© Lisa Beers / Silvestrum Climate Associates.

Estimating mangrove carbon stocks for Belize

An internationally-cooperative effort to provide a comprehensive national assessment of mangrove carbon stocks

Written by Hannah Morrissette (Smithsonian Institution)

Coastal wetland environments, like mangroves, seagrass meadows, and saltmarshes, are known to provide a number of ecosystem services, such as promoting biodiversity and increasing coastal stability. Despite their relatively small global extent, they are disproportionately effective at storing carbon within their sediments.

Countries have responsibilities, as signatories to the Paris Agreement, to produce a plan of action towards mitigating certain aspects of climate change, called nationally determined contributions (NDCs). More and more interest resides in including these mangrove, seagrass, and tidal marsh ecosystems within their NDCs, because they are naturally incredible solutions to fighting many aspects of climate change.

However, despite mangroves being recognized as an important nature-based solution to climate change, many countries still lack local data on carbon stocks and instead must use global or regional averages that might misrepresent the actual amount of carbon within the mangrove ecosystems of that country.

Red mangroves - easy to spot with their extensive above ground root systems - are one of three species of mangroves that thrive on the Belizean coastlines. © Hannah Morrissette / Smithsonian Institution

In September 2021, fourteen institutions collaborated in a field campaign to provide these data, with sampling that occurred at nine sites from the north to the south of Belize, and along the vast network of cayes, to provide as many representative sampling locations as possible. The coastlines of Belize are home to an expansive range of mangroves that are highly varied even within a hundred meters of each other – guaranteed to be immensely different from other regions in which carbon stock data has already been measured. The marked differences observed between the systems over the course of the sampling was clear validation that in situ measurements were necessary to accurately estimate the carbon storage capacity of Belizean mangroves.

Field team participants from Belizean government agencies and local environmental NGOs are trained in taking sediment cores to share knowledge and build capacity. © Jon Lefcheck / MarineGEO, SERC

In total, 102 sediment cores up to three meters in depth were collected and subsampled for bulk density, loss on ignition (LOI), and elemental analysis. Aboveground and belowground biomass were also non-destructively sampled through measurements of diameter at breast height (DBH), canopy width, tree height, and associated allometric equations. These measurements occurred at plots placed 25 meters apart on 125 meter transects. All fieldwork followed standard practices to provide the first national carbon stock estimate assessment that included carbon stored in soils for Belizean mangrove ecosystems.

A sediment core from 2m depth in a dwarf mangrove plot is processed in the field, which includes bisecting the core, recording its characteristics, and collecting many subsamples for further analysis. © Hannah Morrissette / Smithsonian Institution

A sliced surface sediment core is measured for total depth, depth of the organic peat layer, location of roots or other large objects, and any transition to mineral soils. © Hannah Morrissette / Smithsonian Institution

Aboveground biomass is estimated through species identification, tree height, canopy width, and DBH measurements. © Hannah Morrissette / Smithsonian Institution

A 125m transect is the first step of the field day upon reaching location, to set up the uniform sample plots. © Lisa Beers / Silvestrum Climate Associates

An important component of this program was to provide training on methods and build capacity to allow for NDC updates to be conducted by local Belizean institutions. Key partners included the University of Belize Environmental Research Institute (UB ERI), the Pew Charitable Trusts, Silvestrum Climate Associates, World Wildlife Fund (WWF), the Smithsonian Institution, Belize Fisheries Department, Belize Forestry Department, National Climate Change Office (NCCO), Coastal Zone Management Authority & Institute (CZMAI), Toledo Institute for Development and Environment (TIDE), Southern Environmental Association (SEA), Sarteneja Alliance for Conservation and Development (SACD), the Corozal Sustainable Future Initiative (CSFI), and the Turneffe Atoll Sustainability Association (TASA). The high caliber of data and progress was only achieved through the hard work and investment of our Belizean colleagues.

The field team on the final day of sampling in Punta Gorda, Belize. © Stacy Baez / Pew Charitable Trusts

These data support Belize’s commitment to protect and restore mangroves as part of their NDCs. The information gathered from the sediment cores and biomass measurements will help inform coastal policy and management towards the goal of protecting mangrove ecosystems long-term. Our efforts and the dedication of our partners will hopefully serve as a blueprint for other countries seeking to conserve natural blue carbon sinks in meeting their climate targets.

Sediment analysis and data management are ongoing, with plans to present a subsample of the data at the Scottish Blue Carbon Forum following COP26, and to have the data ready to share with the government of Belize to include in their NDC update in 2025.

Check the twitter hashtag #BelizeBlueCarbon for previous posts and updates on the progress of the project, or contact Hannah Morrissette (morrissetteh@si.edu) with any questions.