Plant ecologist Mark A. Davis will not participate in this year’s “Buckthorn Roundups” around his St. Paul, Minn., neighborhood. ­Davis will not tag along as these intrepid crusaders set out to eradicate the common and glossy buckthorn, two ornamental shrubs imported in the 19th century from Europe. The nonnatives have now taken over some Midwestern forests, prairies and wetlands. That is why eco-minded volunteers eagerly wrench young weeds from the soil, hack away at thick stems and douse remaining stumps with herbicides. Their hope: a return of Minnesota to its primeval state.