Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
Describing IPH (a lay Institute of Preparation for the Hereafter) in Canto Three of his poem, John Shade (the poet in VN's novel Pale Fire, 1962) mentions l’if, lifeless tree:
L'if, lifeless tree! Your great Maybe, Rabelais: