How Christmas Trees Could Become a Source of Low-Carbon Protein

In a polytunnel on the Scottish island of Bute, thousands of spindly Scots pine seedlings are growing. On the surface, nothing could appear more commonplace than Scots pine growing in Scotland. But hidden within the soil of these particular seed trays, a symbiotic relationship is forming that could produce the first carbon-negative source of protein grown on a commercial scale.

The seedlings’ roots have been inoculated with ectomycorrhizal fungi, which provide the plants with soil nutrients and water in exchange for carbon, and produce protein-rich, edible mushrooms. When these seedlings are planted out in the spring, they will grow into a tree plantation that could sequester hundreds of kilos of carbon for every kilo of protein produced.

This project is at the forefront of mycoforestry, an emerging practice of cultivating fungi as a food crop from tree planting. Currently, demand for agricultural land is the biggest driver of deforestation worldwide, with catastrophic implications for the climate and biodiversity. But a study published last year by scientists from the University of Stirling in Scotland shows that mycoforestry has the potential to avoid this land-use conflict, by growing trees and food on the same plot.

Saplings growing at Mycorrhizal Systems’ nursery in Bute, Scotland.Photograph: Mycorrhizal Systems Ltd.

Christmas tree plantations could be an ideal use-case, says Paul Thomas, an honorary professor at the University of Stirling and the director of Mycorrhizal Systems Ltd, who is coleading the project with Alistair Jump, dean of natural sciences at Stirling. One of the key species they are focusing on, Sitka spruce, is used as a Christmas tree, and Thomas believes that the way Christmas tree plantations are managed would “work brilliantly” with fungi production. “They’re the ideal species, planted at a good density where mushrooms could easily be collected, and are normally cared for better than forestry trees,” he explains.

Thomas has spent his career developing technologies to cultivate some of the most prized ectomycorrhizal fungi—truffles—in orchards around the world. But he has become increasingly concerned about food production in a rapidly changing climate.

“About five years ago, I started to look at the climate impacts on these species, and that sent me into quite a dark place,” he says. “I started looking at growing other food crops with trees, using our expertise and our knowledge base for truffles. And it just spun out of there.”

Mycorrhizal Systems’ repurposed farmstead.Photograph: Mycorrhizal Systems Ltd.

Now, thanks to a £554,000 ($706,000) grant awarded last year by the UK’s innovation agency, he is raising 14,200 seedlings—11,000 Scots pine, and a mixture of Sitka spruce, silver birch, hazel and English oak—on a former farmstead, home to a picturesque low white building which has been converted into a lab and office. Next to it stands the polytunnel and an orchard, which has panoramic views of the snow-capped mountains of the Isle of Arran and mainland Scotland.

Inside the lab’s incubators are thousands of agar plates, on which 17 different species of fungi—some of which have never been cultivated before—are growing in colorful splashes. The fungi are raised in a liquid medium, “like a kind of fungal soup,” says Jump, and are then added to saplings’ roots. “You can either dunk roots in it, or you can water it on. The fungus will then find the roots and then start to form these mycorrhizal associations with them, like a glove over a hand.” Thirteen thousand seedlings have already been inoculated, allowing these symbiotic associations between the tree and the fungi to develop before they are planted out.

Mycorrhizal Systems is cultivating different strains of fungi to see which have the most potential.Photograph: Mycorrhizal Systems Ltd.

Based on calculations that the two scientists published in the Proceedings of the National Academy of Sciences last March, cultivating ectomycorrhizal fungi while at the same time raising forests could sequester up to 12.8 metric tons of carbon per hectare annually—in stark contrast to producing other protein-rich foods, which all result in net carbon emissions. Even crops like pulses, which you might think would be carbon negative or carbon neutral, are net emitters across their lifetime.

“Pretty much all other crops that we grow will release carbon as they’re grown over their lifecycle. Even if you’ve got a plant that is sucking up carbon as it grows, the overall production system will release carbon,” says Jump. “But when you’re growing mycorrhizal fungi associated with the trees, because the trees suck so much carbon dioxide out of the atmosphere, it means that the system itself is carbon negative.”

