There is a broad choice when it comes to tagging tree specifics, species:en for instance is already used 210k times species:en | Keys | OpenStreetMap Taginfo
species:de even 412k
“species”, which ideally is tagged in latin, which also seems to be the actually the case, has more than 1.6 million: species | Keys | OpenStreetMap Taginfo
taxon numbers are an order smaller but still:
https://taginfo.openstreetmap.org/search?q=taxon#keys
and in latin as well:
https://taginfo.openstreetmap.org/keys/taxon#values
Looking at the history, it seems taxon cannot cope with species:
http://taghistory.raifer.tech/#***/species/&***/taxon/