If you fill the bed under it with rock, eventually the falling pine cones and needles will need to be
cleaned/removed. If this is small rock and you use a leaf blower / vacume to clean/remove, then the rocks will
be disturbed more easily than if you had filled it with large rock.
An alternative is to bed it with a load of pine cones (these are going to fall off and collect under the tree anyway).
Or a load of bark wood chips will color complement the fallen pine cones well.