Diario de fredwatson

I'm not an expert on mistletoes. But I noticed an inconsistent classification of dwarf mistletoes (Arceuthobium) on the Monterey Peninsula pines in iNaturalist, so I researched the literature and have concluded that in iNaturalist, at least for the time being, we should classify all dwarf mistletoes infecting Pinus radiata and Pinus muricata on Monterey Peninsula as Arceuthobium littorum - this being one of the essential conclusions of the last five journal papers that address the taxon, as far as I have been able to find.

I'll attempt to update this post from time to time - e.g. upon receipt of informative comments. I started the posted on 5-Sep-2022 and tagged a few observations with it. If I tagged one of your IDs, and you have an opinion, please chime in!

A timeline of much of the main relevant literature appears near the bottom of this post. To summarize, a portion of the genus has oscillated between "lumped" and "split" perspectives. The epithet "littorum" did not appear until 1990, but numerous other taxa in the "campylopodum complex" were being lumped (e.g. by Kuijt) and split (e.g. by Hawkswoth et al.) for decades prior to 1992. The "split" perspective began as morphological one (Hawksworth & Wiens 1970, 1972) and then became supported by some genetic analyses, but not all of them. In the general trend toward increasing support for littorum as a distinct taxon, there are perhaps two notable contrapoints: one is where Nickrent et al. (2004) reached a lumped conclusion based on genetic analysis; the other is where Kuijt's second edition Jepson Manual treatment re-affirmed a lumped classification in the same year that Nickrent (2012) re-split the complex. The newest work supports the split perspective and pointedly challenges Kuijt's (2012) re-lumping in the latest Jepson Manual. This work is by Mathiasen, Kenaley, and Daugherty in several journal papers (2009, 2013, 2015, 2016) including one review paper (2021) and separates littorum based largely on morphology but noting the genetic evidence reported as early as 1990 by Nickrent & Butler. Of the major first authors on the taxon, recognition of littorum as a distinct taxon in some way (either as a species or subspecies) is supported in the most recent publications on the matter led by Mathiasen (2021), Nickrent (2012), and Hawksworth (1993). Job Kuijt (last relevant publication 2012?) is the outlier; he's 92, but was co-authoring with Dan Nickrent as recently as 2019. iNaturalist uses Plants of the World Online (POWO) for taxonomy, and includes littorum as a species (as I write in September 2022).

In the field in Monterey County, my understanding from reading the literature below is that a dwarf mistletoe specimen could be classified entirely based on the host species. If the host is Pinus radiata or Pinus muricata (as it would be anywhere among native pines of the Monterey Peninsula, Point Lobos, etc.), you have yourself an Arceuthobium littorum. Morphological differences are included in the Monterey County flora (Matthews & Mitchell), but these have barely changed since the 1997 first edition, and presumably could benefit from a revision based on the recent morphological studies by Mathiasen et al. which lead to a new taxonomic key to the genus based on host and morphology (Mathiasen & Kenaley 2015). Interestingly, while color appears early in the keys as an indicative character, it has not yet formed part of the quantitative basis for a morphological analysis. So, arguably characteristics other than color might be more definitive for the time being, if one were to attempt to key a specimen based on morphology and not host plant or location.

Timeline of of much of the main relevant literature:

• 1953 to 1970 - Kuijt single-authored 10 publications on Arceuthobium or relevant to Arceuthobium.
• 1970 - Hawksworth & Wiens - journal paper: "New Taxa and Nomenclatural Changes in Arceuthobium (Viscaceae)". 13 species in "Series Campylopoda" of Genus Arceuthobium. Does not mention epithet "littorum" anywhere.
• 1972 - Hawksworth & Wiens - lengthy Forest Service report: "Biology and Classification of DWARF MISTLETOES (Arceuthobium)". Many species covered, but epithet "littorum" not mentioned. Indicates A. occidentale, not A. campylopodum, as the taxon that infects Pinus radiata.
• 1984 - Hawksworth & Wiens - conference proceedings: "Biology and Classification of Arceuthobium: an update". Epithet "littorum" not mentioned.
• 1990 - Nickrent & Butler - journal paper. "Allozymic Relationships of Arceuthobium campylopodum and Allies in California". First (?) introduction of the eipthet "littorum". Suggested that it be recognized as a species based on morphology (including color) and genetic evidence.
• 1991 - Nickrent & Butler - journal paper. "Genetic Relationships in Arceuthobium monticola and A. siskiyouense (Viscaceae): New Dwarf Mistletoe Species from California and Oregon". Demostrated genetic differentiation between littorum and other taxa in campylopodum complex.
• 1992 - Hawksworth, Wiens, & Nickrent - journal paper: "New Western North American Taxa of Arceuthobium (Viscaceae)". Describes A. littorum as a new species (separating it from A. occidentale).
• 1993 - Hawksworth & Wiens, in Jepson Manual 1st ed.: 12 species and 2 subspecies. Recognizes littorum as a species. Uses common name "Coastal dwarf mistletoe" - which may be the first use of this name.
• 1994 - Nickrent et al. - journal paper: "A Molecular Phylogeny of Arceuthobium (Viscaceae) Based on Nuclear Ribosomal DNA Internal Transcribed Spacer Sequences". Excluded littorum, presumably because it had not yet been described at the time of initiation of their study. Did not cite the 1992 or 1993 publications by Hawksworth et al.
• 1997 - Matthews - book: "An illustrated field key to the flowering plants of Monterey County, and ferns, fern allies, and conifers". Recognizes littorum as a species i.e. following the statewide treatment in Jepson Manual 1st ed.
• 2004 - Nickrent, Garcia, Martin, Mathiasen - journal paper: "PHYLOGENY OF ALL SPECIES OF ARCEUTHOBIUM (VISCACEAE) USING NUCLEAR AND CHLOROPLAST DNA SEQUENCES". Lumped littorum to be part of species campylopodum, based on genetic analysis.
• 2012 - Nickrent - journal paper: "JUSTIFICATION FOR SUBSPECIES IN ARCEUTHOBIUM CAMPYLOPODUM (VISCACEAE)". Recognized subspecies within species campylopodum, including subspecies littorum as the only Arceuthobium to infect Pinus radiata and Pinus muricata.
• 2012 - Kuijt, in Jepson Manual 2nd ed.: Recognized only 3 species of Arceuthobium in California, lumped from previous 12 in Jepson 1st ed., based on "recent molecular studies". Does not recognize any subspecies. Does not recognize littorum as a taxon. This lumped treatment by Kuijt was considered "highly conservative" by Mathiasen & Kenaley (2015).
• 2009, 2013 - Mathiasen & Daugherty - journal papers that now serve as a preface to Mathiasen & Kenaley (2015).
• 2015 - Mathiasen & Kenaley - journal paper: "A MORPHOMETRIC ANALYSIS OF DWARF MISTLETOES IN THE ARCEUTHOBIUM CAMPYLOPODUM –OCCIDENTALE COMPLEX (VISCACEAE)". Elevates littorum back to species level, based on morphology. Provides a new taxonomic key to the genus, including littorum as one of 4 species.
• 2015 - Matthews & Mitchell - book - second edition of the Monterey County flora by Matthews (1997). Recognizes littorum as a species, unchanged from the 1st edition of the county flora, NOT following the most recent Jepson Manual (2nd edition) treatment, but following the emerging studies by Mathiasen et al.
• 2016 - Mathiasen, Kenaley, & Daugherty - journal paper. Similar to their previous publications but broader in scope.
• 2021 - Mathiasen and Kenaley - journal paper. Re-affirmed species status for littorum, in context of a comprehensive review.