Askefruer

An askefrue (Danish for ash wife, plural: askefruer), also known as the askafroa and eschenfrau, is a type of legendary creature in Norse mythology and German folklore. They are tree-dwelling spirits similar to the Greek hamadryad. They were willing to use their powers to heal the sick if given an offering on Ash Wednesday, despite the fact that Ash Wednesday is named for fire ashes rather than ash trees.
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[Norse] The Askefruer, or "Ash Maidens," are a northern variation on a common mythological theme: spirits, often depicted as human women, inhabiting trees. The ash tree was particularly important to ancient northerners: the World Tree, the axis about which the world revolved, was the ash tree Yggdrasil.