For AO, reducing from 2048 to 1024 doesn't introduce too many artifacts. Imagine how big the structure is on screen for 99% of a match. It'll probably take up not much more than 256^2 on a typical 1080p monitor at any given time, so a 1024 AO map is most likely adequate.
For the trees, it might be cool to put a dummy object there when baking to simulate the presence of the average sized or smallest size cypress that goes there, but that step is up to you.