Equisetum sylvaticum; The Wood Horsetail is very rare on Iceland. It is known only from a few (remote) localities. However, it can easily be identified: it is the only horsetail that has the primary side branches split into several secondary side branches. All other horsetails either have no side branches from the main stem or only unbranched primary side branches. The Icelandic name is Skógelfting.
For more information see www.floraislands.is / Wood Horsetail page (in Icelandic)