alder: [OE] Alder is an ancient tree-name, represented in several other Indo-European languages, including German erle, Dutch els, Polish olcha, Russian ol’khá, and Latin alnus (which is the genus name of the alder in scientific classification). Alder is clearly the odd man out amongst all these forms in having a d, but it was not always so; the Old English word was alor, and the intrusive d does not begin to appear until the 14th century (it acts as a sort of connecting or glide consonant between the l and the following vowel, in much the same way as Old English thunor adopted a d to become thunder). The place-name Aldershot is based on the tree alder.
tree related to the birch, Old English alor "alder" (with intrusive -d- 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), from Proto-Germanic *aliso (cognates: Old Norse ölr, Danish elle, Swedish al, Dutch els, German erle), from *el-, the ancient PIE name of the tree (cognates: Russian olicha, Polish olcha, Latin alnus, Lithuanian alksnis).