Lyric discussion by BakerStreetAcoustic 

Cover art for Reverie/Harlequin Forest lyrics by Opeth

Mike uses the word Harlequin as both the meaning that rude_ridims describes and as a joker. He sees the trees as idividuals with independent thought. The character portrayed throughout the Ghost Reveries record is suffering from dillusions for many reasons. Particularly because he is in a state of panic since he's been charged with murder.

The trees have faces to him. The devil is basically contacting the character through them to persuade him to cause an increasing amount of self-loathing as a pathway to other hatred, whether it be on himself or other entities.