Forest pathologist Martin MacKenzie strode forward on a narrow path through California’s mythic bristlecone pine forest in the White Mountains near the Nevada border, methodically scanning gnarled limbs for the invaders that threaten the lives of some of the world’s oldest trees. These intruders are bark beetles, a menace smaller than a pencil eraser, but they bore by the thousands into the bark and feast on the moist inner core, where trees transport nutrients from roots to crown. Then they carve out egg galleries, where hungry larvae hatch. A blue stain fungus carried in by the pests deliver…