the bark/the bite

I have two questions.
1. what is the reason the next dialogue is funny?
2, I'd like to know the meaning of "tree dentist" and "The bark was worse than the bite"

A: I had an uncle once who was a tree dentist.
B: ...A tree dentist?
A: Yeah, until he switched to dermatology.
B: ...Turns out The bark was worse than the bite.

Have a good day!

[Mister Micawber]

There is no such thing as a tree dentist (although there are tree surgeons). However, if there were one, he would work on the tree's 'bite' (which is meaningless, but the joke reader will skip over that realization). When he became a dermatologist, he would then work on the tree's 'skin', or 'bark' instead.

This leads us to the irrelevant proverb that a 'dog's bark is worse than his bite'. Playing on the polysemous nature of 'bark' and 'bite', the joke writer leads us to see why the tree dentist would change to the more lucrative occupation of dermatologist.

Haw, haw, haw! Get it?

[RonBee]

Polysemous?

(It's a funny joke, but you have to be acquainted with American (or British) idioms to "get it".)