Correct names of Ledums in Rhododendron

Discussion in 'Plants: Nomenclature and Taxonomy' started by Harri Harmaja, Feb 4, 2005.

  1. I wish to remind of two names which are mostly cited erroneously (the latter due to an nomenclatural error which unfortunately was approved and thus made very widely distributed through an authoritateive compendium of the genus).

    1) According to the International Code of Botanical Nomenclature, Ledum palustre is 'Rhododendron tomentosum Harmaja'.

    2) According to the International Code of Botanical Nomenclature, Ledum decumbens is 'Rhododendron subarcticum Harmaja'.

    However, if you wish to treat Ledum decumbens at the subspecies, variety or forma levels (in Ledum or Rhododendron), you have to use the epithet 'decumbens' (and not e.g. 'subarcticum') according to the nomenclature rules. Of all synonymous epithets, 'decumbens' was the first that was used at the subspecies, variety and forma levels (no matter of which species or under which genus).
    But I am convinced that Ledum decumbens/Rhododendron subarcticum is worth of the specific rank: it differs from L. palustre/R. tomentosum in morphology, chemistry and distribution, at least.

    Harri Harmaja, University of Helsinki, Finland
    http://www.helsinki.fi/people/harri.harmaja/

    Links to papers on subsect. Ledum:
    http://www.sekj.org/PDF/anbf35/anbf35-263p.pdf
    http://www.sekj.org/PDF/anb39-free/anb39-183s.pdf
     
  2. A few additional remarks to my appeal for the use of correct scientific names of Ledums in Rhododendron. Actually no less than 4 comprehensive, widely known and used treatments of Rhododendron published in 1996-1998 (three books, one web list) used an illegitimate name for worldwide consulting.

    For those who tend to think that the taxonomic inclusion of Ledum in Rhododendron was made because appearing 'cool' or as the result of 'tricks of molecular taxonomy', I firstly remind of the words of a wise, old botanist of the middle of the 1900's: "Ledum is nothing but a Rhododendron with separate petals". By that, the difference between the genera is condensed into a nutshell (in fact, even this sole difference does not hold perfectly). Secondly, actually this taxonomic solution was first made on morphological grounds (Kron & Judd 1990, a cladistic analysis; Harmaja 1990 & 1991). Thereafter, the scientific correctness of this merger has repeatedly been confirmed with molecular (DNA) studies.

    There may be additional characteristics of R. subarcticum (L. decumbens) to confirm its distinctness as an independent species rather than a race of R. tomentosum (L. palustre). The utmost careful studies of Canadian mycologists on the host specifity of rust species of the genus Chrysomyxa, parasitizing ericaceous plants, would suggest a 'narrow' species concept in Rhododendron subsect. Ledum. The chromosome numbers (whether tetraploid or diploid) yielded for these two species deserve more attention; some studies report them different while others do not.

    Lastly, I recommend everybody to read the recent paper by the Swedish Professor Bengt A. Kihlman ("Hybrids between Ledums and lepidote Rhododendrons". - American Rhododendron Society, Vol. 58, Number 2: 74-81, Spring 2004).

    Harri Harmaja
     
  3. Harri Harmaja

    Harri Harmaja Active Member 10 Years

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    It is very disappointing that - in spite of a good amount of guidance given - important databases and floras continue using nomenclature that is against the international rules. A showy component of the arctic tundra vegetation of North America and Asia is called "Rhododendron tomentosum ssp. subarcticum" though that name is illegitimate according to the International Code of Botanical Nomenclature.

    By the way, why this very Ledum is commonly treated as a subspecies though there is no proof that it would be less "good" a species than any other of this plant group?

    For the correct nomenclature of Rhododendron subsect. Ledum, please consult the following pages:

    http://www.ars-grin.gov/cgi-bin/npgs/html/tax_search.pl?Ledum
    http://www.rhodogarden.com/cross/ledum_taxonomy.html

    Best wishes, Harri Harmaja
     
  4. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    Two queries . . .

    1. Since the basionym is Ledum palustre L., why doesn't the new combination in Rhododendron use the existing epithet, i.e. Rhododendron palustre (L.) Somebody? Was that name already in use for a different species of Rhododendron?

    2. What about Ledum glandulosum and Ledum groenlandicum?

    Edit: Ooops! Found the answers in the links above :-)
     
  5. Harri Harmaja

    Harri Harmaja Active Member 10 Years

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    It's OK, Michael F!

    Some people seem to dislike the merging of Ledum in Rhododendron as they believe that this taxonomic decision is made purely on molecular grounds. However, the first authors that proposed this solution (Kron & Judd 1990, Harmaja 1990 and 1991) only had non-molecular arguments. There is no morphological character that would separate between Ledum and Rhododendron in all cases. As features that speak for the merger, I myself moreover emphasized (i) the spontaneous hybridization of Ledum decumbens and Rhododendron lapponicum and (ii) the fascinating ecophysiological phenomenon: the dropping winter position of the leaves present in both taxa.

    Cheers Harri Harmaja
    http://www.fmnh.helsinki.fi/users/harmaja/about_myself.htm
     
  6. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    I don't think this is a very useful character - numerous unrelated broadleaf evergreen species demonstrate the same behaviour (I see it in frequently in e.g. Fatsia japonica and Viburnum rhytidophyllum, just to name two); it is an obvious adaptation to shed snow.

