The surprising way woodstove ashes can help your forest

cozy wood stove with fire burning inside Photo by bluejava1/ Shutterstock

Muskoka’s trees are famously colourful, but, it turns out, are not as strong or productive as they could be. The soil lacks calcium, a vital nutrient for growth, and an Ontario non-profit is working on a clever solution to fertilizing the soil: scattering recycled wood ash. 

“Calcium plays many of the same roles in trees as it does in humans,” says Norman Yan, a retired biology professor at York University. Yan is a board member of the Friends of the Muskoka Watershed, a not-for-profit group that is dedicated to researching and finding solutions to Muskoka’s environmental challenges. With their ASHMuskoka program, they hope to replenish the calcium deficient soils of the region in order to boost forest productivity.

Yan explains that in Eastern Canada, the Northeastern United States, and parts of Scandinavia, a history of glacial retreat has towed much of the soil away, leaving behind low-calcium granite bedrock. “We’ve also had decades of acid rain. It took about a third, sometimes to a half, of the residual calcium away,” says Yan. He estimates that Muskoka soils have lost around half a ton of calcium per hectare, mostly due to acid rain.

Like in humans, calcium plays an important role in all kinds of physiological functions, from basic cellular processes to wound repair. Yan says that wood from trees that are deficient in calcium are actually 20-30 per cent weaker than their non-deficient counterparts, and the phenomenon of calcium-poor soils results in a condition called ecological osteoporosis.“The implications of that are lower photosynthesis, weaker wood, lower rates of oxygen production and sugar production, and weaker regeneration.”

To mitigate the calcium deficiency, the ASHMuskoka program is focused on research and sustainable solutions. Rather than importing limestone or dolomite to restore the lost calcium, the program proposes recycling wood ash from residential wood stoves. “Hundreds if not thousands of people out here heat with wood,” says Yan. “The ash that’s leftover is kind of a waste. It has more or less all the nutrients that the tree needs in the right proportions.” Except, he says, for nitrogen, which isn’t a concern because Muskoka soils already have plentiful amounts of that nutrient.

spreading woodstove ashes in forest
Photo courtesy ASHMuskoka

In the program’s study plots Yan and other researchers have already found that fertilizing forest stands increased calcium and potassium levels in foliage and dramatically improved calcium levels in root systems. “The most interesting result that we don’t quite understand yet is a dramatic increase in sap volume from sugar maples,” says Yan. In one experiment, some maple trees supplemented with wood ash doubled in sap flow.

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 The broader benefits of fertilizing forests with wood ash are multifold. For one, trees supplemented with wood ash transpire—or release water vapour through their leaves—25 per cent more than non-fertilized trees. The added water vapour in the atmosphere could influence the water cycle and mitigate the spring flooding issue the region often faces.

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Critically, boosted forest growth can be vital for capturing carbon dioxide from the Earth’s atmosphere. “This could make a real contribution to Canada’s goal to be carbon neutral by 2050 if we can roll out a program like this across the landscape,” says Yan. A study done in New Hampshire found that calcium-fertilized forests captured a ton more carbon dioxide per year per hectare.  

Now, the AshMuskoka program is looking to collaborate with logging companies that could oversee the widespread  implementation of wood ash fertilization. They’re also interested in raising awareness for recycling wood ash and involving the public in their project.

woodstove ashes in forest

People interested in ASHMuskoka can contribute in several ways. For one the program is planning a citizen science project where property owners can volunteer some of their land as a study plot. ASHMuskoka also runs monthly wood ash drives where volunteers can drop off their ash at the Rosewarne Transfer Station in Bracebridge, Ont. Lastly, people that have groves of maples or other hardwoods can also sprinkle about a yoghurt container per square yard of wood ash in their forest stands. “You’ll see a real benefit for the health of your trees,” says Yan. Just be sure the ash is completely cold to eliminate any risk of starting a forest fire.

“If we look after our forests, our forests will look after us,” says Yan. “The forest could be a lot healthier in mitigating climate change and mitigating spring floods.”


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