Sugar maple abundance already has dropped in parts of the northeastern U.S. and southeastern Canada over the past 40 years, primarily because of high acid levels in soils. The upper Great Lakes have mostly escaped the damage because the area’s soils are rich in calcium, which provides a buffer against acid.
But in an article published this month in the Journal of Applied Ecology, scientists said they had discovered another way that acid rain harms sugar maple seedlings in upper Great Lakes forests.
It causes excessive nitrogen buildups that prevent dead maple leaves from decaying after falling to the ground, hampering growth of new trees, said Donald Zak, a professor of ecology and evolutionary biology at the University of Michigan.
“The thickening of the forest floor has become a physical barrier for seedlings to reach mineral soil or to emerge from the extra litter,” Zak said. “What we’ve uncovered is a totally different and indirect mechanism by which atmospheric nitrogen deposition can negatively impact sugar maples.”
Nitrogen deposition from acid rain is expected to more than double worldwide by the end of the century because of increased burning of fossil fuels, researchers said.
Zak and his colleagues have placed sodium nitrate pellets at three test plots in Michigan for the past 17 years to simulate the expected increase.
The team found that the extra nitrogen boosted the volume of leaf litter on the forest floor by up to 50 percent, causing a significant drop-off of sugar maple seedlings taking root.