That's a good question.
Ultimately (probably not in my lifetime) someone is going to harvest these oaks for sawboards or veneer. And the idea is to have at least nine feet of clear lumber - wood that is largely free of knots. Eighteen feet is even better.
On the larger and more robust oaks they've been pruned to somewhere about chest to six feet in height.
So we're making progress. Never take more than a third-off at any one time.
Before...
And after...
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