@eye-of-horus-42

Create a rectangular triangle over each side.

Afterwards the trees stand in the center of the square area.

Yes, creating triangles on each side will do it.

Each triangle base is the same size as one side, and extend the height of each triangle out 1/2 that. (This gives you 4 right triangles with the hypotenuse of each attached to a side of the square.) What results is a space exactly twice the original area. (With the trees outside the area.)