ANN ARBOR, Mich. -- The number of sugar maples in Upper Great Lakes forests is likely to decline in coming decades, according to University of Michigan ecologists and their colleagues, due to a previously unrecognized threat from a familiar enemy: acid rain.
Over the past four decades, sugar maple abundance has declined in some regions of the northeastern United States and southeastern Canada, due largely to acidification of calcium-poor granitic soils in response to acid rain.
Sugar maple forests in the Upper Great Lakes region, in contrast, grow in calcium-rich soils. Those soils provide a buffer against soil acidification. So sugar maple forests here have largely been spared the type of damage seen in mature sugar maples of the Northeast.
But now, a U-M-led team of ecologists has uncovered a different and previously unstudied mechanism by which acid rain harms sugar maple seedlings in Upper Great Lakes forests.
The scientists have concluded that excess nitrogen from acid rain slows the microbial decay of dead maple leaves on the forest floor, resulting in a build- up of leaf litter that creates a physical barrier for seedling roots seeking soil nutrients, as well as young leaves trying to poke up through the litter to reach sunlight.
"The thickening of the forest floor has become a physical barrier for seedlings to reach mineral soil or to emerge from the extra litter," said ecologist Donald Zak, a professor at the U-M School of Natural Resources and Environment and co-author of an article published online Dec. 8 in the Journal of Applied Ecology. Zak is also a professor of ecology and evolutionary biology.
"What we've uncovered is a totally different and indirect mechanism by which atmospheric nitrogen deposition can negatively impact sugar maples," Zak said.
The new findings are the latest results from a 17-year experiment at four sugar maple stands in Michigan's lower and upper peninsulas.
|Contact: Jim Erickson|
University of Michigan