    Agreed about the hybrids, though.
     
  7. Ron B

    Ron B Paragon of Plants 10 Years

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    I don't think slow adoption of new combinations is unusual.
     
  8. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    Too right! 55 years on from the publication of Platycladus orientalis, uptake of the name is still very far from complete.

    In general, the nursery industry seems to be the worst offender.
     
  9. Harri Harmaja

    Harri Harmaja Active Member 10 Years

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    1) Actually, possessing the genes that generate the winter position of the leaves assists in judging the relationships of the Ledum group; it was not used as a decisive character. It is essential that this feature is present in Rhododendron subg. Rhododendron in which Ledum was inserted on the basis of other characters.

    2) According to what I have learnt from the litetarure, this showy character has developed for another reason. Leaves that are (i) obliquely positioned towards the sun rays, (ii) more compactly packed, and (iii) apparently often slightly incurved, evaporate less water during the most critical season of the year for ledums: the early spring when the sun already warms the atmosphere and the vegetation but the roots are not yet able to suck compensative water as the bog peat still remains frozen.

    Best wishes for the new Rhodo season!
    Harri
    http://www.fmnh.helsinki.fi/users/harmaja/about_myself.htm
     
  10. Harri Harmaja

    Harri Harmaja Active Member 10 Years

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    The scientific names of species of Rhododendron subsect. Ledum are nowadays generally cited in conformity with the rules of the International Code of Botanical Nomenclature. Therefore it is sad to notice the recent "creation" of two quite new names in the species descriptions database of the American Rhododendron Society. The names Rhododendron diversipilosum ssp. tomentosum and Rhododendron diversipilosum ssp. subarcticum are found ïn the online database. However, neither name has been proposed in a printed paper as the rules would indicate. Moreover, such names -- even if published -- would be illegitimate. If you wish to treat the entities tomentosum and subarcticum as subspecies, you cannot treat them under the youngest species level name (diversipilosum).

    Actually, the database simply refers to two valid species: R. tomentosum (Eurasia) and R. subarcticum (almost circumpolar). R. diversipilosum is a third species that is restricted to coastal areas of NE Asia.

    Harri Harmaja
    http://www.fmnh.helsinki.fi/users/harmaja/about_myself.htm
     
  11. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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  12. Chris Klapwijk

    Chris Klapwijk Active Member 10 Years

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    Harri, on March 17th of this year I posted Ledum authors and references to Wikipedia, I've also posted the classification here.

    Michael F, I asked Bob Weissman a couple of years ago to incorporate on the ARS page the Ledums as well as R. eastmanii. He replied: "Perhaps best to wait until the expert taxonomists come to an agreement on where these various species are to be placed."

    The agreement Bob is waiting for may never be reached, given the current state of flux:

    In the "Flora of China 2005" 9 subgenera, Sleumer 1949 based, are listed: Azaleastrum, Hymenanthes, Pentanthera, Pseudazalea, Pseudorhodorastrum, Rhododendron, Rhodorastrum, Therorhodion and Tsutsusi.

    Chamberlain et al 1996 list 8 subgenera: Azaleastrum, Candidastrum, Hymenanthes, Mumeazalea, Pentanthera, Rhododendron, Therorhodion and Tsutsusi.

    In the DNA based study Goetsch et al 2005, the authors proposed to reduce the number of subgenera to 5: Azaleastrum, Choniastrum, Hymenanthes, Rhododendron and Therorhodion.

    In Argent 2006 section Vireya was raised to the subgenus level.

    Davidian's Balfourian type of classification, although not widely used, should be mentioned here also.

    I suspect that after a number of papers are published by different scientists using different genes in their studies, a scientific consensus view will emerge, but whether the more horticulturally-oriented community will accept such a view remains to be seen.

    In the mean time we are stuck with no less than 5 distinct systems of classification, take your pick.
     
  13. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    One bad nomenclatural error in their classification, is that if subgenera Pentanthera and Hymenanthes are to be combined, the older (and therefore valid) name for the merged group is subgenus Pentanthera, not Hymenanthes as they give.
     
  14. Chris Klapwijk

    Chris Klapwijk Active Member 10 Years

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    Michael,

    section Pentanthera pre-dates subgenus Hymenanthes, but subgenus Hymenanthes pre-dates subgenus Pentanthera.

    Rhododendron sect. Pentanthera G. Don, Gen. Hist. 3: 846. 1834.
    Rhododendron subg. Hymenanthes (Blume) K. Koch, Dendrologie 2(1): 170. 1872 [Hymenanthus]
    Rhododendron subg. Pentanthera (G. Don) Pojarkova in Schischkin & Bobrov, Fl. URSS 18: 57. 1952.
     
  15. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    Hi Chris,

    Are you sure of that? I've seen subgenus Pentanthera cited with G.Don as the subgenus author. I've not seen the protologue but suspect it will be one of those cases where the valid rank depends on interpretation of symbols like '§'.
     
  16. Chris Klapwijk

    Chris Klapwijk Active Member 10 Years

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    Michael, I'm pretty sure.
    I herewith request you either substantiate the above claim or recant, thank you.
     

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