MT. AIRY, Md. — Standing in a 40-acre field of Christmas trees, Lisa Gaver traced the path of the high-voltage power line that could one day cut through the heart of her family’s farm. The 500-kilovolt line would skim a parking lot, cross through the woods and over a metal deer fence, before running diagonally across a field of Douglas firs and blue spruces and continuing as far as the eye could see. Gaver, a seventh-generation farmer, wants no part of what she and other landowners in rural