POWO accepts this taxon as a variety of C. michauxii, but Weakley accepts this taxon at species rank. Both are present on iNat at the moment. Croton willdenowii G.L.Webster seems to be used significantly more often than the variety, at least on iNat.

Given how little I interact with the group, I don't have much of an opinion. @benjamin_vanee might though.

@nathantaylor you were the one that added the variety, so I assumed you would have some kind of opinion on it. No matter which we decide on, we can't have both at the same time.

@wildlander do you know of anyone who disagrees with this being a valid species?

I'm trying to figure out which option has the most support. If you wouldn't mind passing this around, that would be extremely helpful.

From doing some quick Google searches, it looks like it was accepted as Croton willdenowii in POWO prior (or at least by Rafael in 2011), and POWO may have changed their treatment after the FNA treatment for Croton was published. @benjamin_vanee and Paul E. Berry wrote the FNA treatment that puts this at var. level in FNA Volume 12, which looks like it was published 29 September 2016. I'm hoping Benjamin leaves a comment here.

Hi everyone, interesting discussion thank you for tagging me, my position is that the two taxa should be treated as distinct. It was back in 2009 when I worked on this publication: (https://bioone.org/journals/harvard-papers-in-botany/volume-14/issue-1/025.014.0110/The-Circumscription-of-Croton-Section-Crotonopsis-Euphorbiaceae-A-North-American/10.3100/025.014.0110.full) as part of preparing the FNA treatment that I spent the most time thinking about this.
I could really go either way on this, such as treating them as two species or as two varieties of one species. Here is a quote from the 2009 paper that I think still quite accurately reflects my taxonomic opinion:

"Our concept of what constitutes a distinct species and what should be considered a variety within Croton is influenced by a consideration of the genus as a whole. There are many species of Croton that are widespread and morphologically variable. If the degree of morphological variation observed between C. michauxii and C. willdenowii were used as a baseline for distinguishing distinct species, many of these could be divided into two or more species. The large number of collections available, which thoroughly cover the geographic and morphological range of these taxa, make it possible to distinguish two very closely related taxa with subtle morphological differences and distinct, yet overlapping, ranges. Based on this, we treat C. willdenowii as a variety of C. michauxii. Pursh (1813) was the first to make this combination, however he combined the two under Crotonopsis argentea Pursh, a superfluous and illegitimate name. Here we make the combination in Croton giving priority to the earliest valid name."

For full disclosure, I am more of a lumper than a splitter. Back in 2009 I looked at hundreds of specimens of these, mostly under the scope, and in some cases it was equivocal which taxon it was. Since I discovered iNaturalist, I've sort of given this pair of taxa a wide berth, because looking at photos of live plants differs in several way from looking at herbarium specimens, and I'm holding off having to retrain myself on how to distinguish these from photos.
I subscribe to the idea that ranks are useful and overall necessary, but that they can be arbitrary rather than "real," and that in other groups they can in fact rather closely approximate phylogenetically equivalent groups (that is, be "real").

In summary, my opinion is that the two taxa should be treated as distinct taxa. My inclination is towards treating them as two taxa within a single species (C. michauxii) in recognition of how much more similar and closely related to each other they are than almost any other pair of North American Croton species. I have no problem with iNaturalist, or any other taxonomic treatment, treating them at the rank of species, but that does not mean that I'm ready to retract my position of treating them as distinct, yet conspecific taxa.
Ben

hi @rynxs I agree that no one is arguing to recognize just one taxon (Croton michauxii), I was probably just overemphasizing that I also agree that there are two recognizably different taxa.
Croton michauxii var. elliptica and Croton willdenowii are homotypic synonyms, and it seems to me that the iNaturalist taxonomic policy (based on what I've seen) is that they cannot both be in use as accepted determinations.

I am in favor of using "Croton michauxii var. elliptica" for "Croton willdenowii." My argument for that would be the "more similar and closely related..." quote you mention, and backed up with the 2009 publication I included a link to above.

