Good news on the environment front, folks: the effects of acid rain on forests in the northeastern US and eastern Canada are finally starting to reverse, nearly forty years after the United States began passing environmental legislation to control the problem.
That's according to a new, USGS-led study, which examined soil acidity and toxin levels at 27 sites in the northeastern US and eastern Canada, all of which have experienced declining levels of acid rain over the past 8 to 24 years. The study, published today in Environmental Science and Technology, finds that aluminium concentrations (a telltale sign of acid rain damage) have declined while pH has increased in the upper soil layers across nearly all sites.
In other words, forests are finally starting to recover from an environmental problem the US identified decades ago and took legislative action to fix.
What is Acid Rain?
Acid rain is caused by sulphur dioxide and nitrogen oxide, two compounds produced in large quantities by coal power plants, and to a lesser extent when we burn petrol. In the atmosphere, these chemicals dissolve in water and undergo a series of reactions leading to the formation of sulfuric and nitric acid. During rainstorms, acid water percolates into the soil, kickstarting a cascade of chemical reactions that disrupt terrestrial and aquatic ecosystems.
"Essentially, all the ecological problems manifested by acid rain start in the soil," lead study author Gregory Lawrence told Gizmodo over the phone.
Image Credit: Wikimedia
There are two key processes at play here: First, acid rain strips clay minerals of calcium, an important nutrient for plants, and one that helps neutralise soil acidity. Next, as soils become more acidic and calcium depleted, aluminium starts to become liberated from its mineral bonds. "That causes problems for trees, forests, and surface waters" said Lawrence. Indeed, aluminium is a big problem for many key tree species in the northeast, most notably red maple and sugar maple. "It impacts their overall health, in terms of seedling survival, and how they deal with climate and insect stress," Lawrence said.
What's more, as aluminium becomes mobile in soils, it starts leaching into nearby surface waters. Over the years, this has led to widespread fish kills, with cascading impacts on aquatic communities.
The link between human pollution, acid rain, and ecological damage has been documented since the 19th century, but public awareness of the problem in the US only became widespread in the 1970s. At that time, the Hubbard Brook Experimental Forest in New Hampshire was publishing seminal studies demonstrating the harmful effects of acid rain on forest ecosystems. Growing public concern prompted Congress to pass a series of amendments to the Clean Air Act designed to dramatically reduce the emissions of sulphur and nitrogen oxides from power plants. These efforts have been widely seen as successful, with acid rain levels in the US dropping dramatically since the mid 1970s.
Despite our success in reducing acid rain, scientists have had trouble telling whether forest ecosystems in the northeast are, in fact, recovering.
"Prior to this study, published research on soils indicated that soil acidification was worsening in most areas despite several decades of declining acid rain," said Lawrence. "However, those studies relied on data that only extended up to 2004, whereas the data in this study extended up to 2014."
The new research offers some of the most promising evidence to date that a rebound to pre-acid rain conditions may indeed be underway. Across all 27 sites in the study, the uppermost soil layers are showing a strong recovery response, with aluminium levels dropping and soil pH increasing. In certain sites, Lawrence and his co-authors found that deeper soil layers are becoming more acidic, but he says this could be a natural part of the recovery process.
The Connecticut River running through Pioneer Valley, Massachusetts. Image Credit: Wikimedia
"I was involved with research in the mid-90s, showing that one of the effects of acid rain was moving aluminium from deep in soils to surface organic layers, where roots do their nutrient uptake," Lawrence said. "In this study, we're seeing that process reverse quite strongly, with aluminium levels decreasing down into the soil. It's sorta coming back where it came from."
Next, Lawrence and his team hope to look at surface water chemistry in northeaster rivers and lakes to see if similar signs of recovery are evident.
"Recovery is happening," he said. "We're still trying to figure out how much capacity these ecosystems have to recover, but there's no question that decreases in acidic deposition are having a positive effect."
Amidst the constant onslaught of news about how humans are wrecking the planet, it's all too easy to become resigned and cynical. But stories like this remind us that our environmental problems aren't insurmountable — nature, after all, is incredibly resilient. A good lesson to keep in mind as we approach the most important climate conference of the century.
[Read the full scientific paper at EST h/t USGS]
Top image: An acid-rain damaged forest stand, via Shutterstock