According to the Irish-Canadian botanist and author Diana Beresford-Kroeger, trees release aerosols that are narcotic, anesthetic and relaxing, which is how I might describe the pleasant enough advocacy documentary in which she stars. Screening for one night at TIFF Bell Lightbox (July 13), Call of the Forest: The Forgotten Wisdom of Trees mixes the not-incompatible voices of Beresford-Kroeger and narrator Gordon Pinsent, who describes trees as the "secret to our existence." The film is laidback, folksy and educational, with a poetic sort of alarmism about disappearing forests. We get an eco-chemistry lesson and lovely photography covering Japan, northern California and Canada's boreal forest. And apparently there's more to Germany's Black Forest than cake. Beresford-Kroeger's earnestness grows on you slowly (like moss); after a while, her saying that forests are "haunted by silence and a certain quality of mercy" no longer seems so annoyingly New Age. Passionate and gently insistent, Call of the Forest carries its important message with grace.