In all the bird's eye views of the street with Lester talking, all of the trees (except pine trees) are completely bare. But later when Jane and Ricky are walking home from school, all the trees have green leaves.
The movie takes place over the course of a year so it was quite possibly different seasons at different times. What did you think that Lester got in shape in a matter of days?
The movie chronicles the last year of Lester's life and when he is narrating (at the beginning and at the end) he is dead. The trees should go from losing leaves to bare. I think this is correct.
When Lester narrates, it is after when he is already dead. The movie takes place over the previous year, so I would think it would be a different season when they walk home.
The others are right on this point, but when Lester is jogging with the two neighbors, the leaves start out bare, but when Lester stops running to talk to Ricky the trees are covered in leaves again. This is all within one scene.