People living in Ontario’s premier cottage country of Muskoka have traditionally scattered the remains from their wood stove fires on their gardens, but some are outfitting themselves with 80-litre cans bearing the slogan “Save Your Ash”, where they are stockpiling this material for recycling. Once the contents are dropped off at an approved region waste storage facility, a group called Friends of the Muskoka Watershed will be transporting it to a local sugar bush in an attempt to correct the damage caused by decades of acidic rain.

Muskoka was among the parts of the province most affected in this way by air pollution generated by large American power plants, which poured enough NOx and SOx into the atmosphere to lower the pH of downwind precipitation. This phenomenon peaked in the 1960s and 1970s, prompting much stricter regulation that forced these operations to take measures such as installing scrubbers on their smoke stacks.

But while the acidity of the area’s rainfall is now just a fraction of what it had been, lakes and forests are still struggling to recover from the original impact. Norman Yan, who spent 25 years as a research scientist with Ontario’s Ministry of the Environment and another 15 as a professor in York University’s Department of Biology, has dubbed this challenge “ecological osteoporosis”.

“As acid rain fell out of the sky, much of it was neutralized by cation exchange reactions,” he explains, pointing to resident calcium as a primary buffering agent, which became severely depleted. He cites calcium levels in Muskoka lakes as being an average of 25% lower than were 50 years ago, and in some cases as much as 50% lower.

Since calcium is an integral component for the health of plants and animals, such scarcity in places like Muskoka has made it difficult for many species to survive. According to Yan, who chairs the board of the Friends of the Muskoka Watershed, restoration efforts have to do much more than offset pH imbalances in lakes. The missing calcium must also be replaced and wood ash turns out to be a good source of this material.

Yan recently co-authored an article in the Canadian Science Publishing journal Facets, which outlines a pilot project called HATSEO (Hauling Ash To Solve Ecological Osteoporosis), which explored the potential of recycling residential wood ash for this purpose. This initiative, called ASH Muskoka, was led by Shakira Azan, who completed her doctorate at Queen’s University’s Department of Biology under Professor Shelley Arnott, who has spent much of her own career studying the effects of acid rain on the Ontario landscape.

Arnott recalls the challenge of distinguishing between ecosystem effects caused by a drop in pH and those caused by a drop in calcium.

“We did experiments to tease apart the effects of biotic interactions, pH, additional stressors like invasive species, and also the legacy effect of regional acidification, and declining calcium,” she says. “We were busy looking at those first factors and it was only recently that we realized that declining calcium could be holding back recovery. We then did some experiments and found that low calcium had a huge impact on zooplankton, often affecting the same species that were sensitive to low pH.”

Eventually the team concluded that addressing calcium deficiency in the surrounding forests was fundamental, since trees hold most of this material and it survives combustion in wood stoves. The Facets paper, based on data from contributions by 10 local sources of residential wood ash, found it to consist of about 30% calcium by weight and much lower amounts of potentially problematic substances like metals.

“We think that we need to add somewhere between one-and-a-half and two tonnes of wood ash per hectare of forest, just to make up for the amount of calcium that’s been lost,” says Yan.

The next phase of this work, now called ASH Muskoka was launched in January 2019 with more than 130 participants and a subsequent analysis of this scaled-up trial could point the way for even more ambitious efforts to replace forest calcium. Based on the initial enthusiasm, Yan is optimistic that process will continue.

“It’s amazing,” he observes. “People are absolutely thrilled to come into the office and pick up their ash cans and get their picture taken with them.”