December 20th, 2011 by

When was the last time you heard anything about acid rain? It’s been awhile, right? Though the issue had a good deal more coverage back in the 90s, it certainly hasn’t gone away. And now, scientists have discovered a new and unexpected effect it will have on some forests here in the U.S.

Maple leaves (Credit: Flickr/LizWest)

Acid rain occurs when the pollution in the atmosphere — especially sulfur dioxide and nitrogen oxide — mixes with precipitation and falls back down to earth as water with higher levels of nitric and sulfuric acids. Soil, especially in forests, has to maintain a careful balance of acidity and nutrients for it to support the local flora. So when acid rain enters the picture, it throws everything out of whack. That’s what has been happening to sugar maples in the Northeast. The species is especially vulnerable to nutrient imbalances and soil acidification, and much of its range is in areas with calcium-poor soil. Without this natural buffer to neutralize the acid, the trees absorb it through the groundwater and are poisoned. Over the past few decades as the sugar maples in the Northeast started to decline, one consolation has been that the sugar maple forests of the Great Lakes region would be safe because of the high calcium content in the area’s soil. And while that soil has protected the forests from absorbing too much acid, a recent study has found that those forests may soon have a whole new problem to worry about.

By simulating the effects of increased levels of acid rain on sugar maple forests, scientists found that as the acid falls on the layer of dead leaves and other waste that litters the forest floor, it slows the decomposition process. In a healthy ecosystem, this process breaks down the leaves and other organic matter, clearing them away to make room for new trees. When the acid slows this process, the dead leaves pile up, creating a much thicker layer than the forest understory is used to — in some places increasing it by 50 percent — which prevents new trees from taking root. This means that as current generations of sugar maples die off, there will be fewer and fewer trees to take their place.

These findings, published in the Journal of Applied Ecology, are the result of a 17-year study by ecologists at the University of Michigan, the Michogan Technological University and the University of Idaho. Although the research reveals a new, negative effect of acid rain, its discovery could prove extremely important, influencing the management of sugar maple forests to give seedlings a fighting chance and giving foresters and scientists a push to look more closely at the ecology that takes place on the forest floor. Because of the continued burning of fossil fuels, nitrogen deposits from acid rain are expected to double in the next 100 years worldwide, so it is crucial that we fully understand the consequences they can have on our forests.