Research finds link between invasive ferns, soil acidity

University Park, Pa. -- A researcher in Penn State's College of Agricultural Sciences has found a strong correlation between acidic soils and the prevalence of hay-scented ferns in Pennsylvania forests, shedding new light in the search for the reason desired tree species are not regenerating at historic levels in the state's woodlands.

Bill Sharpe, professor of forest hydrology, who has been doing acid rain research for more than 30 years, recorded the occurrence of hay-scented fern (Dennstaedtia punctilobula) in 18 stands of sugar maple trees in Centre, Huntingdon, Potter, Somerset and Sullivan counties. According to Sharpe, sugar maples (Acer saccharum), which are sensitive to acidic soils, are dying and not regenerating on hundreds of thousands of acres across Pennsylvania. Sugar maple mortality is happening, he points out, in areas where soils are extremely acidic and have low availability of critical plant nutrients calcium and magnesium, and high levels of aluminum, which is toxic to plants.

For decades, Pennsylvania has experienced the most acidic precipitation in North America, and forest soils became increasingly acidic over the last half of the 20th century. For his research, Sharpe selected sugar maple stands in areas with soils ranging from slightly acidic to very acidic. In stands of sugar maples in very acidic soils, he found the largest infestations of hay-scented ferns. The ferns are known to thrive in acidic soils.

"In areas where we had full blown sugar maple mortality -- where the canopy was only 60 percent due to large openings being created by sugar maple mortality -- we saw the highest prevalence of hay-scented ferns," Sharpe says. "Those areas also had the most acidic soils. There is no question that light reaching the forest floor was also a factor. Where the soil was most acidic, the canopy was less dense, and ferns thrived."

"Soil acidity and light -- we couldn't separate those two factors," Sharpe adds. "It's a chicken and egg situation. I'm convinced that both more light and soil acidity were strongly related to the abundance of ferns and to each other. Where the soils were most acidic, sugar maples were declining and canopies were not as thick."

The importance of his findings, Sharpe explains, is that they provide an alternate theory explaining why hay-scented fern is the dominant forest floor plant in declining sugar maple stands. Many scientists believe that overbrowsing of preferred plants -- such as sugar maple seedlings -- on the forest floor by too many white-tailed deer has left only hay-scented ferns, which deer do not prefer to eat. But Sharpe's research suggests that acidic soils caused by acid rain are at least partly to blame for fern prevalence.

"Some scientists blame the growing abundance of ferns on elimination of competing plants by deer overbrowsing," Sharpe says. "But I believe hay-scented ferns are well adapted to acidic soils, otherwise this plant would not be so successful. Deer browsing may be part of the reason hay-scented fern is proliferating, but there are other factors such as soil acidity involved as well."

Rain falling on Pennsylvania these days is not as acidic as it was a decade or so ago, says Sharpe, who with colleagues has monitored five Pennsylvania streams for the U.S. Environmental Protection Agency for the past 12 years to help evaluate the effectiveness of the Clean Air Act. "The rain is better, there is no question of that," he says, "but it has not yet translated into significant water-quality improvements."