Formerly known as Parmelia robusta
As its name suggests this is a robust lichen, though in other respects is very similar to the smaller and much more common Parmotrema perlata. It is grey-green in colour and has a lobed “foliose” habit. The lobes are much larger than those of P. perlata being 5-15mm wide and are more elaborately cut and inrolled, giving a ragged appearance to their margins. Sparse cilia are scattered on the margins (cilia are abundant in P. perlata). Soralia are sparse, rounded and often clustered on small extensions of the cut lobed margins.
It can also be distinguished from P. perlata by chemical tests: the medulla of P.robustum gives a rust-red reaction to Pd and a dirty yellow-brown reaction to K. In P. perlata Pd gives orange and K gives a bright yellow-orange.
In Wales it grows on damp rock faces. It has been found epiphytic elsewhere, and on the Lizard Peninsula in Cornwall, and in SW Ireland, it occurs on more open coastal rock faces.
P. robustum is known from only three sites in Wales – Coed Cwm Einion (SSSI and SAC) and Coed Cwm Cletwr (SSSI) in Ceredigion; and Coed Crafnant (part of Rhinog SSSI) in Meirionydd. (The NBN Gateway and the BLS database have an additional Merionydd record, from Ganllwyd, but this appears to be erroneous).
Elsewhere in the UK it is recorded only from the Lizard peninsula in Cornwall. Outside of the UK it is known from a small number of locations in SW Ireland, S.Europe and W. France, Macaronesia, N. & S. America, Africa, Australia, New Zealand and the Pacific Islands.