If you attempt to grow a sapling on grass with bone meal in a place where a tree cannot grow (with a block on top of the sapling, for example), the grass beneath the sapling will be converted to dirt even though no tree spawns. This works with Oak, Birch, Spruce, and 1x1 Jungle saplings, but not with 2x2 Jungle, Acacia, or Dark Oak (obviously 2x2 as well) saplings.