Symbolism, Imagery, Allegory
What a waste. The black and yellow striped trunks of the Truffula Tree are just left to die, and for what? The Once-ler doesn't ship them off to the paper mill, or use them to build things. Even his shop/home and factory are made mostly of rocks. (The illustrations make that pretty obvious.)
Now head over to lines 71-78, where the Once-ler has his first interaction with a Truffula Tree. He obviously isn't attracted by the trunks. Nope, he reaches out of his Once-ler Wagon and grabs him a big hunk of Truffula Tuft. That's right: he successfully removes the tuft, without cutting down the tree.
But next thing you know, he's inventing the "Super-Axe-Hacker," and taking out trees four at one pop. Any kid (or Shmoopy adult) can see that it's possible to remove the tuft without cutting down the tree. The genius Once-ler easily could have invented a machine to do just that. Instead, the wasted trunks become a symbol of the Once-ler's shortsightedness and limited vision.
And hey, even if he was going to cut down the whole tree, he could have made use of the trunks, too. This guy has clearly never heard of Head to Tail.