Exotic plant species release 150 percent more carbon dioxide from the soil than native New Zealand plants, according to a new study from the Bio-Protection Research Centre published in Science.
The research is the latest development in an extended scientific debate over whether to prioritise planting native or exotic species to increase biodiversity and fight climate change.
While it doesn’t upset the longstanding scientific consensus that faster-growing plants sequester more carbon – and that exotic species planted outside their usual range will grow faster – the study does complicate the picture of the carbon cycle.
Carbon cycling and the soil
So what is the carbon cycle and how does CO2 get into the soil in the first place?
“It’s really important to think of it as a cycle,” the study’s lead author Dr Lauren Waller told Newsroom. Waller is a researcher at Lincoln University and a postdoctoral fellow at the Bio-Protection Research Centre.
While most people understand that plants remove carbon dioxide from the atmosphere, they don’t always realise what happens next.
“Plants are incredible at taking CO2 out of the atmosphere and they put it into their tissues but then all that material does go into the soil. It can be stored in a lot of different ways,” Waller said.
How the carbon is stored in the soil affects how long it remains there before being processed and rereleased by microorganisms. The longer the carbon is sequestered, the more time we have to continue planting trees and investing in other forms of sequestration. Once released, the carbon can be sequestered again or remain in the atmosphere, where it will contribute to climate change.
“I think that’s the part that is missing from a lot of people’s knowledge of the carbon cycle, that there are stable forms and unstable forms. In the end, that carbon is going to be decomposed and utilised by soil microorganisms and when they use that, that CO2 is released back into the atmosphere,” she said.
“That just happens at really different rates. It depends on temperature, it depends on the plant species, it depends on the microorganisms, it depends on the land use history before the plants got there. It’s complicated but it’s really all about carbon acquisition and release.”
Study concludes native plants store for longer
Waller’s study was a gargantuan effort. It involved setting up 160 “experimental plant communities” – arrangements of different native and non-native plants in pots where the carbon in the soil could be easily measured – and ran from 2016 to 2019 to allow for long-term study of carbon cycle.
“It was quite a journey and it aged me a lot,” Waller joked.
“When the results started coming out, it was just remarkable how consistent they were – complicated, but very clear and very consistent. And that was just so exciting for us because when you put that much work into something, you really hope that you get something out of it. It kind of came through in spades.”
What were those results?
Carbon was sequestered far more stably by native plants than exotic ones.
“Non-native plants’ interactions with herbivores and soil microorganisms [resulted] in 2.5 times as much CO2 being released from the soil compared to native plants,” a media release from the Bio-Protection Research Centre stated.
While Pinus Radiata – a favourite of foresters and a mainstay of the One Billion Trees scheme – was included in the study, the sample size was too small to draw solid conclusions, Waller said.
“It’s hard. I don’t know that we can really say anything about Pinus Radiata plants. But I think what we can say is that exotic nitrogen-fixing plants like acacia and alder – alder’s another one that’s being planted in the Billion Trees programme quite a bit – I think that we can say those are probably not our best choices. Whereas I think we can say that the native plants, they can stabilise the carbon a little better than particularly those nitrogen-fixing species.”
More work ahead
Waller was happy to admit that this project wasn’t the end-all, be-all.
“There’s probably a right answer for every different ecosystem. New Zealand is likely to be very, very different than a lot of other places. So the solutions here could be very different,” she said.
Euan Mason, a professor at the University of Canterbury’s School of Forestry, shared his preliminary thoughts on the study with Newsroom.
“I’m still mulling over the results from their study. Insofar as the impact of their results on carbon sequestration, really what they’re trying to say is there might be some loss of soil carbon with exotic species,” Mason said.
“It’s not altogether clear why that would be but the fact that exotic species behave differently when they’re outside of their range, frequently, means that maybe there is something about having acclimatised to the soil. But I think if we were to make any broad statements about loss of soil carbon under exotic species per se, we would want to do studies that were in the field and much more extensive and then target the species that we’re actually considering growing, such as having Radiata pine growing next to Tōtara and next to Rimu and next to other species.”
Mason said the study appeared to be more indicative of the benefit of native bushes and shrubs than of trees.
“Soil carbon is really quite a small component of forest carbon storage, but it’s a potentially much larger component of small plant storage: Grasses and small shrubs and so on, which is what they were actually dealing with. And so it possibly has far greater consequences for those smaller species than for the species of large stature, which actually have a tremendous amount of biomass above the ground,” Mason.
“Clearly there are a lot of things about carbon sequestration of forests that we probably should take into account that we don’t.”
