alder (n.) Look up alder at Dictionary.com
tree related to the birch, Old English alor "alder," from Proto-Germanic *aliso (source also of Old Norse ölr, Danish elle, Swedish al, Dutch els, German erle), from the ancient PIE name of the tree (source also of Russian olicha, Polish olcha, Latin alnus (French aune), Lithuanian alksnis), from root *el- (2) "red, brown," used in forming animal and tree names (see elk).

The unetymological -d- was added 14c.; the historical form aller survived until 18c. in literary English and persists in dialects, such as Lancashire owler, which is partly from Norse.