‘Treble’ means the pitch range of the highest female voice. Here it refers to the chirping of birds. The poet wants to say that the song of the birds that live on a tree is harmonious (well matched) to heaven’s own tunes.

The person who plants a tree, actually plants a home for birds near the heaven. Those birds sing in hushed and happy hours of twilight. When we hear that song, it sounds like a heavenly note. So this comparison between the birds’ song and heaven’s own tunes is fitting and beautiful.