birchyoudaoicibaDictYouDict[birch 词源字典]
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]
birch (n.)youdaoicibaDictYouDict
Old English berc, beorc (also the name of the rune for "b"), from Proto-Germanic *berkjon (cognates: Old Saxon birka, Old Norse börk, Danish birk, Swedish björk, Middle Dutch berke, Dutch berk, Old High German birihha, German Birke), from PIE *bhergo (cognates: Ossetian barz, Old Church Slavonic breza, Russian bereza, Lithuanian beržas, Sanskrit bhurjah, Latin farnus, fraxinus "mountain ash"), from root *bhereg- "to gleam, white." Meaning "bunch of birch twigs used for flogging" (1640s) led to verb meaning "to flog" (1830). Related: Birched; birching. Birch beer is by 1827, American English.