birch: [OE] Old English bi(e)rce came from a prehistoric Germanic *berkjōn, source also of German birke. The word goes back ultimately to an Indo-European *bhergo, but as is often the case with ancient tree-names, it does not denote the same type of tree in every language in which it has descendants: Latin fraxinus, for example, means ‘ash tree’. It has been speculated that the word is related to bright (whose Indo-European source was *bhereg-), with reference to the tree’s light-coloured bark.
It could also be that the word bark [13] itself is related. The verb birch ‘flog’ (originally with a birch rod or bunch of birch twigs) is early 19th-century. => bark, bright[birch etymology, birch origin, 英语词源]