We've recorded Campylium stellatum along a number of forest tracks, e.g. there is a significant population in the Rhigos/Cwm-hwnt forestry on the border with RCT and others in the Rheola/Resolven forests (and doubtless, elsewhere). We haven't been able to check all the populations, but we've reviewed the Cwm-hwnt plants and one of the Resolven populations.The habit and morphology of these plants are the same, so I am satisfied that, at least, they are the same taxon.
The photograph below shows the Cwm-hwnt habitat with scattered Campylium colonies.
Campylium habitat, Cwm-hwnt
Campylium habit, Cwm-hwnt
The plants, which have a very nice scent (I've not seen that described elsewhere), are small but their size falls within a range for both protensum and stellatum. They are not as large as some plants I've seen in dune slacks. The are not prostrate but some plants have some pinnate branching.
The Campylium key in Smith separates the two species on the basis of habit (prostrate = protensum versus upright = stellatum), the relative length of the acumen (in abruptly narrowed leaves) and in the anatomy of the leaf - whether alar cells form distinct auricles.
In all the colonies I examined, there were no prostrate plants.The longest acumen I could find was 65% of the total leaf length. This is at the upper limit for stellatum (the acumen can be much longer in protensum). Some leaves show a squarrose tendency, which is a protensum characteristic, but this is not a prominent feature (see photo above):
Abruptly narrowed Campylium leaf showing acumen
And the alar cells form distinct auricles (which is a feature of stellatum):
Campylium leaf showing alar cells and auricles (note slightly squarrose leaf tip).
On balance, I think these forestry track plants are Campylium stellatum, although the habitat may seem suitable for Campylium protensum. Comments and opinions on my interpretation please.