Here's a plant I have never seen here before now: Japanes angelica-tree (Aralia elata), very similar to our native Hercules' club or prickly-ash (Aralia spinosa) that is not found here. Don't let common names fool you—the native Zanthoxylum americanum also goes by the names Hercules' club and prickly ash but has pinnate leaves not bipinnate like the aralias. That's one leaf—and not even all of it—spread out on a table in our visitor center above.
I believe we have the Japanese species since the veins go all the way to edges of of the leaflets, per Rhoads and Block, Flora of Pennsylvania, page 152-3. The other differences can only be seen when the tree is in flower.
I don't know how it got here, other than by bird-dispersed seed. It can be found in the area where we have created a demonstration site of old-growth acceleration and are promoting oak regeneration—the disturbance of prescribed fire and selected thinning has created the conditions for it (Rhoads and Block note that it is "naturalized in disturbed woodlands; especially SE [Pennsylvania]").
Look at the thorns at the base of each pair of leaflets and even on the midrib of each leaflet. This species is not noted as terribly invasive—though I recall there is a lot of it in the Wissahickon Valley of Philadelphia's Fairmount Park—but we will probably remove it since it isn't native or desirable in our woods.
