John Bunker normally searches for heirloom apple trees in the fields and forests of rural Maine, but on a trip to Boston, he stumbled upon one in an unexpected place: an ice-cream-parlor parking lot. An expert on American heirloom apples, particularly those of Maine, Bunker has been investigating, preserving, and growing nearly forgotten apple cultivars since he graduated college and immediately bought a parcel of Maine farmland in the 1970s.“I could spend the rest of my life studying, tracking down, learning to identify, and preserving historic apples from Maine, and…