How invasive plants can alter the carbon cycle

Invasive plants can interact differently with local insects and microbes in the soil, releasing more carbon dioxide into the atmosphere than native plants, according to new research in Science.

Why it matters: The cycling of carbon between the land and atmosphere is a key process in the regulation of Earth's climate and global temperature. Understanding how — and how much — nonnative plants alter that carbon cycle is important for climate forecasting and efforts to restore ecosystems and address climate change.

"It isn't enough to look at how nonnative plants affect climate change — you also have to consider how these plants interact with native and nonnative insects and soil organisms."

Background: When plants expand their range — which is increasingly happening due to climate change — they encounter new microorganisms above and below the ground that affect whether their invasion is a success.

• Researchers have known invasive plants increased carbon cycling but they didn't know how.

What they did: Waller and her colleagues created 160 experimental mini ecosystems.

• Each had a different combination of invasive and noninvasive plants, soil (either "home" soil containing microbes typically associated with the plant species or "away" soil with foreign microorganisms), and insects (weevils, aphids, moths and others).
• They then measured how the different combinations of plants affected the mini ecosystems, including the flux of carbon dioxide from the soil.

What they found: In the mini ecosystems with "home" soil and without insects, there was no change in how much carbon dioxide was emitted when native and nonnative plants were compared.

• In the mini ecosystems with insects and "away" soil, 2.5 times more carbon dioxide was released from the soil if the setup was dominated by exotic plants species compared with native plants.
• "This is likely because the interactions between exotic plants and biota were so much stronger," Waller says.
• Exotic plants interacted more strongly with certain types of soil bacteria and more weakly with certain types of fungi, compared with native species.

• As the proportion of invasive species increased so did the number of insects.

Plant traits — the thickness and density of their leaves, for example — may play a role in attracting insects and speeding up the rate that bacteria and fungi decompose them, releasing CO2.

• Waller says studying a broader range of native and exotic species could help to determine whether it is these traits of the plant or if it is something else about being invasive that drives the differences.
• And it is not known whether the effect of invasive plants in the lab translates to natural ecosystems or if the rate of carbon cycling changes as invasive plants establish themselves, she says.

The big picture: A better understanding of how invasive plants drive ecosystems is important, as proposed efforts to plant massive numbers of trees in places around the world are debated, says David Wardle, a professor of forest ecology at Nanyang Technical University in Singapore.

• "Are all trees good? Do we really want trillions of trees if they are nonnative species that are transforming the ecosystem? Probably not," Wardle says.