Carbon cycling is the process by which plants take carbon out of the air, and then transfer it to the soil, where animals and microbes release it as carbon dioxide (CO2). For decades, scientists have assumed that the high abundance and unique traits of non-native plants – such as faster growth, higher biomass and leaf thickness – explained how they increased carbon cycling where they invaded.
Now, an international team of researchers show that it is the different interactions that these plants form with herbivorous insects and soil microorganisms that drive this faster carbon cycling.
“All organisms interact with others, and the interactions we see today have been shaped over evolutionary timescales,” says Dr Lauren Waller, lead researcher from the Bio Protection Research Centre in New Zealand. “When exotic plants arrive in a new place they interact very differently with their new neighbours, such as insects and microbes.”
Humans have been transporting plants across countries and continents for centuries. So many of these have become invasive that some regions now have more invasive plants than native.
Between 2016 and 2019, scientists from six international research organisations and universities – including Lancaster University – conducted a large-scale experiment, looking at how plant invaders, and their associated insect herbivores and soil microorganisms, affect carbon cycling.
The scientists created 160 experimental plant communities, with different combinations of native and non-native plants in New Zealand. They then studied how those plants interacted with insect herbivores and soil microorganisms.
The results were dramatic, with non-native plants’ interactions with herbivores and soil microorganisms resulting in 2.5 times as much CO2 being released from the soil compared to native plants.
Dr Filipe França, a co-author from Lancaster University, said: “Our results suggest that exotic plants have traits that allow them to grow faster than natives, which means they can incorporate carbon into their tissues more quickly, before this carbon is then lost through the relationships with native insects and microbes.
“This research provides valuable insights not just for New Zealand, but in many countries around the world with high numbers of non-native plants.”
The study also shows that the same traits that allow faster growth in exotic plants also support microorganisms that return CO2 to the atmosphere at a faster rate.
“We were amazed to discover that novel interactions among non-native plants, herbivores, and soil microorganisms were the strongest drivers of carbon losses from soil into the atmosphere,” says co-author Dr Warwick Allen, also of the Bio-Protection Research Centre.
“We believe the increased release of CO2 from soil was driven by the higher quality and quantity of non-native plant leaves,” says Dr Waller. “These were more palatable to insect herbivores, and sped up rates of decomposition by soil microorganisms such as bacteria and fungi.”
Besides providing scientific advances in clarifying the mechanisms responsible for the effects of non-native plant invasions on carbon cycling, the study also brings insights for decision making in ecosystem restoration.
“There are many ongoing initiatives focused on nature-based solutions to fight climate change and restore ecosystems at a large scale – e.g. Plant for the Planet and Trillion Trees. Our research highlights that such approaches should consider how planted species will interact with other organisms as these can have further implications for carbon cycling,” explained Dr Waller.
Dr França added: “In future work, we would like to see how our results compare to what we might see in communities growing in the field and in other ecosystems with different non-native plants and environmental conditions.”