The Moreton Bay fig tree's role in the film "is about nature, how small we are compared to nature, a feeling which is particularly strong in Australia," Bertuccelli told AFP.
"The Tree" is not in competition for the top Cannes award, the Palme d'Or.
But nature plays a leading role in others of the 19 films vying for the Sunday prize -- in critics' favourite Mike Leigh's "Another Year" and late entry from Thailand, "Uncle Boonmee Who Can Recall His Past Lives", by Apichatpong Weerasethakal.
To a backdrop of Australia's harsh climate -- drought, water restrictions, cyclones -- the movie based on Judy Pascoe's bestseller "Our Father Who Art In A Tree" is a poignant tale about mourning.
Fighting off sadness over her father's death, a little girl seeks comfort and his ghost in the branches of the sprawling tree.
Ghosts and the afterlife wafted through several Cannes movies this year.
Oscar-winner Javier Bardem, star of Mexican Alejandro Gonzalez Inarritu's "Biutiful", another festival favourite, is gifted with the power of talking to the dead. Thailand's tropical forest "Uncle Boonmee" movie is chockfull with monkey spirits, talking fish and reincarnated cows.
"Finding the right tree was a challenge, we travelled thousands of kilometres before we spotted this one," said Bertuccelli.
"Few were as welcoming to climb up, and the surrounding countryside west of Brisbane was just perfect."
Starring Gainsbourg as a Frenchwoman brought up in Australia and seven-year-old Morgana Davies in a first role, Bertuccelli said shooting in a foreign language and foreign country was a boon.
"Exile is like mourning," she said.
"You have to tear yourself away from your roots. It gives you distance, as does directing in another language because you focus on the acting, on universal human feelings, rather than on dialogue," she said.
The film is Bertuccelli's second after her award-winning 2004 movie "Since Otar Left".
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