If an oak woodland has matured and closed its canopy, then there will be no light reaching the floor, hence fallen acorns cannot grow into saplings. Oak has a fascinating history in the UK - it used to regenerate within woodlands easily, but now takes the opposite strategy. As for oak dominance, it is all about light - see how beech suddnely appears when oak is felled: the seed was always there, but never got the chance to grow due to oak preventing light penetration.