NEW LONDON — Frank Gustafson spent six summers during college and law school with a chainsaw, a skid loader and a brush cutter, removing invasive buckthorn from 100 acres of oak savanna. The work is part of a larger-scale ecological restoration he’s pursuing with assistance from the U.S. Department of Agriculture’s Natural Resources Conservation Service. Gustafson is a third-generation attorney-by-day, farmer-by-night, and the third generation t…