I have 3 Korean Starkers Dwarf balsam firs that have suddenly dropped virtually all of their needles in the last week. They are in partial shade within the canopy of an large oak tree. I don't see bugs on them or any sort of scale. I put down cocoa bean mulch (just an inch thick) a couple of weeks ago --- could it be related to the mulch, or is there something going around? I'd appreciate any insights.

Laura