I am familiar with the Weakley treatment of Croton in the Flora of the Southeastern United States.
In that treatment Croton floridanus is recognized at the species rank, I recognize it at the variety rank (Croton glandulosos var. floridanus, together with several other varieites).
In that treatment Croton berlandieri is recognized at the species rank, I consider it a synonym of Croton humilis.
In that treatment Croton willdenowii is recognized at the species rank, while I consider it to be one of the two varieties of Croton michauxii.

Consider this: up until somewhat recently the name Croton capitatus was widely applied across all of the United States, and that usage started to take hold in iNaturalist. I think @nathantaylor was key in getting the name "Croton lindheimeri" into much more common usage, and if the photo includes a pistillate flower, the two taxa are relatively straight-forward to distinguish.
Based on that experience, I'm not ready to accept that just because a name gets a lot of usage that means it is the correct taxonomy.

If you really want the best treatment, I encourage you to consider the evidence: the Croton taxonomy in the FNA treatment is backed by a series of publications with far greater detail, including molecular data in some cases, namely:
Van Ee, B. and P.E. Berry. 2010. Taxonomy and Phylogeny of Croton section Heptallon (Euphorbiaceae). Systematic Botany 35(1): 151–167.
Van Ee, B. and P.E. Berry. 2009. The circumscription of Croton section Crotonopsis (Euphorbiaceae), a North American endemic. Harvard Papers in Botany 14(1): 61–70.
Van Ee, B., P.E. Berry, and S. Ginzbarg. 2009. An assessment of the varieties of Croton glandulosus (Euphorbiaceae) in the United States. Harvard Papers in Botany 14(1): 45–59.

The Weakley treatment, in my opinion, contains no additional evidence than what is found in the previous works, primarily FNA, and simply reaches a different taxonomic conclusion based on taxonomic judgement.

Before making any decision on Croton michauxii, I encourage you to get @nathantaylor 's opinion on whether Croton berlandieri should be recognized, and @jayhorn 's opinion on whether Croton floridanus should be recognized.

As for Croton berlandieri, I'm actually not familiar with that one yet. I'll have to look at it! From a species philosophy perspective, I think I usually tend towards lumping when barriers are uncertain but towards splitting when there's some sense how well biological, ecological, or morphometric-based species criteria are or might be met. In this case, I'm not familiar enough with the evidence or if the evidence is there for me to have much of an opinion one way or the other. For example, if these two occur sympatrically without intermediates (a somewhat informative proxy for reproductive isolation), then I'd tend towards species. If not, I might just leave them at varietal level.

Sorry for forgetting about this flag. I was going to swap C. willdenowii into var. elliptica after doing a bit more research and waiting for a response from the people Benjamin tagged, but school got in the way.

POWO has changed the name to var. ellipticus: https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77102410-1

@benjamin_vanee @nathantaylor do you have an opinion on the name POWO uses? Since Nathan created the taxon, it's possible to change it without a swap.

Sorry, I meant elliptica (what iNat currently uses) vs. ellipticus (what POWO has). I knew we were moving in the direction of variety, but I was hoping to get the spelling straightened out first.

Would you mind reaching out to Rafael (bi@kew.org)?

hi @rynxs this is what Rafaël responded:
"Dear Ben,
The epithets at all ranks need to follow the gender of the genus so only -us can be correct as with the species:
https://powo.science.kew.org/results?q=Croton%20ellipticus
Best wishes,
Rafaël"

In the meantime, I had been considering this, and I had come to the same conclusion.
Therefore, the correct termination is "Croton michauxii var. ellipticus." When we made the transfer to the rank of species in anticipation of the FNA treatment, we made a correctable, orthografic error, which means that all is well, the taxon is valid and all, just the ending needs to be corrected to "ellipticus."

Ben

Thanks! I have swapped C. willdenowii into C. michauxii var. ellipticus.