Hello,
New aspen stems won't grow under old stems, even if there is enough sunlight, unless you cut the old stems. If you cant cut the old stems, new aspen is very easy to propagate from roots, which you can then replant in the middle of the grove given there is enough light. If there is almost any canopy cover at all the aspen will likely not take. If you have any more questions, just let me know.
Source: I am a forester who has done aspen management
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