When conditions are right the forest looks after itself. One of a foresters jobs is to make sure the balances are kept in check.
Among other things at the moment I'm trying to sort out a particularly dense patch of regenerated Ash trees that decided that they like growing with Oak. They have grown from seed from old hedge Ash trees that existed while the land was still in pasture. This is a great and a positive thing in relation to the forest floor but this patch is so dense I have to intervene or else some of the preexisting planted Oak will die. I want to encourage a diversified tree cover but sometimes you have to just give things a chance.
I've known foresters who originally planted Oak seeing Ash completely take over their stands and the Ash in their opinion was of a better quality than the Oak so they just cut down the oak and turned it into a Ash stand.
My oak that have survived are not of a poor quality so I'm trying to keep some Ash for cover/diversification while taking out the general overwhelming amount. It's a tricky thing to get right especially if you have very good regenerated Ash and medium quality Oak. The greenery to the left and right of the photos are the brashings, gives you an idea of the amount.