In 1999, scientists spread about 40 tons of calcium mineral over a New Hampshire forest damaged by acid rain; maple-seedling survival rose from 10.2% to 36.6%

Acid rain spent much of the twentieth century quietly stripping something out of northeastern American soils that nobody could easily see or replace. It was not just acidity itself doing the damage, but the way that acidity washed away the calcium the forest needed to grow. By the late 1990s, sugar maples across parts of New England were struggling so visibly that scientists at Hubbard Brook decided to test a direct fix, putting the missing calcium straight back into an entire watershed and watching whether the forest could recover.The target watershed was Watershed 1 at the Hubbard Brook Experimental Forest, chosen because decades of monitoring had already shown its soil and stream water were chronically acidic and calcium poor. In November 1999, a helicopter spread 45 tons of pelletized wollastonite, a calcium silicate mineral, across the 11.8-hectare watershed. Researchers deliberately calculated the dose to roughly match the calcium that scientific estimates suggested had leached out of the soil over the previous fifty years of acid deposition, rather than adding an artificial fertiliser boost. As detailed in project records held by the Hubbard Brook Ecosystem Study, the pellets were delivered by a computer controlled hopper slung beneath the helicopter, with the watershed boundary mapped out beforehand using GPS to keep the application precise. Within a couple of years, the treatment had already raised soil pH significantly, giving scientists a live before-and-after comparison against a neighbouring untreated watershed that continued to receive the same acid rain with no intervention.Healthier crowns and surviving seedlingsThe clearest signs of recovery showed up in the forest's sugar maples, a species that had been declining across the region for years. Foliar calcium levels in the treated trees rose sharply within two growing seasons, and by 2005 researchers assessing the health of individual tree crowns found them markedly improved compared with maples on the untreated reference watershed. The most striking numbers came from the forest floor itself. A study published in the journal Ecology and later summarized by the US Forest Service found that seedlings germinating in 2003 survived through the following two years at a rate of 36.6 percent on the treated watershed, compared with just 10.2 percent on the untreated one. Those young maples also grew roughly half again larger and carried noticeably higher chlorophyll levels than their counterparts struggling on calcium starved soil nearby.A fix with an unexpected side effectThe experiment did more than confirm that calcium loss was holding back the forest; it also produced a genuine surprise more than a decade later. A follow-up analysis published in the Proceedings of the National Academy of Sciences found that roughly ten years after the calcium addition, the treated watershed had unexpectedly flipped from absorbing nitrogen to exporting more of it than it took in, a shift researchers had not predicted when the project began. That twist made Watershed 1 a rare long-term experiment in ecology: an intervention designed to restore calcium unexpectedly revealed that changing one part of a damaged forest ecosystem could alter nitrogen cycling in ways researchers had not anticipated.Catch the latest World News and Live updates. Download the TOI app.