They also calculated that 1 kilogram of mushroom protein would be produced per 668 square meters of land, making it potentially more efficient than beef production, which ranges from 37 to 2,100 square meters per kilo. But to achieve this at scale, they must develop techniques that can easily be adopted by the forestry industry.

“The goal is that we can go into an existing tree nursery and slot into their practices, and work out how we can get the mycorrhiza grown on the root system for pennies,” Thomas explains. “If we get it that cheap and easy, that’s where we can make really big gains in terms of the impact for the environment.”

The trial is initially concentrating on tree types most commonly used in UK forestry plantations. But given the predicted increases in global temperatures over the coming decades, ensuring the viability of the tree and fungi species over their lifetimes is also vital. “We have a whole range of distribution maps that we’ve produced for decades going forward where we can demonstrate, according to all the data that we’ve got at the moment, that these are viable systems running forward through multiple forestry rotations,” says Jump. The team is currently modeling at the level of the woodland stand, a unit of forest made up of a relatively uniform group of trees, to assess the optimal distance between trees.

The trial passed its first major hurdle in November, when analysis of the saplings’ roots under a microscope revealed that their tips were covered in fungus. “The process we developed is working, which is brilliant,” says Thomas.

Matt Taylor, a forester, will be planting 10,000 inoculated seedlings at a site in Northumberland in February. He is used to producing timber, and has never produced food. When asked about his returns on the project, he laughs. “I don’t know, in truth, because it’s brand new,” he says. “My motivation for doing it isn’t economic, it’s because it seems like a good idea. Someone’s got to try it to demonstrate either that it works, or find out why it doesn’t and allow the technology to develop.”

Mushrooms should start to appear in the soil around the trees three years after planting, with the crop potentially peaking between eight and 12 years. Having an annual mushroom crop, Taylor says, would incentivize tree-planting by addressing two key challenges that farmers and landowners level at woodland creation: that it makes land unavailable for food production, and the long wait before tree-growers see a return on their investment in the form of harvestable timber. “It has the potential to generate income from year three to four, rather than waiting for 40 years,” he says. “The difficulty with getting people into the forestry sector is that they’re unlikely to see the benefit of their investment in their working life.”

Mycorrhizal Systems is focusing on using fungal strains that are native to the UK.Photograph: Mycorrhizal Systems Ltd.

Planting forests while growing protein-rich food seems like a win-win, so are there any potential drawbacks? Lynne Boddy, professor of fungal ecology at Cardiff University, believes that mycoforestry “absolutely” has the potential to produce significant food crops while sequestering carbon. However, she advocates for “huge care” in doing so. “To my mind, whenever we inoculate anything, it has to have local provenance,” she says of the fungi. “There’s the worry of an invasive genotype if it comes from elsewhere. Even if it comes from Britain, and you’re just planting the same thing in an area there’s a worry of diluting the gene pool. You’re also likely to decrease local species diversity.”

Thomas says these are valid questions, but points out that this project is focused on using fungal species native to the planting area, and with less aggressive strains than those already currently inoculated by many nurseries to encourage tree growth—although he acknowledges there could be potential for another company to look to use some form of genetic modification in the future, which would require regulatory approval.

There’s also the question of what happens to the trees. If they are left standing or produce timber that’s used as a building material, then that draws down carbon from the atmosphere for the long haul. But if the wood ends up decomposing or being burned—as is often the case with Christmas trees—then much of the carbon drawn down could end up back in the atmosphere. Even so, this cycle would still produce protein with a low net carbon footprint.

This process could make future Christmas trees more sustainable, Jump believes. The conventional way of growing Christmas trees is “quite an inefficient system, if you look at it from the carbon perspective,” he says. “You might be soaking up the carbon into the tree, but then you’re trashing the trees after Christmas. Getting the fungi associated with the Christmas trees is actually quite a nice way to reduce the carbon impact.”

A milk cap mushroom.Courtesy of Mycorrhizal Systems Ltd.

But the main question has to be, how do the mushrooms taste? “Amazing,” says Thomas. Some of their common names suggest their flavor and appearance, including the “delicious milk cap” and—Jump’s favorite—“penny buns.” “They’re just so cute,” he explains. But Thomas won’t be drawn into choosing a favourite. “I love all of them,” he says, laughing. “There’s too many really cool ones.”