Symbolism, Imagery, Allegory
Ben knows a lynching tree when he sees one. Or he does once Abraham shows him one anyway, because prior to meeting up with Abraham, all Ben actually notices is segregation. Oops. But then Abraham comes along and shows Ben what racism actually looks like in Eudora:
A cool grotto tucked back in the woods away from the road. Big branches interlaced overhead to form a ceiling. The dirt was packed hard as a stone floor from the feet of all the people who had stood there watching the terrible spectacle […] Even without his guidance, I would have recognized it as a lynching tree. There was a thick, strong branch barely a dozen feet from the ground. The low dip in the middle of the branch was rubbed free of its bark by the friction of ropes. (34.4)
Okay, so Ben has seen a lynching tree before. And because of this, while he needs Abraham to show him where one is, he doesn't need Abraham to explain what he's looking at. As he describes the tree here, we see evidence of human intervention in nature—the "branch […] rubbed free of its bark by the friction of ropes," and the "dirt packed hard as a stone floor from the feet of all the people who had stood there watching." These sort of scars left by humans remind us that lynching is a human intervention in nature, too; it's humans choosing when other humans die instead of nature running its course.
As much as this passage makes it clear that lynching isn't natural, though, it also makes it clear that the people of Eudora are big fans. A tree doesn't lose its bark because just one rope's strung around it, nor does the earth get tamped down so much by a single gathering. This tree shows wear, making it clear that lynching is an oft-repeated and socially-supported event around these parts. Yikes.