Panarctic Flora

740104-05 The Rhododendron tomentosum (Ledum) aggregate R. groenlandicum, R. tomentosum

Geography: Circumboreal-polar.

Notes: Elven and Murray: Kron and Judd (1990) analysed the molecular relationships among the genera close to Rhododendron and argued for inclusion of Ledum in Rhododendron, with as low rank as a subsection of one of the eight subgenera. This assignment found support in a later study (Kurashige et al. 2001) and is becoming established in the literature. This Rhododendron-Ledum clade excludes the Therorhodion group, see below. Ledum is a distinct and probably advanced segregate with its main range outside that of Rhododendron s. str. However, P.M. Jørgensen (in comment) stated that there are eastern Asian species connecting Ledum with core Rhododendron.

The account below - as two species, one with two races - is based on the following arguments: (a) R. groenlandicum and R. tomentosum (subsp. decumbens) are largely sympatric in North America without any reported transition, whereas R. tomentosum subsp. tomentosum and subsp. decumbens are allopatric to parapatric in parts of Russia, (b) R. groenlandicum is disjunctly different from R. tomentosum s. lat. in several characters (see, e.g., Savile 1969), whereas the two races within R. tomentosum are difficult to keep consistently apart, and (c) R. groenlandicum is diploid, whereas R. tomentosum s. lat. is tetraploid or at least predominantly so.