Tales of a Fourth Grade Nothing is a hilarious book. I find it funny for a variety of reasons, but each reason more or less comes back to the fact that Peter is a hilarious narrator. He moves from topic to topic at a blinding speed, but it makes sense. Maybe it makes sense because that is what my own first grade child does. It's complete stream of consciousness from my own kid, and that's what Peter does do.
For example, the answer to your question can be found in chapter four. The chapter begins with Peter talking about mugging. At one point, he admits that he doesn't think being mugged would be that bad, because he would get to go to the police station and look at mug shots of criminals. Peter stays on topic for a bit and talks about how his dad has been mugged and why his mom won't take the subway anymore. Mugging is a topic on Peter's mind, because woven through the mugging scenarios is Central Park. What else is in central park other than muggers? Trees of course. Peter rapidly shifts from mugging to his description that New York's tree leaves just turn brown and fall off. They don't change to red and orange like country trees because of all of the pollution in the city.
In the fall the leaves turn darker and drop off the trees. Sometimes there are big leaf piles on the ground. It's fun to jump around in them. I never saw bright red, yellow, and orange leaves until the day my father took us for a drive in the country. The reason the leaves don't turn bright colors in New York is the air pollution. And that's too bad. Because yellow and orange and red leaves really look neat!
The answer to this is that, according to Peter, leaves in New York City do not turn bright colors in the fall because of how there is so much air pollution in the city.
You can find this answer early on in Chapter 4. It is on page 31 in my copy of the book -- this is the third page of the chapter. Peter says this when he is talking about how he likes to jump in the leaves in the fall. He says that he never knew that leaves turned bright colors until one time when he and his family went driving in the country.