Several missives came from as far afield as Russia, Singapore, Brazil, Denmark and Hong Kong - and some admit they have never even visited the city.
"Dear Algerian oak," one letter sent in February read. "Thank you for giving us oxygen. Thank you for being so pretty. I don't know where I'd be without you to extract my carbon dioxide."
Another letter was written from the perspective of a tree in the United States. It said: "How y'all? Just sayin how do ... Y'all can call me Al. I'm about 350 years old and live on a small farm in N.E. Mississippi, USA."
It continued: "I'm about 80 feet [24m] tall, with a trunk girth of about 16 feet. I don't travel much (actually haven't moved since I was an acorn). I just stand around and provide a perch for local birds and squirrels."
Among the most popular trees in Melbourne is ID 1028612, a 70-year-old golden wych elm on Punt Rd. One admirer said the wych elm reminded them of their "slightly strange" old driving instructor - who "smelled like cat food" - because he used to tell them it was his favourite tree.
It is feared that four in 10 of Melbourne's trees will be lost in the next 20 years because of old age, though the city has tried to mitigate the decline by planting more than 12,000 new trees.