> A birch is a thin-leaved deciduous hardwood tree of the genus Betula

https://en.wikipedia.org/wiki/Birch

You can see that the Mastodon account is made in the style of the bark of this tree

https://fosstodon.org/@betula

Betul (with umlaut) is a common Turkish name as well, from Arabic, of similar meaning. Also name of birch tree and related things, e.g. Fomitopsis betulina for birch fungus.