What is a "Tree hugger"?

So I was having a conversation after school with my 9 year old son, Riley.  He was talking about his day and he said to me it was an efficient way to waste a day.  I asked him why.   

He said, “Well, we went to DNS.”  

DNS is our cities nature studies. 

I said, “That sounds like it would be fun to me, not a waste of a day."

He said, “Yeah, but we were taught by a bunch of tree huggers.”  

“What’s a tree hugger?” I asked teasing him.   

“You know, someone who loves the earth and doesn’t want to destroy it.”  

 “I love the earth and don’t want to destroy it,” I replied. “Does that make me a tree hugger?”

His answer, “Well, a tree hugger is a DEMOCRAT who loves the earth and doesn’t want to destroy it.” 

I see I’ve taught my son well.