I think what you are looking for is a cover to keep debris out in the summer?

If there is any such thing -- other than the winter covers you mention -- I haven't seen it. The problem is, a cover that would do what you want would be as heavy and expensive as the winter covers . . . and also as hard to get on and off.

Other than a lot of skimming and vacuuming, I'm not sure what the best way to care for a pool under a bunch of old oaks would be. I love trees, but I don't like them close to pools I'm servicing!

Ben