My wife bought a potted "Christmas Tree" last year in the week running up to Christmas. It wasn't in the best of shape when we got it and about a week after being inside the house, it shed the majority of its needles, leaving the bottom 1/3rd of the tree "naked".
Having moved it outside and letting it enjoy a lot of sun and rain, it's been putting out lots of new growth at the top of the plant and generally seeming a lot healthier. However, very little has happened in that bottom third. The very tips of the branches have produced some new growth, but there's no sign of regrowing needles anywhere on the tree.
Is there anything we can do to stimulate the tree to re-grow its lost needles and rejuvenate the sad parts?