Acid rain has long been known to acidify the calcium-poor soils in parts of the Northeast, resulting in declines in sugar maple and other important tree species. New research by ecologists at the University of Michigan has identified another threat from acid rain that could cause an even greater decline of sugar maple forests in coming decades.
Scientists led by Donald Zak have concluded that excess nitrogen from acid rain slows the microbial decay of maple leaves on the forest floor, resulting in a build-up of leaf litter that creates a physical barrier for seedlings trying to reach sunlight and a barrier for their roots seeking soil nutrients.
“We had previously documented that experimental nitrogen deposition at rates that will happen by the end of the century slowed the decay of plant litter,” Zak said. He reasoned that if the thickness of the litter layer is increasing, the sugar maple seedlings will struggle to survive.
According to Zak, nitrogen deposition from acid rain is expected to more than double worldwide by the end of this century, due to the increased burning of fossil fuels. For the last 17 years, Zak and his colleagues have added sodium nitrate pellets to three test plots in each of four sugar maple stands in Michigan to simulate the amount of nitrogen deposition expected by the end of the century.
He found that the extra nitrogen increased the amount of leaf litter on the forest floor by up to 50 percent, causing a significant reduction in the successful establishment of sugar maple seedlings. The abundance of second-year seedlings declined from 13.1 stems per square meter under present nitrogen conditions to 1.6 stems per square meter under simulated nitrogen deposition. The abundance of seedlings between three and five years of age also declined from 10.6 stems to 0.6 stems per square meter.
“Increased nitrogen has a negative effect on the fungi that are the primary agents of litter decay in the forest,” Zak said. “It slows their ability to decay dead leaves.” Sugar maples in the Great Lakes region, where this study was conducted, have been spared the type of damage that has already taken place in the Northeast, because the calcium-rich soils provide a buffer against soil acidification. This new threat is raising considerable concerns.
“Increasing nitrogen deposition has the potential to lead to major changes in sugar-maple-dominated northern hardwood forests in the Great Lakes region,” said botanist Sierra Patterson, who conducted most of the fieldwork for the study. “In terms of regeneration, it looks like it will be difficult for new seeds to replace forest overstory in the future, so the populations of sugar maples in this region could potentially decline.”
Zak said that although his research did not examine forests in the Northeast, the logical assumption is that forests there will suffer from the combined effects of acid rain – deteriorating soils and declining leaf litter decomposition.