Q: What do you get when you cross a bear with a forest?
A: Fur trees!

Fir tree with a pinecone (Needpix.com)

Explanation: Can you imagine a tree with fur growing on it?  No, fur grows on animals, specifically, on mammals. Trees can’t grow fur, but bears can. All bears have fur covering their bodies.

Trees, on the other hand, grow leaves.  Even pine trees have leaves because their needles are a type of leaf.  Fir trees are a type of pine tree with needles.

This joke is funny because it plays with the words fur and fir which sound the same.  Thinking about a fur tree, a tree with fur, is funny.

Here are some bears in trees (and there are fir trees nearby, I think):