Adiantum venustum

Incidentally, it is usually easy to distinguish Adiantum venustum from A. tibeticum, which is why the latter as described as a species, not subspecies, which I personally prefer to maintain (as with other species in that group, like A. davidii etc.). Some previous confusion led to perceived difficulty in telling them apart.
In this case, difficulty lies in the inadequate photographs, rather than any overlap of the two species.
So are they the same plant or not? – As someone says, they do look a bit as if they might be off different plants, though difficult to see.

Since it is difficult to say with any amount of assurance to which species they belong to, unless the altitude and size of the fronds are mentioned, to guess (?) the name of the species even by experts of the stature of …, why have them as two species?
Why not treat these as A. venustum subsp. tibeticum and the other as sub sp venustum. What’s wrong with this treatment?

As … says – there is nothing WRONG with deciding to lower them in rank – it is everyone’s choice as to what to do. However I believe the statement that it is difficult to identify them [and without knowing altitude] does not reflect reality. They are normally immediately and easily identifiable – as I mentioned before. This is why A. tibeticum was recognised as a species by Lin, You-Shin, who is a competent pteridologist and specialist in Adiantum – and it appears rather obviously sufficiently distinct to be more appropriately and practically ranked as a recognisable species. It was only that the two photos were inadequate that made these particular photos difficult to identify (and we still don’t know if the two were the same plant or not!). True, frond-size in a full-sized specimen is important, but nearly all the other diagnostic features have been ignored by that statement.
The reason for confusion there is that in his previous accounts … had the descriptions and names transposed, and on my pointing this out, the next mention had partly mixed characters. This was why they may have seemed to be more difficult to identify than they actually are. The characters are easy and I can usually spot them from a distance. Let’s have it the right way round and clear now – I think that would be the most helpful thing to do – as a summary:.
A. venustum – large, more erect and more herbaceous frond (usually 30-40 cm. when full grown), with many pinnae and masses of segments; segments smaller, narrower, often rather asymmetrical, teeth at the apices long, narrow, markedly flabellate [but as there are some fronds where the teeth don’t develop properly, one then needs to look at another frond where they are properly developed]. More of a high-forest species.
A. tibeticum – small, stiffer frond (usually up to c. 20 cm. when full grown), with few pinnae and not an abundance of segments; segments larger, obviously wider, usually symmetrical, teeth at the apices acute but with wide bases, so rather widely deltate, thus not appearing flabellate. Usually above the forest line among rocks.
One has to be careful about the occasional fronds that occur in A. venustum where the teeth don’t develop much – but one can then find them better on another frond of the specimen. Sometimes they get even narrower and longer than usual in some populations of A. venustum (described as A. fimbriatum, a synonym of A. venustum) and are then very easily recognisable.
One comes across A. venustum much more commonly in the Central Himalaya (Pithoragarh to Sikkim), with A. tibeticum rare and right up on the edge of the dry inner-Himalayan zone – but in the West Himalaya and in the dry zones of the NE in Bhutan, A. tibeticum is more common or equally common to A. venustum.
Hope this helps to clarify them. There are about 4 or 5 species in the whole A. venustum group – but so far I can only recognise these two in the Indo-Himalaya though I have seen all the relevant herbaria for Indian ferns and plodded all over the Himalaya.

Fwd: Adiantum venustum in cultivation in New York Botanical Garden? : 2 posts by 1 author. Attachments (1)

Yes, I reckon that is A. venustum – as opposed to the related A. tibeticum.  It has the narrower segments and longer, narrower teeth characteristic of A. venustum sensu stricto.  It should also develop a larger fronds with more segments than in A. tibeticum and is more of a woodland species. Both are grown commonly in N. American and British gardens.
    In general Adiantum in India is one of the more stable genera with fewer critical taxa and less revision needed.  A. pedatum is here and extends further west than the related A. myriosorum.
     Recent treatments are in my 2008 book Taxonomic Revision etc. and in Nepal vol 1 (2015), and the full Indian situation is in the forthcoming Annotated Checklist of Indian Pteridophytes Part 1 (in press 2016).   A aethiopicum was a mistake – not present in Asia.  A. capillus-veneris is everywhere!  There are also some widespread New World adventives.  15 native species, 8 adventives or widely cutivated in India, and various subspecies.