The sugar maple is the most economically valuable tree species in the eastern United States because of its high-priced lumber, syrup and tourist-attracting fall colors.
The study, whose lead author, Stephanie Juice '04, was an undergraduate when the research was conducted, suggests that because acid rain makes the soil more acidic, unfavorable conditions are created for sugar maples. In acidic soils, sugar maples produce fewer seedlings that survive and mature, and more adult trees die, the researchers found. They drew these conclusions after adding nutrients to soil in a test plot and reproducing the favorable soil conditions that existed prior to 20th century industrial pollution. The result: Sugar maples on the plot rebounded dramatically.
The study provides "the most conclusive evidence to date" that the decline of sugar maples is linked to the effects of acid rain produced by human activity, said Timothy Fahey, professor of forest ecology at Cornell and co-author of the study, which is published in the May issue of Ecology. Juice wrote the main part of the paper as part of her senior thesis.
"The research addresses how a long-term, human-caused change in the environment is affecting sugar maples, which are valuable both ecologically and economically as one of the dominant species in our region," said Juice, who now works for the Institute of Ecosystem Studies in Milbrook, N.Y., as a project assistant and is applying to the Peace Corps for next year.
The research was conducted at Hubbard Brook Experimental Forest (HBEF), a 3,160-hectare reserve near North Woodstock, N.H., where scientists have measured soil composition for the past 50 years. The scientists added nutrients in a test plot to replicate soil conditions that existed prior to the loss of sugar maples over the past 25 years.
Nitric and sulphuric acid in acid rain leaches calcium from the soil. Calcium is the second most abundant plant nutrient (after nitrogen). In addition, the loss of calcium leads directly to more acidic soils. When soils become too acidic, such trees as sugar maples become stressed and have a harder time growing or producing seeds and seedlings.
"Because of the detailed measurements for the last 50 years, we know almost exactly how much calcium was removed by acid rain," said Fahey. "Our treatment was designed to replace just that amount of calcium to bring the system to what it was like prior to the acid rain era."
The study used two 10-hectare (25-acre) watersheds. On one 25-acre site, a calcium-rich mineral called wollastonite was spread in pellet form by helicopter in October 1999. The other site served as a control.
While the pellets dissolve slowly over five to 10 years, the researchers were surprised to find that by summer 2002, the soil acidity in both the top and lower layers of the test plot neutralized from being highly acidic to more acceptable levels for sugar maples. The researchers also found that by the second year, calcium levels in the maples' leaves had risen.
Acid rain also increases levels of the trace nutrient manganese in the soil, which can be toxic to trees in higher doses. By year four and five, manganese in the leaves had declined to healthier levels. In addition, seed production and the density and size of sugar maple seedlings all increased in the few years following treatment, compared with the untreated neighboring plot.
The researchers also found that the communities of mycorrhizae -- soil fungi that help provide more nutrients to plant roots -- were substantially greater around the roots of both seedlings and mature sugar maples in the treated plot. Future research will explore the relationship between mycorrhizae and acid rain.
Though pollution-control legislation has helped reduce sulphuric acid by one-third since its high point in the 1960s, nitric acid from automobiles has not significantly declined.
This research was supported by the National Science Foundation and the Hubbard Brook Ecosystem Study.
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Journal
Ecology