Oaks

Though Tolkien's references to these trees are generally simply 'oak', he must surely have normally had in mind the tall and strong English Oak, Quercus robur. A significant exception is the holm-oak, a smaller evergreen variety that is explicitly identified as growing in Ithilien.

The original Indo-European word for 'oak' was something like dreu, but in the Germanic language branch that word evolved to describe any tree (and indeed is the source of the word 'tree' itself). As that change developed, oaks received their own specific name (probably originally aiks), but the origin of that name remains unclear.