About the words ‘beaver’ *bebhrus and ‘oak’ *perkus

The Proto-Indo-European language had this adjectival formation with a *u-ending. Two main derivational processes can be identified:

- Type1 is reduplication, like in *bebhru- ‘beaver’, from *bhru- ‘brown’.

- Type2 is suffixation, mostly by *ka, as in whelk from *welu-ka ‘spiral-shaped shell’.

An intriguing set of words relates to the oak-tree: *perkus, *kwerkus: Latin quercus f. ‘oak’; possibly Welsh perth f. ‘bush, hedge’. Another formation is attested in Celtic Hercynia silva from *Perkunia, possibly borrowed in Germanic *ferguniō, O.H.G. Fergunna ‘Erzgebirge (mountain range)’, M.H.G. Virgunt f. ‘mountain range in Bohemia’, Gothic fairguni n. ‘mountain range’, Old Englis firgen ‘mountain forest’.

Derivative with new meaning: Old Indian parkatī- ‘sacred Ficus’ with -ati suffix.

Another deadjectival suffix -ka is shown in Old High German fereheih, langobard fereha ‘horse chestnut’ from *per(i)kika, O.Ice. fjorr m. ‘tree, man’; o-grade in O.H.G. forha ‘pine tree’, Old English furh; Old Icelandic fura f. ‘pine’, fy:ri n. ‘pinetree forest’, O.H.G. *forh-is.  

This suggests that the root *kwerk- was originally some kind of adjective.