From Reddit:
Reader: The tree on top of Shutenmaru's head is modelled after the Rakugo [Head Mountain] right? In Oda-sensei's case, where would your tree sprout from?
Oda: That's correct! The Rakugo [Head Mountain] program is a strange story where Sakura trees sprout from the head of humans. They explain that as the reason for why people like to gather around for flower watching. It's quite the surreal story. Eh, in my case, if you ask where my tree would sprout from the answer is obvious. My anus!! Let's see everybody come for flower watching!! Hm? I can hear police siren....