Acid rain's blind spot


University of Michigan researchers say future generations of sugar maple trees are at risk unless soft spots in the federal Clean Air Act are strengthened to address an old nemesis: acid rain.

Precipitation that is highly acidic from burned fossil fuels has been largely under control since the early 1990s. In 1989, the federal government adopted a system to control acid rain through large reductions of sulfur dioxide. Electricity-producing coal-fired power plants were allowed to meet tougher limits through swaps of so-called emission credits.

Industrial leaders and environmentalists agree that cap-and-trade has worked well for acid rain. But it isn't suitable for all forms of pollution. In particular, it wasn't designed to address nitrogen.

New research suggests that sugar-maple seedlings will be less able to regenerate forests as more nitrogen is pumped into the sky and brought back to Earth in raindrops. Scientists say nitrogen deposition from acid rain is on pace to double worldwide by the end of the 21st century.

The UM research was based on a 17-year study of four sugar-maple stands in Michigan. They found that excess nitrogen in acid rain inhibited decomposition of leaves on the forest floor. The buildup of dead maple leaves blocked seedling roots reaching for the soil and seedling leaves reaching for sunlight, slowly killing the forest.

This weakness in federal regulation needs to be addressed through the Clean Air Act. The U.S. Environmental Protection Agency is calling reasonably for utilities in 27 states, including Ohio and Michigan, to modernize their plants to reduce nitrogen emissions.

It's a step in the right direction.