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