VVM,
The issue of competition and the issue of protection are two different issues; however, they are related. Competition deals with the battle for water and nutrients (sunlight / not an issue in open pasture) that occurs between a tree and surrounding vegetation (grass, weeds, grains, other trees, etc). It effects growth rate substantially; therefore, it is usually most problematic for a younger tree. Studies at KS University have shown competition can retard the growth rate of a young tree by as much as 60-70%. Your oak tree likely won't die ... it will just grow veeeery slowly. For me, chemical control or the use of mulch represent the best methods to ensure growth potential is realized. Protection, on the other hand relates to damage to trunk/limbs by a variety of critters (deer-eating or-rubbing, mice, rabbits, squirrels-if-you-direct-seed, voles, etc), and is essential - I believe - if your DPSQ (density) is relatively high. I'm a cage (6' dia - one and done) plus mulch (fabric-pea gravel-mulch) planter. Higher cost, more work ... but again, one and done! 20 trees a year per property adds up quickly. How are protection/competition related? The more vegetation, and its height and closeness to tree trunk, provide mice/rabbit habitat and better security (hides) from predators for critters who damage trees. That's my take on it.