- Much research has been done on the impact of mangrove restoration projects, but because such studies typically have their own distinct contexts, their results are not easily generalized.
- To determine the ecological and economic benefits of mangrove restoration across studies, researchers analyzed 188 peer-reviewed articles from 22 regions, mostly in East and Southeast Asia.
- They found the ecosystem functions of restored mangroves to be higher than bare tidal flats, but lower than natural mangroves.
- They also concluded that the economic benefits of mangrove restoration projects largely outweighed their costs, even at high discount rates.
SINGAPORE — In the green and dimly lit mangrove forests of West Papua in Indonesia, towering Rhizophora trees loom more than 40 meters (130 feet) overhead into the canopy, their tangled roots taller than a human. Oceans away in the Caribbean, mangroves of the same genus reach a maximum of 2 m (6 ft) in height, their shrubby stuntedness belying decades of growth.
That mangroves come in such varied forms is evidence of their adaptability, says Dan Friess, associate professor and head of the Mangrove Lab at the National University of Singapore.
Yet mangrove restoration projects have some of the highest failure rates around. Adaptable as mangroves may be, straddling the border between land and sea is uniquely stressful. Misguided restoration efforts — planting the wrong species in the wrong places in the wrong densities — push their stress levels further to the breaking point.
“There’s a common misconception that because mangroves grow by the sea, they love seawater. But they’re just tolerating it,” Friess said. “It’s a very stressful environment, and that’s why so many mangrove restoration projects fail.”
Despite the difficulties, cultivating these salt-tolerant jungles provides more ecological benefits than bare tidal flats, according to new research from Friess and his colleagues. From ecotourism income to coastal protection, successful restoration also promises economic returns, the study published in Nature Communications found.
Studies on mangroves have long assessed the impact of restoration projects on ecosystems and livelihoods, from capturing and storing carbon to serving as a nursery for fisheries. But because each study tends to have its own distinct context and methodology, results are not easily generalized.
To compare and summarize the ecological and economic benefits of restored mangroves across studies, the researchers analyzed 188 peer-reviewed articles from 22 regions, mostly in East and Southeast Asia. They found ecosystem functions of restored mangroves to be higher than unvegetated tidal flats, but lower than natural mangroves.
“We knew natural mangroves would be top, restored mangroves in the middle, and unrestored ones at the bottom. But what we didn’t know was where restored mangroves fell within these two extremes,” Friess said. “What’s surprising is that, for some ecosystem functions, restored mangroves were quite close to natural mangroves — even if they were only 10 to 15 years old.”
Further quantifying the economic benefits of these ecosystem services and comparing them with plantation, engineering, labor and other costs, the researchers found mangrove restoration projects to be overall cost-effective. Benefit-cost ratios ranged from 10.50 to 6.83 under discount rates of -2% to 8%, they calculated.
Even so, protecting and maintaining existing mangroves was still most cost-effective, with benefit-cost ratios as high as 16.75 under a discount rate of -2%, the team found.
As the planet warms, investor interest in mangrove restoration and conservation projects is heating up. Mangroves sequester up to four times as much carbon as rainforests, making them attractive nature-based climate solutions in the booming carbon offsets market.
Governments are also increasingly paying attention. Against frequent tropical storms, mangroves serve as natural and effective buffers for coastal communities. As wildlife hotspots and carbon sinks, they contribute to global commitments on biodiversity conservation and climate change mitigation. In Southeast Asia, Indonesia is targeting the restoration of about 600,000 hectares (1.5 million acres) of mangrove forests by 2024.
Thus far, conventional mangrove restoration efforts, which involve the mass planting of seedlings, have failed more than they have succeeded. Misguided projects have led to mangroves being planted on habitats such as seagrass meadows and mudflats, which are not only unsuitable, but also disrupt existing ecosystems.
In recent years, however, newer ecological engineering methods have yielded better outcomes. Such projects modify water and sediment flows to reduce flooding and provide a more suitable environment for mangroves to regenerate naturally.
“It takes more preparation, but if you fix the physical environment before letting mangroves grow, you get more complex and species-rich ecosystems with better functions,” Friess said.
Even though restoration methods are becoming more sophisticated, restored mangroves provide less ecological and economic benefits than natural mangroves, and restoration cannot replace conservation, he added.
“Conservation should always be the first step,” Friess said. “Many countries are starting to establish policies around mangrove conservation and restoration … we have to make use of this momentum, but we have to do it right.”
Banner image of mangroves in Puerto Princesa City in Palawan, Philippines. Image by Kino Obusan via Wikimedia Commons (CC BY-SA 4.0)
Citations:
Bayraktarov, E., Saunders, M. I., Abdullah, S., Mills, M., Beher, J., Possingham, H. P., … Lovelock, C. (2015). The cost and feasibility of marine coastal restoration. Ecological Applications, 26, 1055-1074. doi:10.1890/15-1077
Su, J., Friess, D. A. and Gasparatos, A. (2021) A meta-analysis of the ecological and economic outcomes of mangrove restoration. Nature Communications, 12(1). doi: 10.1038/s41467-021-25349-1
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