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Changing the genetic makeup of trees could supercharge their ability to suck up carbon dioxide. But are forests of frankentrees really a good idea?

Of all the potential fixes for the climate crisis, none has captured hearts and minds quite like tree planting. It’s a goal that seemingly everyone can agree on: Scientists, politicians, even billionaires are putting their heft behind efforts to green the land with new forests that will capture carbon and—hopefully—lock it away in trunks and soil for decades.

But no climate fix is ever that simple. Multiple studies have found that tree-planting campaigns don’t always deliver the benefits they promise. If newly planted forests aren’t properly cared for and monitored, the trees can die and any carbon they stored will be released back into the atmosphere. Sometimes there aren’t enough seedlings for these programs in the first place. The mass enthusiasm for tree-planting programs has sparked a partial backlash, with scientists arguing that planting trees is important, sure, but we shouldn’t kid ourselves that it’s a silver bullet for the vast challenges of the climate crisis.

Other scientists point to a different problem with mass tree-planting efforts: the trees themselves. What if existing trees just aren’t good enough at storing carbon? If scientists could find a way to increase trees’ carbon-sucking potential, we’d be unlocking more cost-effective carbon capture with every tree planted. A better tree could be what we’ve been waiting for. We just have to make it.

Maddie Hall, CEO and founder of the climate startup Living Carbon, is looking for the Tesla of trees. “Not just a tree that’s better for the environment, but a tree that grows faster and might be able to survive or perform better in climates than traditional varieties,” she says. “A lot of that comes down to how you could improve the growth rate and also carbon-capture potential of trees.”

The way that plants take carbon dioxide and sunlight and turn them into living material is nothing short of miraculous, a biological alchemy that supports almost all life on Earth. But this process—photosynthesis—is also woefully inefficient. Only a tiny fraction of sunlight that falls on leaves actually gets turned into living material—in the case of most plants around 95 percent of all that energy is wasted. For plant scientists like Amanda Cavanagh at the University of Essex, UK, this waste looks like an opportunity. If she can find a way to get plants to cut out some of this inefficiency, trees might put that energy into growth instead. Like most researchers in this area, Cavanagh’s focus is on faster-growing crops that can feed more people, but the same approach could be a boon for pulling carbon from the atmosphere, too. Photosynthesis-enhanced trees should be quicker at turning atmospheric carbon into trunks, leaves, and roots. That’s the theory, at least.

In 2019, Cavanagh and her colleagues published a paper in Science that strongly suggested they were on to something. By inserting a couple of new genes into tobacco plants, the scientists could get them to recycle a waste product of photosynthesis back into a molecule the plant could use to grow. Once they were planted, Cavanagh’s edited tobacco plants were 40 percent more productive than their non-edited equivalents. (Tobacco plants are the lab rats of the plant science world—the ultimate goal is to repeat this trick with crops like wheat or soy.)

Now a Californian startup has taken the same approach, but this time with poplar trees. In a non-peer-reviewed preprint first posted on February 19, scientists at Living Carbon claimed that by inserting new genes into poplar trees, they can make the plants grow 53 percent more quickly than their non-edited equivalents. Both sets of trees were grown under controlled conditions that differ significantly from the ones the plants would face in the wild, but Hall hopes that the edited trees will supercharge tree-planting plans by drawing down atmospheric carbon more quickly.

“Our belief is that climate change is a problem of relative rates. And also it’s one that we can’t just solve with man-made, intensely managed human processes like direct air capture,” she says. (Direct air capture means building devices that could scrub atmospheric carbon dioxide—or others that might trap methane—but by one recent estimate it could take 10,000 such machines to make a difference in CO2 levels.) Living Carbon’s eventual business model will be to plant its genetically engineered trees on land leased from private landowners, then give those landowners a share of the money earned by selling carbon credits earned against the growth of the trees.

