Posted on June 30, 2025
Study finds that efforts to replace an invasive weed could spike greenhouse gas emissions in the region
In 2023, China embarked on the largest coastal restoration project ever attempted. Threatened by an invasive, fast-growing weed known as smooth cordgrass (Spartina alterniflora), which was overrunning clam farms, bird habitats, and shipping channels, the country planned to remove the plant and replace it with environmentally friendly species, such as native reeds and mangrove trees.
But such efforts would have a huge downside, increasing methane emissions 10-fold, researchers report this month in Geophysical Research Letters. The mangroves would eventually counter these effects, but it could take 5 decades for these native plants to absorb the increasing greenhouse emissions.
The study highlights the inadvertent environmental cost of coastal restoration, says Steven Pennings, a coastal ecologist at the University of Houston not involved with the work. Bo Li, an invasion ecologist at both Fudan and Yunnan universities, also not involved with the research, says countries considering such efforts in the future “need to look at the bigger picture and weigh all the pros and cons.”
China brought smooth cordgrass to its shores from the United States in 1979 to protect its beaches. The plant has a strong root system, which binds sediment and stabilizes the coast. The stabilized area was then used for aquaculture development, such as breeding and harvesting fish in ponds. Straw from the weed is also commonly used to produce biofuel and fertilizer.
But the weed has now grown out of control. The country’s 18,000 kilometers of coastline are now overrun by cordgrass, which has clogged shipping channels and damaged the habitats of migratory birds.
China launched its Spartina control plan in February 2023, with a goal of completing the project by the end of this year. Apart from digging cordgrass-filled soil, workers widely use physical control measures such as cutting and tilling to eliminate the weed. After the removals, they plant mangrove seedlings. In the first year alone, workers cleared 37,000 hectares—more than half of the total area occupied by this cordgrass—putting these goals well on track.
Previous studies have shown that disturbing coastal land can have undesirable consequences, such as drastically increasing greenhouse gas emissions. To measure the impact in China, wetland ecologist Xudong Zhu and marine biogeochemist Minhan Dai, both at Xiamen University, and their colleagues studied a cordgrass wetland in the Zhangjiang River estuary. The estuary is part of the country’s coastal restoration project.
The scientists employed the “eddy covariance method,” where towers equipped with sensors analyze gas and wind speed, measuring the exchange of greenhouse gases between the land and the atmosphere. They installed these towers in November 2021 in the wetland and a mature mangrove forest 1 kilometer away, ensuring greenhouse gas flux differences were due to the distinct habitats, and not the atmosphere. In October 2022, as part of its coastal restoration projects, like the management of harmful species, China conducted intensive Spartina removal here, using mechanical shovels and diggers that are suitable for removing plants with strong root networks. In June 2023, workers used tools such as hoes to remove any seeds or root fragments brought to the surface during the removals. Mangroves are now being planted here as part of the restoration.
The researchers calculated carbon dioxide and methane emissions for these areas for 2 years, until October 2023, to account for any differences in atmospheric greenhouse gas emissions before and after the digging. The mangrove forest consistently provided environmental benefits: It absorbed more carbon dioxide than it emitted, and it released little methane over the 2 years, the team found. The wetland showed similar benefits before being excavated.
But after being dredged, this advantage disappeared: The emissions were more than the absorptions before the diggings. Worse, methane emissions increased 20 times here after the government used mechanical shovels, according to the researchers. In the following year, these emissions were 10 times higher than before the excavations, and 100-fold that of median methane emission levels in similar wetlands. In all, the excavations unleashed 11,680 tons of greenhouse gases into the atmosphere over the following year—the equivalent of burning nearly 6000 tons of coal.
It takes up to 16 years after the diggings for the wetland to absorb more greenhouse gas and produce fewer such emissions than it did before the digging, according to a computer model. However, the mangroves need up to 48 years after being planted to absorb these increased emissions collected over 16 years, as they need time to reach maturity.
The team doesn’t recommend abandoning the project, because of the devastating impacts of cordgrass on the region. Instead, it proposes a combination of methods, such as cutting combined with tilling, or smothering the cordgrass with plastic sheets to suppress the regrowth, as they are more environmentally friendly.
Donald Strong, an ecologist at the University of California, Davis, suggests using herbicides, as they’ve been effective and environmentally friendly for Spartina control in the U.S., Australia, and other countries. Li says herbicide use is a concern because of the potential harmful impact on bird habitats and aquaculture. But now, some safer chemicals are being approved for usage in China to eradicate this weed, he notes, similar to their usage in California. Li believes it’s an example of how countries can learn from each other to find better ways to deal with such invasive species.
