Two Blue Cohosh Species

Caulophyllum thalictroides and Caulophyllum giganteum are separate species according, as far as I can tell, mostly to this paper. However, it occurs to me, reading this paper, that physiological effects of early emergence could confound the determination of the morphological details used in this study.

For example, if the emergence and flowering phenology of C. giganteum is earlier, and if the vegetative characters that supposedly distinguish the species continue to expand and grow, as do many forest herbs, then the vegetative characters could appear larger for supposed C. giganteum plants just as a consequence of their head-start. This could be a problem as long as all the plants in this part of the study were sampled on the same day-of-the-year, rather than day-since-emergence (they were).

"On the collection date of the mass sample, 11 May 1982, C. giganteum had completed flowering while C. thalictroides was in anthesis." But how were such plants assigned species-identifications, then? Hopefully not by the same morphological characters that were used in the PCA!

The vegetative morphological characters in the single-population experiment:

Vegetative differences in C. giganteum (all longer and/or bigger):

  1. leaflet length and width of the first two leaves
  2. leaflet sinus length of the first two leaves
  3. primary petiolule length of the first two leaves
  4. terminal inflorescence length
  5. and a decrease in the degree of compounding of the second leaf.
The authors also show that flower size differences distinguish the species. (This part is from herbarium specimens across the ranges of all three species in the genus.)
  1. stamen length
  2. sepal length
  3. pistil length
  4. petal length
  5. ratio of filament length to anther length
    (but you shouldn't use ratios in this type of analysis)

In this case, they found convincing evidence of a bimodal distribution along PCA1 (composed of the above 5 characters, in decreasing order of importance, and with the same positive valence), which suggests two morphologically distinct species, one big-flowered and one small-flowered. However, this shows no evidence of the claimed phenological separation, and doesn't really show evidence of other traits that supposedly differentiate the species (flower number per inflorescence, perianth color). Herbarium specimens are not always the most representative examples of a given population, and there may well have been plants in the C. giganteum populations that had smaller flowers that were less conspicuous to the collectors.

A common greenhouse experiment might be necessary to determine whether there really is separation here, and I'd like to see evidence that organ expansion has completed by the time of its determination in the first part of this study. But maybe first I should observe some of these populations for myself:

The closest Caulophyllum locations to me:

For my late April trip to Shenandoah:

on the way down:

in and around Shenandoah:

Publicado por ddennism ddennism, 28 de marzo de 2018



Good thoughts and questions. But I am confident these are distinct species.
My experience in the field with these two taxa is that they are generally distinctive. I find style length to be the most distinctive character, and it is evident both during flowering and even in developing fruit. Puny (.25 - 1 mm) in thalictroides, longer (1 - 2 mm) in giganteum. At this point I can eyeball difference in the field, though a small ruler is also helpful. You didn't mention the difference in color - purplish on young plants in giganteum, green in thalictroides. At the site of my pic from yesterday both species grow together. Generally I find one species or the other at a site, so this place is interesting. Differences are evident once they are in flower, and phenology does generally hold up, though there are outliers of both species. I'd need to have close up of flower for a pic that really cuts the ID muster.

Publicado por stevendaniel hace casi 3 años (Marca)

Hi -
@stevendaniel - Thanks for your guidance. What "pic from yesterday" were you referring to?

Publicado por ddennism hace casi 3 años (Marca)

Now that’s a good question. I took pics of emerging C. giganteum in bud but never uploaded it, figuring that it really wasn’t diagnostic for ID. I guess I forgot and thought that I had Included it with the other wildflowers!

Publicado por stevendaniel hace casi 3 años (Marca)

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