Aspen Sapling Defoliation
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Do you see any leaves at the base of the sapling? Or are they completely gone?
Could you send a closeup of the ends of some of the newly defoliated stems? Is there any damage at the base of the leaf?
My first hunch would be that you have a vertebrate pest that is eating the leaves, and putting a cage that protects the length of the sapling would be a good way to prevent further damage.