Answer: In my experience, if you store them dry in the bags, they will be in essentially the same condition in spring as when you put them into the bags. If you want them to break down somewhat over winter you will need to shred them, add some moisture (approximately enough to feel like a damp, wrung out sponge) and a source of nitrogen (such as fresh grass clippings or a nitrogen fertilizer) to the bags and seal them up, then keep them in a nonfreezing location all winter. Roll the bags around periodically to try to mix the ingredients a bit. A warmer location will help them compost faster since composting slows with cooler temperatures. Come spring, they should be partially composted and can be used as mulch. Leaf mold (composted leaves) is an excellent soil amendment, too.
Alternatively, you can shred them and use them as mulch now, or put them in a pile to rot down on their own and use them next spring or whenever they are rotted enough to suit you. (Shredding will speed up the process, as will following general composting practices.)
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