There's no reason to let your out of commission fireplace go to waste - there are plenty of creative ways to use the space and make it as visually appealing as a fire, or maybe more so. Here are some ideas for decorating your unused fireplace.
You can take the fire out of the fireplace, but maybe consider leaving the logs in. Arranging birch logs lends a rustic element, and what's more fitting than stowing them in your hearth? You can stack them as though they're prepared to make a fire, or stand them up on varied risers to give the space some dimension. Real Simple recommends putting a wooden crate in your fireplace with the bottom facing outward, then cutting a collection of logs to about one-fourth of an inch less than the opening of your fireplace and stacking them until the crate is out of sight. This is a creative way to bring nature into your home as a design element.
Your empty hearth can also be a valuable space for storing books. Consider piling them up to different heights, or installing shelves within the fireplace so you can also use it for vases or picture frames.
Rather than putting parts of trees into your unused fireplace, why not use the whole plant? Turn the area into a mini garden, but be sure to use plants that don't require a lot of light, like mosses and ferns. The plus side of keeping greenery on the hearth is that you can let the vines spill out over the sides of the pots, as they'll still be contained in some way.
You may not be able to have a full-fledged fire, but you can always keep candles in your hearth, so long as they're out of the way of your custom window treatments! There are several ways to go about this: You could place an antique candelabrum in the center and use tea candles or tall thin candles, or you could use large pillar candles. The best visual effect would be to either keep them absolutely level, or to strategically vary the height. Modern Housing Insight magazine recommends putting the candles on top of birch logs and leaving some on their own. You can use risers to make the candles in back higher and create facets.