I was also puzzled by this tree story. I'll try to get my hands on the book and see if he provides any references.
Rustam's memoirs unfortunately have not been translated into english (to my knowladge), it's an autobiography by Napoleon's mamluk bodyguard. While it's relatively short it was extremely educating for me, for it's may be the only autobiography of this kind I know about. There is a french version:
Roustam Raza
"Souvenirs de Roustam, mamelouck de Napoléon", 1er.
Introduction et notes de Paul Cottin. Préface de Frédéric Masson (Paris, 1911) 302p.
Here is Napoleon with Roustam in the background:
http://digital.library.mcgill.ca/nap...g/8NIII058.jpg