When most plants photosynthesize, they produce a toxic byproduct called phosphoglycolate, which they then have to use energy to break down—a process called photorespiration. Living Carbon’s edited trees have extra genes from algae and pumpkin that help the plant use less energy to break it down, as well as recycling some of the sugars created by this process. This pathway was an obvious target for making plants more efficient, says Yumin Tao, Living Carbon’s VP of biotechnology. “You channel that byproduct into energy and nutrients for plant growth,” says Tao. And more plant growth means more carbon captured.

Tao and his colleagues grew the genetically engineered poplars for 21 weeks in a lab before harvesting and weighing them to see how much biomass they’d accumulated. The best-performing seedling had 53 percent more above-ground biomass than non-edited plants. Tests also showed that the edited plants took up more carbon than their non-edited cousins, an indication that these plants had a higher rate of photosynthesis.

“It’s a really exciting first step,” says Cavanagh, who was not involved in Living Carbon’s research. But she cautions that we don’t know whether these trees will be better at storing carbon in the long run. Living Carbon’s poplars were harvested after only five months, but in the wild, the trees can live for more than 50 years. Only further studies will reveal whether the edited trees will continue to grow quickly as they mature. Their growth rate might slow, or they might become so unhealthy that they fall over and release all that carbon back into the atmosphere when they rot. “Is the effect you see at the seedling phase the same at different stages of maturity, or does the plant fight back?” asks Cavanagh.

Soon this will be put to the test. Living Carbon has already planted 468 of its photosynthesis-enhanced trees in central Oregon, part of a field trial it’s running with Oregon State University. The company will analyze how quickly the trees grow over longer periods of time and also how they perform in different environments. It has also secured agreements to plant poplars created using a slightly different technique on around 3,500 acres of private land in the US, with the first plantings scheduled to start in late 2022, according to Hall.

But releasing genetically engineered trees into the wild is still controversial. Researchers at the State University of New York have engineered a chestnut tree that is resistant to a blight that has ravaged the species in the US, but the tree has still not been approved by the Department of Agriculture. Only two genetically engineered trees have been approved in the US: varieties of virus-resistant papaya and plum trees. The trees that Living Carbon is currently working with don’t produce pollen, which should limit the problem of genetic material from the edited plants mixing with wild trees.

But some plant scientists think there’s a simpler path to making a better tree: cultivating them the old-fashioned way. Humans have been breeding better crops for thousands of years, says Richard Buggs, an evolutionary biologist who studies plant health at Kew Gardens in London. “I’m totally in agreement with that core premise that we need trees that are more productive and fix carbon faster. I just think there are fantastic opportunities to do that by variation that already exists in nature,” says Buggs. Typically, cultivation means either hybridizing two varieties through cross-pollination—fertilizing the flowers of one tree with the pollen of another—or reinforcing a desirable trait within a species by self-pollinating a plant with that trait.

Rather than meddling with something as fundamental as photosynthesis, Buggs suggests there are other traits that might be useful for making more efficient trees. “There are actually lots of things that are already happening in nature that affect the growth of a tree that we could be working with,” he says, like variation in how quickly trees grow, how straight their trunks are, and when they drop their leaves. All would affect their suitability for carbon capture, Buggs says. “I would prefer that kind of approach. I think it’s more realistic, and you’re much more likely to end up with a tree that will survive and fix carbon in the long term in natural environments.”

Hall doesn’t actually envisage giant public forests of genetically engineered trees. She says that most of the time her trees will eventually be cut down for timber—another reason to find a way to speed up their growth. Other companies are interested in fast-growing trees too: In 2015 the Brazilian government approved a eucalyptus tree engineered by a paper-producing firm to produce 20 percent more wood than conventional trees.

The debate between natural breeding and genetic engineering has been bubbling along in the agricultural world for more than half a century. Now a similar conversation is starting to play out when it comes to forestry. We might be able to cultivate more productive trees, but that approach could take decades. We might not have that kind of time, Cavanagh says. “Thirty years will be the end of my career,” she says. “I would like to know I’ve done everything I can to make sure that there are solutions if things look as bad as the worst-case projections.”