carbon12,

That's a good question. I spent some time trying to figure it out myself when I first encountered it. I'm not 100% certain, but I think it is what is called a Hercules Club Tree (Zanthoxylum clava-herculis). Also known as Prickly Ash, Tooth Ache Tree, Tingle Tree. Not all Hercules Clubs' bark are as densely knobbed as this one.