Forest pathologists often use the word decline to refer to a specific disease complex responsible for a gradual deterioration in the health of individual trees. But in the past century, decline of a different sort has been affecting entire oak ecosystems on a subcontinental scale. The cause is usually not a pathogen, but rather a combination of factors still imperfectly known. Fire suppression, increased consumption of acorns by burgeoning mammal populations, herbivory of seedlings, exotic pests, and climate change have all been implicated.

As with other scientific issues involving long-term change, definitive evidence is not easy to come by. Data obtained in a typical two- to three-year thesis project are often inconclusive or even misleading unless gathered as part of a long-term study. Despite these limitations, the implication of scores of studies...