Invasive grass turns the tide on coastal carbon loss in China

A new study from researchers at the South China Botanical Garden, Chinese Academy of Sciences provides a complex view of the ecological changes in China's largest coastal wetland. By analyzing four decades of satellite imagery and field data from the Yancheng saltmarshes, the team found that the invasion of an exotic cordgrass, Spartina alterniflora, led to a net increase in the region's "blue carbon" storage, despite significant carbon losses from land reclamation projects.

A Landscape in Flux

The research team mapped landscape changes in the Yancheng saltmarshes from 1982 to 2021. Their analysis documented a dramatic 90 percent reduction in the area of the native Suaeda salsa marsh, famously known as the "Red Beach." This loss was driven by two main factors: human-led land reclamation for agriculture and infrastructure, and encroachment by a native reed, Phragmites australis. During the same period, the non-native smooth cordgrass, Spartina alterniflora, aggressively expanded its territory, primarily by colonizing open mudflats.

An Unexpected Carbon Gain

Despite a slight decrease in the total saltmarsh area over 40 years and the extensive loss of the "Red Beach," the ecosystem’s ability to store carbon actually improved. The study reports that total carbon storage increased from 3,152 gigagrams to 3,529 gigagrams. This counterintuitive result is almost entirely due to the spread of Spartina alterniflora. The total amount of carbon buried annually also rose substantially, from 60.35 to 93.57 gigagrams per year.

The Invasive Plant's Carbon Power

Spartina alterniflora proved to be an exceptionally efficient carbon sink. The study confirmed that this exotic grass has a much higher plant biomass and a carbon accumulation rate more than double that of the native marsh plants. Its dense root systems and tall stalks are effective at trapping sediment from the tides, which builds up the soil and locks away large amounts of organic carbon. The study showed that as the cordgrass establishes itself, surface soil carbon stocks can increase by nearly five times within the first decade.

Indirect Impacts on Native Marshes

The investigation also clarified the complex interactions between the invasive and native plants. The researchers found that Spartina alterniflora primarily expanded into previously non-vegetated mudflats along the seaward edge. This expansion altered the local environment by increasing sediment elevation and reducing soil salinity further inland. This created conditions that favored the native reed Phragmites australis, which then outcompeted and replaced the salt-loving Suaeda salsa, causing the "Red Beach" to shrink. The carbon gain from the invasive species was large enough to offset losses from both land reclamation and this native plant transition.

Reassessing Coastal Management

The findings present a new perspective for coastal ecosystem managers. While Spartina alterniflora is classified as an invasive species with known negative environmental impacts, its remarkable capacity for carbon sequestration offers a significant ecological service. According to corresponding author Faming Wang and lead author Jinge Zhou, this requires a balanced management approach. Instead of complete eradication, strategies could be developed to manage the plant's spread while leveraging its carbon-capturing ability to help meet national and global carbon neutrality goals.

Corresponding Author:

Faming Wang

Original Source:

https://doi.org/10.1007/s44246-023-00070-4

Contributions:

Wang Faming and Zhou Jinge designed this study. Zhou Jinge, Wang Faming, and Zhang Jingfan discussed analysis methods. Zhou Jinge collected the data, conducted the analysis, and drafted the manuscript. Chen Yaping and Wang Faming edited this manuscript. All authors discussed the results and contributed to writing the manuscript. The authors read and approved the final manuscript.