Thanks for all your replies some of which would be well worth to improve documentation in the wiki. A final remark to avoid misunderstandings: My question did not aim at replacing=removing existing species tags but to replace the species tag in favour of species:wikidata for new objects only.
One of the reasons to look at this tag specially is the fact that it is quite challenging for everyone not being a biologist or the like as already mentioned in earlier posts. Pushing mappers to look for species in wikidata would make sure that the value entered into the OSM list of tags is a valid species at least and not just a genus or anything else. I have looked up quite a list of various species in wikidata and did not find a single mistake so I would not say the data quality found there is worse than the data quality found in OSM tags.
Anyhow I agree to your explanatory statements - under the given circumstances it makes sense to use species:wikidata as an additional tag but not as a replacement.
All my previous objections apply to this too! It’s still multiple levels of indirection, the name on wikidata may not match that of the book in your hand …
If you’re not competent to identify trees to species then don’t try, but you can add leaf_cycle, leaf_type and perhaps know the genus. Almost everyone actually mapping these things will be interested in them, own books, perhaps have been on courses, or learnt from others. Additionally, quite a lot of planted trees do not have species names anyway : e.g., most flowering cherries, such as Prunus ‘Kanzan’. Some, such as this very well-known variety will have entries in wikidata, but others will not, although quite a few have recently been added. Others can easily be identified more precisely than the species, e.g. copper-leaved European Beech trees.
Supporting validation pick lists is entirely different, and a task for editors, but one can easily make use of one of the many well-developed observation apps which already provide lists correctly tailored to individual recording areas (iNaturalist, observation.org, iRecord …), such as the UK Species Inventory curated by the Natural History Museum in London. It is also possible to make suitable extracts from such lists which could be used with OSM editors.
I can easily identify an orange tree without memorizing Citrus × sinensis, a weeping willow without ever having heard of Salix babylonica, a palm tree without having an opinion on this menagerie of tags. Applications like iNaturalist are very useful but shouldn’t be a prerequisite for adding this sort of information to the map.
Editors already give mappers the opportunity to jot down the common name using keys like species:en. But data consumers require a more dependable scientific name. It’s that translation that typically requires an external resource, if not Wikidata then something more encumbered. If the editor could persuade the mapper to volunteer a little more detail, then I don’t see how this would be worse than telling mappers not to bother. If iD’s existing field for the species’ Wikidata ID would display descriptions like JOSM or images like Wikidata itself, then mappers and data consumers would be less likely to confuse plantain the weed with plantain the fruit tree.