Flora is the collective term for all plant life found in a specific region or period of the Earth's history, including vascular plants, mosses, lichens and fungi. Iceland's flora is thus the sum of all plant species native to Iceland, while Tertiary flora refers to the plant species of the Tertiary period.
Iceland's flora is believed to comprise some 5,000-6,000 plant species in the broadest sense of the term (i.e., flowering plants, ferns, mosses, lichens, fungi and alga). Fungi and alga account for around 2/3 of the total. The species composition of Iceland's flora is closest to that of mainland Scandinavia; a lesser similarity can be seen with the flora of the British Isles (Scotland in particular) and Greenland. Local flora differs considerably within Iceland. Regional differences in climate largely account for this variation. Other factors include altitude and the duration of snow cover.