Another part about Iceland vegetation is that developing soil on top of fresh lava takes a long time. Tens of thousands of years in some cases. Large sections of Iceland gets covered up with lava flows pretty often. But you can get some great biodiversity on the lava pretty quickly in a wet place like Iceland with lots of moss and lichen growing. On an 8,000 year old lava flow in the middle of Iceland near Leirubakki, the moss is half a foot thick[1] in places. You find a few old gnarled birch growing here and there in the hollows where wind blow dirt accumulates, but mostly just huge mats of moss and lichen with a bit of grass.

[1]https://njarboe.com/iceland/IcelandMoss.jpg