No, it radiant damage, it burns but not like fire. I don't think a cantrip would be enough to set fire to a tree even if it did fire damage. As anyone whose been camping can attest to, getting wood to burn takes effort. Ideally you use dead wood that had a chance to dry, cut it up and arrange it in a specific shape, along with kindling and even fire starters. A cantrip can certainly char/mark a tree regardless of damage type, but to actually get it properly burning you need something stronger.