How mangroves could store more carbon as oceans rise

In South Florida, the sea level has been edging upward by roughly 2.4 to 3.7 millimeters a year, reshaping shorelines and pushing salt water farther inland.

A new analysis of Everglades wetlands shows that this slow creep of salt can boost total carbon storage, if managers leave space for migrating mangroves, while other plant communities stumble.

Sparkle Malone of the Yale School of the Environment and Amanda Richey from Florida International University led the field team that measured carbon flows across three habitat types in 2021.

Rising oceans create what ecologists call climate debt, the lag between an ecosystem’s present state and the state best suited to new conditions.

When that debt grows unchecked, coastal wetlands can drown, a scenario that could claim up to 78 percent of the world’s tidal marshes by 2,100 under high‑sea‑rise projections.

Mangroves excel at carbon capture

On Everglades transects, the team tracked the net ecosystem exchange of carbon dioxide, a direct measure of whether a landscape breathes in or exhales CO₂.

The researchers found that compact “scrub” mangroves, dominated by Rhizophora mangle, kept absorbing about 294 grams of carbon per square meter each year even when water levels covered a quarter of their stems, outperforming sawgrass prairies more than six‑fold.

“We expect that mangroves will need to continue moving further inland. If this occurs, it could enhance carbon capture in those new locations,” said Malone.

The team’s models suggest that as ecotone zones creep inland at roughly 25 yards each year the net carbon uptake of the southeastern Everglades could climb another 12 percent, adding about 131 metric tons annually.

Freshwater plants struggle in floods

Freshwater marl prairie grasses thrive when their leaves stay dry for part of the year. Once water sits above the soil for weeks, photosynthesis drops by a third and the prairie’s climate debt balloons, signaling low resilience to longer floods.

Ecotone mixtures of sawgrass and mangrove seedlings fall in the middle: tolerant of brief floods yet still losing ground when tides linger.

Because the ecotone acts as a moving front, its health now sets the timetable for wholesale habitat turnover across the landscape.

Moving mangroves to boost carbon storage

Mangroves lock away carbon three to five times more densely than most tropical upland forests thanks to deep, anoxic sediments that slow decay.

Allowing mangroves to colonize higher ground provides room for trunks and roots to pile up sediment, a natural levee that can keep pace with moderate sea‑level rise.

Blocking this migration with roads or seawalls traps the forest against the tide, turning a productive carbon sink into open water.

Managers who once fought encroaching salt now face the opposite task: clearing pathways for salt‑tolerant species to race inland ahead of the water.

Steps to protect mangroves

First, map low‑lying barriers that could choke future mangrove expansion, from old spoil berms to abandoned canals.

Second, adjust freshwater releases so that salinity gradients shift gradually; sudden pulses of salt can kill sawgrass before mangroves take hold, leaving bare mud.

Third, prioritize restoration projects that connect patches of existing mangrove to potential refuge sites upslope, reducing fragment stress and fostering seed dispersal.

Finally, track disequilibrium indicators such as leaf‑level photosynthesis and soil elevation change so that intervention happens before large carbon debts accrue.

Balancing carbon capture and biodiversity

Although scrub mangroves are efficient carbon sinks, they support a narrower range of species compared to freshwater prairies.

As salinity increases, native amphibians, fish, and wading birds adapted to low-salt environments may be pushed out or face population declines.

Land managers will need to weigh these biodiversity losses against the climate benefits of carbon storage.

In some zones, maintaining a mosaic of habitats might offer the best of both worlds: storing more carbon while protecting ecological diversity.

Lessons for other coastlines

The Everglades aren’t the only place where mangroves are pushing inland. Similar shifts are being observed in the Mississippi Delta, Australia’s Northern Territory, and parts of Southeast Asia where sea level rise, salt intrusion, and human disturbances are reshaping wetland boundaries.

Each site has different constraints, tidal range, land use, and species pools, but the patterns of mangroves shifting and carbon storage changes echo those seen in South Florida.

This study offers a practical way to assess when to defend current habitats and when to help mangroves move inland for carbon benefits.

The study is published in the Journal of Environmental Management.

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