However, on the Disc, Dryads do exist, and they get pissed off when you damage their tree. The act of carving a heart and initials on a tree-trunk is therefore fraught with danger in certain regions, for instance the forest of Skund. Had Rincewind thought on, and remembered fairly recent experience, he should have been relieved that the trees in Skund only ever spoke to him and nothing manifested.
For in the woods around the temple of Bel-Shamharoth, Rincewind, whilst escaping mad bears, swarming bees, and a pack of wolves trying to get away from the former and enraged by the stings of the latter, climbed a tree for sanctuary. Alas, a branch gave way and he found himself plummeting downwards, whilst the branch didn't break cleanly: it tore a strip of bark and living tissue away with it. It was at this point that an arm emerged from nowhere - well, out of the otherwise solid bark - and drew him into the tree.
This was Druellae, a dryad with, as Rincewind noticed, a nasty fresh scratch on her arm. Drawn into the palatial interior of the living tree, which, Tardis-like, was far bigger on the outside than the inside, and populated mainly by female dryads: beautiful, naked, and in various shades of green.
This distracted Rincewind's attention for long enough for him not to notice the male Dryads - very obviously male, for e.g. in details of strength and musculature. Who were standing behind him blocking his exit.
Rincewind had been saved from Death By Bear, Bee or Wolf for a definite purpose: rubbing her scratched arm, Druellae informs him he has hurt the tree, for which the penalty is death.
However, the Great Spell is present - Rincewind invites Druellae to read his mind. She does - and sees the Spell. In the panic that follows, he escapes - only to run into the Ichor God and Dread Sender of 7a, Bel-Shamharoth.
(Sometimes it's just not your day.)
It is thought these Dryads all perished in the explosion of pent-up Time that followed the death of Bel-Shamharoth, in which trees grew from acorn to decrepitude in a matter of apparent minutes.
However, the Discworld being what it is, there are bound to still be some out there somewhere, guarding the Tree and diligently making acorns. So mind where you carve your initials.
These events are to be found in The Colour of Magic.
Dryads on Roundworld were nature spirits who took the form of beautiful young women, and it was believed that they lived only as long as the trees they inhabited. They were in fact the tree in human(ish) form, and could revert at will to whichever corporeal state they wished. They were often pictured with Silenus and Bacchus (Dionysis) dancing and cavorting. They were often seen as friendly spirits, kindly-disposed to humans, but forest encroachment was apt to draw forth dendritic vengeance whence the offender's body was never returned. This has some resonance with the Ents in The Lord of the Rings.