When Lewis and Clark crossed America's heartland, they tramped through wide swaths of oak forests. But today, the oaks are in decline, and as they vanish so too does a whole coterie of native herbaceous plants critical to forest ecosystems, according to a unique analysis, which lays the blame on efforts to suppress fires. "We're losing the plants that characterized a rich and diverse ecosystem that existed here for thousands of years," says Thomas Rooney, an ecologist at Wright State University in Dayton, Ohio, and one of the study's authors.
Although other studies have recognized that the oak trees are in decline in the East and Midwest, this new work is the first to look at how the loss of the oaks has affected the understory plants, those that grow alongside the trees. Such research is usually impossible because scientists don't have the data necessary to compare present understory plants with those of the past. But in 1949 and 1950, John Curtis, an ecologist at the University of Wisconsin, Madison, and his colleagues carefully surveyed the flora of southern Wisconsin's oak forests. "Curtis's book is an iconic record," says lead author David Rogers, an ecologist at the University of Wisconsin, Parkside. Rogers and his colleagues resurveyed 150 of the original sites after suspecting that the increasingly fragmented oak woodlands have lost more than just trees.
In the 50 years since Curtis's study, two species of red oaks have declined nearly 50%, the team's count reveals, and white oaks have dropped more than 31%.
Even more striking are the changes in the understory species. Fifteen percent of the 200 native plants Curtis recorded are now gone, while non-native species are moving in, the team reports in the current issue of Ecology. Exotic plants were found at only 13 oak stands in 1950, while today they occur at 76 of the sites. Shade-tolerant vines and shrubs have supplanted the flowers, grasses, and sedges that lived in the sunnier oak forests. Similar changes have likely occurred in the understory of the oak forests of other Eastern and Midwestern states where oaks are also in decline, says the team. The native plants are crucial to the forest ecosystem, Rogers says, because they provide food for a host of other species, from wild turkeys to insects.
What's driving the changes? Lack of fire is the primary factor. Fire-resistant oaks need sunlight to sprout and thrive. Yet older oaks cast shade that enable maple seedlings to grow—and maple trees block the light that young oaks need. Fires help oak trees by removing the maple seedlings and opening up clearings for acorns to sprout. They are also believed to kill oak pathogens (ScienceNOW, 9 August 2004).
The Native Americans who once inhabited the East and Midwest oak forests regularly set fires to maintain the trees, which provided acorns, one of their food staples. But European and American settlers suppressed the fires, mistakenly believing that burning harmed the oaks. "The oak woodlands need to be disturbed to stay healthy; fire is one of the best ways to achieve that," says Rooney.
The oaks also suffer from deer eating their sprouts and acorns. Without wolves or other predators, many of which have been wiped out by hunting, the number of deer in these regions has soared.
"These are grim trends," says Gregory Nowacki, an ecologist with the U.S. Department of Agriculture Forest Service in Milwaukee, Wisconsin. "To counteract them will require active management," such as prescribed burning and reducing white-tailed deer populations.