They look like arrowwood, or Viburnum dentatum seedlings. Did you start them from seed?
This is pretty common native plant in Pennsylvania, with a lot of value for birds and butterflies. It has been decimated by the arrival of viburnum leaf beetle, but seems to be making a recovery as beneficial insects are helping to control that non-native pest. Here is more information: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=m720