Here's some more information that arrived via a listserve:
"Timothy Block of the Morris Arboretum near Philadelphia wrote: Aralia elata (Japanese angelica tree) is very prickly and becoming common in the woods .... This plant is a rapidly spreading invasive. In most cases, it was formerly misidentified as Aralia spinosa (devil’s-walking-stick) which is native to western and central PA and widely cultivated. The only completely reliable way to tell the two species apart is by the structure of the inflorescence. Aralia spinosa (the native) has a pyramidal inflorescence with a long central axis, while the inflorescence of Aralia elata (the Asian species) has a short central axis attached to which are long branches, giving the inflorescence the appearance of a fireworks burst. In both cases, the inflorescence may be three feet or more across, bearing thousands of flowers and fruits. The seeds are bird-dispersed."