Carbon stocks of tropical coastal wetlands within the karstic landscape of the Mexican Caribbean

Abstract

Coastal wetlands can have exceptionally large carbon (C) stocks and their protection and restoration would constitute an effective mitigation strategy to climate change. Inclusion of coastal ecosystems in mitigation strategies requires quantification of carbon stocks in order to calculate emissions or sequestration through time. In this study, we quantified the ecosystem C stocks of coastal wetlands of the Sian Ka'an Biosphere Reserve (SKBR) in the Yucatan Peninsula, Mexico. We stratified the SKBR into different vegetation types (tall, medium and dwarf mangroves, and marshes), and examined relationships of environmental variables with C stocks. At nine sites within SKBR, we quantified ecosystem C stocks through measurement of above and belowground biomass, downed wood, and soil C. Additionally, we measured nitrogen (N) and phosphorus (P) from the soil and interstitial salinity. Tall mangroves had the highest C stocks (987±338 Mg ha(-1)) followed by medium mangroves (623±41 Mg ha(-1)), dwarf mangroves (381±52 Mg ha(-1)) and marshes (177±73 Mg ha(-1)). At all sites, soil C comprised the majority of the ecosystem C stocks (78-99%). Highest C stocks were measured in soils that were relatively low in salinity, high in P and low in N∶P, suggesting that P limits C sequestration and accumulation potential. In this karstic area, coastal wetlands, especially mangroves, are important C stocks. At the landscape scale, the coastal wetlands of Sian Ka'an covering ≈172,176 ha may store 43.2 to 58.0 million Mg of C.

Figure 1. Sample locations within Sian Ka'an Biosphere Reserve.

Mangrove forest area map (dwarf+medium+tall) was obtained from CONABIO ; the map for “Peten” mangroves (tall mangroves associated with freshwater springs) was obtained from the Series III, INEGI (2005) and the map of marshes from INEGI (2000) .

Figure 2. Ecosystem C stocks of coastal wetlands of Sian Ka'an Biosphere Reserve.

The stocks are partitioned by A) aboveground (trees and down wood) and B) belowground (roots and soil) components. Lower case letters represent significant differences among sites and vegetation types (n = 6 per site, p≤0.0001). Note different scales between panel A and B.

Figure 3. Relationship among mangrove C stocks, interstitial salinity and surface soil phosphorus.

Seven mangrove sites were sampled within Sian Ka'an Biosphere Reserve, Mexico; three dwarf, two medium, and two tall mangroves, one of the latter associated to a fresh water spring. Soil phosphorus (P) was measured in the 0–15 cm soil horizon. The correlations are significant with R ² = 0.54, F = 31.3, p<0.0001 and R ² = 0.58, F = 26.3, p<0.001 for C stocks against salinity and soil P, respectively. Collectively, salinity and soil P explained 86% of the variance in mangrove C stocks (F = 45.6, p<0.001; VIF = 2.2).

Figure 4. Comparison among C stocks of coastal wetlands of Sian Ka'an Biosphere Reserve with terrestrial forests in Mexico.

Terrestrial forests are represented by a dry forest (Chamela, Jalisco [50]), a floodplain forest and an evergreen forests (Los Tuxtlas, Veracruz; [62]). C stocks include aboveground (trees, vines and wood) and belowground (soil and roots) stocks of up to one meter in depth. The tall mangrove forest in the graph was associated to a freshwater spring.