> Look at this image of Pitfall, and notice how the tree canopy, tree trunks, ground, pit, and underground cave all show left-right symmetry. These are all built from the playfield (plus additional tricks to be described later). The only sprites are Pitfall Harry, the vine he’s swinging from, the rolling log, and the scorpion.

I read in a similar article that the branches of the trees are too finely detailed to be drawn with the playfield layer. So, they are added on with sprites.