Examples specific to the 2012 film adaptation go in their own page.
That wasn't the last truffula seed
The Onceler had a change of heart but never actually learned how ecology works. That's why he didn't think twice about trying to repopulate the truffula trees from a single seed (which would create a severe genetic bottle-neck even if truffula trees reproduced asexually). He assumed a seed he found in his factory was the last one because the environment outside looked like a barren wasteland. In reality, there was a healthy, diverse seed bank left behind from bar-ba-loots eating the truffula fruits.The seeds remained dormant until conditions were habitable again. Thneedville was clean and well-lit enough but there was no exposed soil until Ted made room for the Onceler's seed. That seed was the first truffula sprout. However, others came up while it was still a sapling. When the people started to reduce their pollution and clean the surrounding areas, more truffulas sprung up outside of Thneedville. None of them were the offspring of the first tree which was still too young to fruit.
The 70's cartoon adpatations of Cat in the Hat and The Lorax are set in the same universe
.The boy who gets visited by the Cat in the Hat is Ted's son or other relative, being identical except for hair color. Their fish is a Humming Fish, meaning he can speak and breath out of water because he had already done it, along with the other fish, when leaving the truffula forest. Also, there are regular trees in Cat in the Hat, meaning whatever happened to them for the city to be damaged even when it didn't have the Truffulas (which the Once-ler had never seen before coming to the forest, implying they only existed there), it went back to normal. The book is a kid-friendly adaptation of “The Colour Out of Space”.
No, really, think about it for a moment. Both stories begin with an outsider coming to an area that is completely polluted and hostile to life, who learns about the whole tragedy from the last surviving person to have seen it first hand. The Once-ler is Ammi Pierce. The Truffula forest was once the Gardner farm, and the Gardner family represented the Truffula field’s wildlife. Both start out as lush, unspoiled nature before they are destroyed. The meteor carrying the Colour is represented by the Once-ler creating the first Thneed, thereby beginning The Corruption. Remember, the Colour was only unloosed upon the world once Ammi and the scientists broke open the meteorite. At first, things are fine, as the Once-ler’s business grows, and the Gardner farm’s crops grow huge. But then troubles happen, as the Colour has poisoned the ground, making the crop inedible, and the Lorax warns the Once-ler to stop immediately. But he doesn’t listen. But as this continues, the animals start to leave — a euphemism for death — in several successive waves as things get worse and worse, the last one being the fishes in the lake fleeing. Similarly, Nahum Gardner’s wife, then his three sons, then himself go mad and then die, with the last warning before the point of no return being that the Colour has infested the water supply. After Nahum dies, Ammi must flee before the Colour drains the entire farm—now the blasted heath— and ascends back into the sky above. Likewise, the Once-ler is alone in the former Truffula forest— now the Grickle-grass field— when the Lorax lifts itself up and flies away in the sky.