Craig G. Lorimer
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 seems reasonably clear: Oaks are gradually relinquishing their long-term historical dominance in forests of eastern North America, except on the less productive sites. Sudden events such as careless logging or windstorms can convert an oak forest to one dominated by other species almost overnight, but slower tree-by-tree replacement is occurring even in parks and reserves.
Oak regeneration troubles have been the topic of several review papers, but in the article beginning on page 927, Marc Abrams focuses on a particular trend that has received little attention. Abrams points out that in the presettlement forest, white oak was generally the premier species, but in the past century it has suffered steeper declines than other oaks. Using several lines of inquiry, he reviews evidence on the original physiographic distribution of white oak, its patterns of recruitment, and its relationship to fire history. He also considers physiological attributes that enabled it to prosper in earlier times but that also may have rendered it more susceptible to subsequent environmental changes.
What should we do about the trend that Abrams describes? First, we need long-term experimental evidence to clarify the relative importance of causal factors. For example, solutions to the problem might differ markedly, depending on whether fire suppression, excessive deer, or climate change is determined to be the most important cause of the decline. Further evidence may also influence our perspective on the problem. Lightning fires are uncommon in oak forests, and so if reduced fire frequency is a major cause, perhaps we are witnessing the natural restoration of late-successional forests after thousands of years of anthropogenic fires. Nature, in other words, may want more maples and beeches, not oaks.
If appropriate oak restoration techniques can be identified-and there are encouraging signs that they can-the public must decide whether to implement these programs and cover the cost. Historically, the small private landowners who hold much of the eastern forest have been reluctant to invest time and money in their woods. Even on public lands, methods for perpetuating oaks, such as prescribed burning and creation of sizable openings in the forest, are sometimes unpopular and provoke significant opposition. As is so often the case in scientific matters and public policy, the future composition of the eastern deciduous forests may be decided largely by politics, budgets, and human motivation.
CRAIG G. LORIMER
Department of Forest Ecology and Management
University of Wisconsin-Madison
Madison, WI 53706