Panarctic Flora

860711-14 The Erigeron uniflorus aggregate E. alpiniformis, E. eriocalyx, E. eriocephalus, E. uniflorus

Geography: Circumpolar-alpine.

Notes: Elven and Solstad: This is a little investigated group where four taxa have been proposed, either as species or three of them as races of Erigeron uniflorus. The morphological differences are not very pronounced. The geographical ranges are overlapping (parapatric) to some degree. Erigeron eriocephalus is high-arctic circumpolar but its range overlaps with or touches the ranges of the Greenlandic E. alpiniformis, of the arctic-alpine European E. uniflorus in Iceland and Scandinavia, and that of the mainly alpine E. eriocalyx in Siberia. Fertile intermediates between E. eriocephalus and E. uniflorus are suggested from Scandinavia and Iceland, whereas the other zones of overlap have not been investigated. Nesom (2006b) considered E. eriocephalus a variety of E. uniflorus, probably due to previous American accounts (not least Aiken et al.) rather than to comparison with European specimens. In wait for a combined morphological and molecular investigation, we provisionally accept four species.

Authors disagree about where the range boundary between E. uniflorus and E. eriocephalus runs. The majority of authors have considered E. uniflorus to occur both in central Europe and in Fennoscandia, Iceland, and Greenland. Cronquist (1947) and Hultén and Fries (1986) were of another opinion. Cronquist excluded E. uniflorus s. str. from Greenland as did Nesom (2006b). Hultén and Fries restricted E. uniflorus s. str. to the central and southern European mountains and the Caucasus and considered all the northern European and arctic plants as subsp. eriocephalus. Circumscribed in this way, the morphological differentiation between E. uniflorus s. str. and E. eriocephalus becomes impossible to draw. We have compared plants in several herbaria and find the majority of Fennoscandian and Icelandic plants inseparable from the central European ones. A minority of the northern plants have features resembling the arctic E. eriocephalus but are usually not nearly as 'eriocephalous' as these. The distinction between the two in Fennoscandia and Iceland is not clear, and from a regional (northern European) viewpoint it might be most appropriate to consider them two subspecies. However, in a circumpolar context they are clearly different. The situation in Fennoscandia and Iceland may be a case of local introgression due to postglacial secondary meeting. Note that triploid, sterile hybrids between E. humilis and E. uniflorus are documented to be frequent in Fennoscandia (Engelskjøn 1967), whereas no such hybrids between E. humilis and E. eriocephalus are recorded (as far as we know), even if the two latter species co-occur throughout large parts of the Arctic.

The Greenland E. alpiniformis is superficially more similar to the northern European E. borealis than to E. uniflorus and E. eriocephalus. However, it differs in not being 'trimorph', i.e., it does not have a ring of non-ligulate female flowers between the female ligulate and the bisexual disc flowers. Such heads with 'trimorph' flowers characterize E. borealis, E. acris, and their relatives (the Trimorpha group). We therefore assign E. alpiniformis to the E. uniflorus aggregate. Nesom (2006b) seems to have transferred all previous Greenland records of both E. borealis and E. uniflorus (e.g., Böcher et al. 1978) to this species. However, except for the 'trimorphy', Greenland plants are very similar to many northern European plants and this matter should be studied further in a trans-Atlantic investigation.