With the exception of trees, most plants that establish after their taller peers do not try to dominate the canopy. They have already lost that fight. Instead they either stay low and work with the limited light available, or they aggressively grow in the spring to complete their entire lifecycle before the deciduous canopy closes at the beginning of summer. They might steal a little fertilizer, and some water, but the jury is still out on whether some plants conserve as much water as they use. It's a fuzzy enough area that you can find people who claim that some plants increase total available water.
We are just beginning to fully appreciate all of the ways trees have to ladder up from the forest floor to the canopy, and some of the things we perceive as competition may be a misunderstanding.