamerion, J. Holub 1972b; W. L. Wagner et al. 2007). P. H. Raven (1976) and most others who treated this group as a section of Epilobium divided it into two corresponding subsections. Both species of Chamaenerion in North America are placed in sect. Chamaenerion (not treated formerly here), along with two species endemic to the Himalayan region, C. conspersum (Haussknecht) Kitamura and C. speciosum (Decaisne) Hoch & Gandhi (Chen C. J. et al. 1992); the other four species, all endemic in Eurasia, comprise sect. Rosmarinifolium Tacik.

Chamaenerion differs from Epilobium in leaves nearly always alternate, rarely subopposite or verticillate near stem base (versus opposite at least on proximal stem); absence of a floral tube (versus a distinct floral tube); slightly zygomorphic flowers with subequal stamens that are erect, later deflexed, and a style that is deflexed, later erect (versus actinomorphic flowers with erect stamens in two unequal series and erect style); and pollen shed in monads (versus pollen shed in tetrads, or monads in one species). Molecular analyses (D. A. Baum et al. 1994; R. A. Levin et al. 2004) provided strong support for Chamaenerion as a monophyletic group separate from Epilobium, and for sect. Chamaenerion (only C. angustifolium and C. latifolium sampled) as monophyletic.

The superfluous and illegitimate name Pyrogennema Lunell pertains here.

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

Subspecies 2 (2 in the flora).

Chamaenerion angustifolium, which in North America is known mostly as fireweed, is a widespread circumpolar, circumboreal species that can be locally dominant, often in disturbed habitats, particularly following fires (T. Mosquin 1966, 1967; G. Henderson et al. 1979). In addition to producing numerous, highly vagile, comose seeds, aggressive rhizomatous growth enables it to survive clonally and spread rapidly after fires (L. E. Vodolazskij 1979). As succession proceeds, production of the familiar spikes of magenta flowers is inhibited, but fireweed often persists in non-flowering condition even in relatively mature forests, where it is able to sprout and spread quickly following disturbance. The species is one of the few within the genus and tribe to form true rhizomes (R. C. Keating et al. 1982), enabling it to colonize large areas very rapidly.

Polyploidy is well documented in Chamaenerion angustifolium, but unlike the similar situation in C. latifolium (T. Mosquin and E. Small 1971), diploid and tetraploid populations differ morphologically and have partially overlapping, but distinct, geographical ranges (Mosquin 1966; Chen C. J. et al. 1992). The diploid subsp. angustifolium occupies the northern part of its range, in North America across Canada and interior Alaska, and in Asia across Siberia and northern Europe, ranging southward at higher elevations. Farther south in Eurasia and North America, it is replaced by the tetraploid subsp. circumvagum (Mosquin 1966). Hexaploid (n = 54) populations have been detected only in Japan, and cannot be distinguished morphologically from tetraploids. B. C. Husband and D. W. Schemske (1998), Husband and H. A. Sabara (2004), and Sabara et al. (2013) have documented population segregation and mixing in contact areas between the two ploidy levels, including populations with a single cytotype, a high proportion of mixed populations, and presence of triploids (n = 27), but at low levels, indicating strong reproductive isolation between cytotypes.

The floral phenology of Chamaenerion angustifolium was described in 1793 by C. K. Sprengel (P. Knuth 1906–1909, vol. 2) as a classic example of protandry. The flowers are presented in tall spikes, opening from the base of the spike, initially with stamens erect and style sharply reflexed. By the second or third day after opening, the stamens reflex as the style straightens into the floral axis, the four lobes of the stigma spread open, and nectaries commence production. Bees start at the lowest flowers in search of nectar, moving up the spike until there is no more nectar, by which time they are well dusted with pollen, which they bring to the lower functionally (female) flowers of the next spike.

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