Growth of large oak trees not obstructed by intervening block

If I place a spruce wood block such that there are 6 air blocks between it and the grass/dirt below, and place an oak sapling on the ground beneath the spruce wood block, a large oak tree will sometimes grow in snapshot 13w36b, whereas after a fair amount of repeated testing, I do not believe this will occur in 1.6.2. When the large oak tree does grow in this situation, the spruce wood block is replaced by an oak wood block that is part of the oak tree. This situation occurs when the spruce wood block is in either the vertical or horizontal orientation.