The Cars Universe has several trees and forests present in the movies.
Shouldn't they die from all the environmental pollution from so many cars moving all day? How could trees exist in world of living metal?
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One approach could be:
Cars universe is basically human world, humans being replaced with various automotive types.
Ex: Italy is populated by popular Italian cars,mostly. And some cars have accent like humans do.
So about trees, Trees are part of life as we know it. So there should be trees. As you see, environmental pollution is everywhere, but there are still trees around us,isn't it?
And taking some points from movies, Some cars are using organic fuel (I forgot the name of the car, who lives in Radiator springs and friends with Mater and Mcqueen), Some converting themselves into electric cars.(in Cars 2, Sir Axelroad was faked but it shows some kind of notion that converting own-self to electric is very common thing) Most of the new cars are without need of bio-fuel (Mater's girlfriend,again I forgot the name).
So there should be no such huge environmental pollution as you suggesting. And as I mentioned previously, Trees are basic sign of life form anyone can identify and familiar with. So where there is life, to make it familiar to targeted audience(us humans) there should be trees.
Think for a minute, there is a world with only talking cars and basic buildings. will you like it? Will it be a good story to watch? Will it be familiar to you,your environment anyhow?
Those are the key points that made cars franchise so popular.
Some can argue that trees in that universe are little bit different and they can live with higher percentage of carbon-dioxide in the air.