But some conifers and other trees theoretically could live forever, according to a recent essay that reviews accumulating evidence on extremely long-lived trees—and calls for more scientifically rigorous methods to determine their age and study their longevity. Across the board, trees do not die so much as they are killed, write the authors of the review essay, entitled “On Tree Longevity.” Their killers are external physical or biological factors rather than old age alone. That is, there is no evidence that harmful genetic mutations pile up over time or that trees lose their ability to produce new tissue. “Trees can indeed live indefinitely, but this does not happen,” says co-author Franco Biondi, an ecoclimatologist and tree-ring scientist at the University of Nevada, Reno. “Because eventually an external agent, biotic or abiotic [a living thing or a nonliving one such as a physical condition], ends up killing them.”