fagus

fagus (n.): botanical genus of beech trees, from Latin fagus "beech," from PIE root *bhagos "beech tree" (cognates: Greek phegos "oak," Latin fagus "beech," Russian buzina "elder," Old English bece, Old Norse bok, German Buche "beech"), perhaps with a ground sense of "edible" (and connected with the root of Greek phagein "to eat;" see -phagous). Beech mast was an ancient food source for agricultural animals across a wide stretch of Europe.
The restriction to western IE languages and the reference to different trees have suggested to some scholars that this word was not PIE, but a later loanword. In the Balkans, from which the beech started to spread after 6000 BC, the [Greek] word means 'oak,' not 'beech.' Yet 'oak' and 'beech' are both 'fruit-bearing trees,' so that a semantic shift from 'oak' to 'beech' appears quite conceivable. The word itself may then have been PIE after all. [de Vaan]