In the Lorax Once-Lers are moving to the home of the Loraxes. There he sees truffle trees and decides to make clothes with the leaves. Old man Once-Ler calls his family and they call friends until all the trees are gone and all the sky's are gray from the smoke. The Lorax tries to stop them but they simply move him away from that spot. In the end there are no more trees or animals left in that area.
My first implication is that I think that Dr. Seuss was talking about environmentalism and how humans were destroying nature. I think the Lorax was added because very few people were trying to stop environmentalism like the Lorax was the only one trying to stop the the Once-Lers from cutting down the trees. The Lorax wanted them to stop but the Once-Ler kept inviting more and more people until finally where all the trees were there was now manufacturing companies.
"Then we heard the tree fall.The very last Truffula Tree of them all! No more trees. No more Thneeds."
My second implication is that Dr. Seuss is talking about how our factories are destroying nature and animals homes with pollution. In the story all the skies turn gray and all the animals have to move away because of the smog. Like now our Ozone layer is being destroyed by the smog that factories set off when they produce something. In the story the Lorax talks about how the Once-Lers were destroying all of the nature and how he had to move all the animals away because the smog was suffocating the animals.
"Once-ler! You're making such smogulous smoke! My poor Swomee-Swans... why, they can't sing a note! No one can sing who has smog in his throat."