A student asked me what the rarest tree in the world is. I consider myself somewhat knowledgable about trees, but wasn't sure what to say. I've heard Ginkgo is extinct in the wild, but it's pretty commonly cultivated so surely it's not the rarest in terms of absoute numbers. Then I thought about Wollemi, which i think is just one population in a remote part of Australia, but then again it's been cultivated too. Any ideas?