2.2 | Biological material
Four large compound thalli of P. britannica were collected at Heiðmörk nature reserve in southwestern Iceland, about 10 km southeast of Reykjavík in August 2017 (specimen 1 and 2: 64°4’5.04728”N, 21°43’38.87947”W; specimen 3: 64°4’5.19143”N, 21°43’39.02599”W; specimen 4: 64°4’5.44984”N, 21°43’40.89662”W). The lichen thalli were growing among mosses in shady and moist spots, usually in the opening of crevices of lava rocks or under birch shrubs. As compound thalli ofP. britannica are not common in Iceland, they were only taken from sites where additional ones were present in order to prevent overharvesting.