Using replicate or extrapolate or manually placing the trees will consume a lot of memory. Using ecosystem instances is much more efficient, memory wise. Here's how I would do this:

Create one cube and use the replicate or extrapolate command it in order to have 30 cubes in the place where you want the trees to be. Then, using the ecosystem paint command, paint one tree at a time right next to each cube.

EDIT: in the end, of course, delete the cubes. :-)