What factors determine wetland carbon stocks?

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What factors determine wetland carbon stocks?

Research into the ‘Carbon Stocks of Tropical Coastal Wetlands within the Karstic Landscape of the Mexican Caribbean’ has revealed the role of different environmental factors in determining the amount of carbon stored per unit area for mangroves and coastal marshes.

In a landmark study, Adame et al. (2013) measured whole-ecosystem carbon stocks at nine sites in the Sian Ka’an Biosphere Reserve (SKBR) (fig. 1). The SKBR, a designated UNESCO World Heritage and Ramsar site, is located along the eastern coast of the Yucatan Peninsula, Mexico. The peninsula projects northwards into the Gulf of Mexico, west of Cuba and the Cayman Islands. Mangroves and marshes line its eastern coast, underlain by permeable carbonate bedrock called Karst.

Adame et al. (2013) divided these coastal wetland ecosystems into the following categories of vegetation:

  1. Tall Mangroves (up to 14 m in height)
  2. Medium Mangroves (up to 11 m, mostly around 5 m)
  3. Dwarf Mangroves (up to 1.5 m)
  4. Coastal Marsh

Standing tree biomass, downed wood and soil carbon were measured in order to calculate carbon stocks. The concentration of nitrogen and phosphorus, and interstitial salinity was calculated using soil samples.

Key Research Questions and Findings

A. How do ecosystem carbon stocks of different vegetation types compare?

The figures below, calculated by Adame et al. (2013), show the amount of carbon (megagram – Mg) stored per unit area (hectare per year – ha-1) for each vegetation category and the associated error range. The highest carbon stocks were found in Tall Mangroves, and the lowest in Coastal Marsh.

  1. Tall Mangroves – 987 +/- 338 Mg ha-1
  2. Medium Mangroves – 623 +/- 41 Mg ha-1
  3. Dwarf Mangroves – 381 +/- 52 Mg ha-1
  4. Coastal Marsh – 177 +/- 73 Mg ha-1

Dwarf Mangroves are the most widely distributed ecosystem in the SKBR. The whole ecosystem carbon stocks calculated for this ecosystem are greater than Mexican tropical dry forest, which consists of trees up to 15 m tall (Jaramillo et al. 2003 cited in Adame et al. 2013, p.10) (fig. 2). Whilst the aboveground biomass of dwarf mangroves is low, there are high concentrations of carbon in the surface soil, leading to relatively large carbon stocks.

B. What factors affect the potential of coastal ecosystems to store carbon?

Variation in the carbon stocks within each vegetation category was identified. This indicates that factors other than vegetation type determine the size of carbon stocks. Adame et al. (2013) found that carbon stocks for mangroves were best explained by both salinity and soil surface phosphorus:

Salinity

Low salinity was found to relate to higher mangrove ecosystem carbon stocks. The highest ecosystem carbon stock per unit area was identified at a place called ‘Isla Pitaya’, where tall mangroves are associated with inflowing fresh groundwater from springs and channels sourced inland. Conversely, the lowest carbon stocks were found in saline dwarf mangroves.

Soil Surface Phosphorus

Phosphorus (P) is a nutrient utilised for photosynthesis. Therefore, the mangrove ecosystem carbon stocks were found to significantly correlate with the concentration of P in surface soil. In the case of the tropical wetlands sampled in the SKBR, high surface soil P concentrations were associated with increased carbon stocks. This relationship is particularly pronounced due to the karstic landscape, which is associated with low concentrations of P.

Marsh carbon stocks were not closely related to salinity or soil surface phosphorus. Adame et al. (2013) suggest that inundation regimes have greater influence.

This study shows that vegetation type is not the sole factor that determines the size of whole-ecosystem carbon stocks in tropical wetlands. Variation in the amount of carbon stored per unit area within a vegetation community is caused by factors such as soil surface phosphorus and salinity. The study as a whole identifies large carbon stocks associated with mangroves in karstic regions – equivalent to 40-46% of Mexico’s 2009 carbon emissions in the SKBR alone (IEA, 2011 cited in Adame et al. 2013, p.12). It is possible that mangroves in karstic regions could account for over 10% of mangrove cover worldwide and so the findings of this study will have relevance beyond the SKBR (Valiela et al. 2001 cited in Adame et al. 2013, p.2). It also highlights the close and complex coupling of these ecosystem carbon stocks to environmental change, which raises the question of what impacts will result and how this will affect global climate change in the future.

Reference: Adame, M.F., Kauffman, J.B., Medina, I., Gamboa, J.N., Torres, O., Caamal, J.P., Reza, M., Herrera-Silveira, J.A. 2013. Carbon Stocks of Tropical Coastal Wetlands within the Karstic Landscape of the Mexican Caribbean. PLoS ONE 8(2): e56569. Doi:10.1371/journal.pone.0056569.

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