Proposal to consolidate bracken taxonomy

In light of a recent publication in American Journal of Botany by Wolf et al., (https://bsapubs.onlinelibrary.wiley.com/doi/10.1002/ajb2.1365), I'd like to rearrange our bracken (Pteridium) taxonomy in line with the molecular phylogenetic approach taken by Thomson and others. Of particular value in understanding the situation is Thomson's 2008 article in the Fern Gazette: https://www.ebps.org.uk/wp-content/uploads/2014/05/FGV18P3M3.pdf which describes the "aquilinum" and "latiusculum" morphotypes; most continents have at least one taxon corresponding to each morphotype, but genetic studies, such as Wolf et al., have shown that brackens on the same continent are more closely related to each other than to other taxa of the same morphotype.
Under the approach of Thomson, we would divide Pteridium into four species: Pteridium aquilinum, diploid, of the northern hemisphere and Africa, Pteridium esculentum, diploid, of the southern hemisphere, and the hybrid allotetraploids Pteridium caudatum, of Florida and Central and South America, and Pteridium semihastatum, of southeast Asia and northern Australia. Within P. aquilinum, 11 subspecies would be recognized, most of which already exist in our taxonomy as subspecies or varieties: ssp. aquilinum (Europe), ssp. capense (Africa), ssp. centrali-africanum (Africa), ssp. decompositum (Hawaii), ssp. feei (North America), ssp. japonicum (Asia), ssp. latiusculum (North America), ssp. pinetorum (Europe), ssp. pseudocaudatum (North America), ssp. pubescens (North America), and ssp. wightianum (Asia). Within P. esculentum, 2 subspecies would be recognized: ssp. esculentum and ssp. arachnoideum. (P. esculentum ssp. arachnoideum, in this sense, was recently further divided into subspecies by Schwartsburd et al. on morphological grounds, but these are not yet in our taxonomy and I would prefer to wait for molecular evaluation before recognizing them.)
Aside from the promotion of some varieties to subsspecific rank, this would require swapping our current P. arachnoideum to P. esculentum ssp. arachnoideum, P. latiusculum to P. aquilinum ssp. latiusculum, P. pinetorum to P. aquilinum ssp. pinetorum, and P. revolutum to P. aquilinum ssp. wightianum. P. tauricum would be submerged in P. aquilinum ssp. aquilinum. P. aquilinum ssp. lanuginosum, of somewhat unclear application, would be merged into P. aquilinum, while P. aquilinum var. champlainense, of doubtful distinctness, would be merged into P. aquilinum var. latiusculum, as in Cody (Ferns and Fern Allies of Canada).
This taxonomic arrangement seems fairly reasonable to me: the genetic data in Wolf et al. suggest that species should be delimited rather broadly, while the plethora of recognized subspecies allows for continued recognition of morphological variation on different continents, e.g., the distinctness of ssp. pinetorum from ssp. aquilinum. However, I'd like input from other curators: @apseregin, your thoughts would be particularly welcome, as the Russian iNat community has been making heavy use of P. pinetorum lately.

Posted on October 26, 2019 05:23 AM by choess