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January 15, 2025

As blue carbon dynamos, mangroves can elevate soil to adjust to rising sea levels

Study highlights the vital role of coastal wetlands in building land with rising seas, protecting their substantial carbon stores

New research has revealed the unique ecological role of coastal wetlands in the face of rising sea levels, capturing carbon, adjusting their soil elevation and protecting the coastline.

Led by ¾«¶«´«Ã½ of ¾«¶«´«Ã½ (UOW) PhD candidate Brooke Conroy, the , published in Plant and Soil, measured root growth in mangroves, saltmarsh and adjacent tidal forests over a two-year period at a coastal wetland within Westernport Bay in Victoria, near the southernmost limit of mangroves in the world. 

Ms Conroy discovered that coastal wetlands naturally adjust their soil elevation by root growth to protect against rising sea levels, with mature mangroves storing the greatest amount of carbon in their root systems.

“Mangroves, saltmarsh and adjacent tidal forests, which are known as ‘blue carbon’ ecosystems, have the unique capacity to capture and store carbon within vegetation structures and soils over long time scales,” said Ms Conroy, who conducted the research alongside study co-authors and , both from UOW’s School of Science.

“We wanted to understand the capacity of coastal wetland plants to store carbon and build soil elevation through root growth.”

“We set up a field-based experiment across a tidal landscape with mangrove, saltmarsh and an adjacent tidal forest. We placed sand-filled mesh cylinders into the ground to a depth of one metre and later collected them after two years.

“The mass and volume of roots that had grown inside the cylinders were measured in a laboratory. Conversion of these estimates to carbon and values of elevation change at the soil surface demonstrate how quickly these plants can replace losses in carbon due to decomposition and increase soil elevation.”

Being at the land-sea interface, coastal wetlands are an important line of defence against rising sea levels.

The study showed that coastal wetlands can use root growth and external sediment input supplied by tides to build their soil elevations and adjust to rising sea levels. This study also demonstrates root growth can occur rapidly, even where plant growth may be restricted by cooler temperatures.

Mangroves and adjacent tidal forests have deeper roots that extend up to a metre below the surface – adding considerable root volume to the soil.  

Ms Conroy said the research further underlined the critical ecological role of coastal wetlands, storing carbon and protecting coastlines which are essential to building resilience to climate change.

“As sea level rises and mangroves shift into saltmarshes, root volume will increase, storing more carbon and assisting with soil elevation adjustment,” she said.

“Mangrove forests, in particular, build soil elevation, preventing mangroves from drowning in the face of rising sea levels.

“With more than 85 per cent of the Australian population living on the coast, it is vital to protect and improve the condition of coastal wetlands to maintain livelihoods and preserve the ecosystems that we depend on.”

Saline coastal wetlands support a diversity of plants at the interface between the land and sea. Mangroves are distributed across tropical to subtropical and temperate climates, however, are the most diverse in the tropics and thrive in the intertidal. Saltmarshes and tidal forests grow in the upper coastal intertidal zones and adjacent areas that receive infrequent tidal inundation.  

About the research

‘Root productivity contributes to carbon storage and surface elevation adjustments in coastal wetlands’, by Brooke Conroy, Jeff Kelleway, and Kerrylee Rogers, was published in Plant and Soil.

The research was funded in part by the Victorian Government and the Australian Research Council through a Discovery Project.