Coastal wetland ecosystems deliver large carbon stocks in tropical Mexico
Introduction
Tropical wetlands are unique systems contributing to a multitude of ecosystem services (Posa et al., 2011, Keddy et al., 2009, Evers et al., 2017). Among tropical wetlands, peatlands, a type of wetland containing large deposits of partially decomposed organic material (peat), hold ≈ 20% (119 Gt) of global peatland C (C) belowground (Leifeld and Menichetti, 2018). Although aboveground biomass also contributes to the overall store (Sjögersten et al., 2014). Inaccessible peatland areas in the Congo and Amazon are still relatively intact, while in other regions, e.g. South East Asia and southern Mexico, the role of tropical peatlands as C stores is currently under threat from large-scale expansion of drainage based agriculture and cattle ranching (Phillips et al., 1997, Moreno-Casasola et al., 2012a, Leifeld and Menichetti, 2018, Cooper et al., 2019). This is a concern, as peatland degradation results in a greater release of greenhouse gases to the atmosphere (GHGs), e.g. CO2 and N2O (Couwenberg et al., 2010, Hernández et al., 2015, Hoyos-Santillan et al., 2016a, Hoyos-Santillan et al., 2016b) albeit emissions of CH4 is reduced following drainage (Prananto et al., 2020).
Mexico has large wetland systems, distributed throughout its Atlantic and Pacific coasts, some of which are peat forming, and are under considerable pressure from land use change (LUC) (Moreno-Casasola et al., 2012a, Kauffman et al., 2016, Valderrama-Landeros et al., 2017). Overall, it has been estimated that Mexico has lost ≈ 60% of its wetlands area (Landgrave and Moreno-Casasola, 2012). Since freshwater wetlands can be important C stocks (e.g. Kauffman et al., 2016), the loss of this ecosystems may potentially represent a substantial release of C into the atmosphere (Guerra Martínez and Ochoa Gaona, 2006). The impact of land conversion on C storage and GHG emissions remains poorly constrained, but C losses, in the form of GHG emissions, have been reported to increase following conversion (Hooijer et al., 2012, Couwenberg et al., 2010, Pendleton et al., 2012).
To date most work on wetland C storage in Mexico has focused on mangrove ecosystems which store 80–1200 MgC ha−1, with a median of 380 MgC ha−1 (Moreno Cáliz et al., 2002, Adame et al., 2013, Marín-Muñiz et al., 2014, Hernández et al., 2015, Moreno-Casasola et al., 2017). In Mexico, many mangroves are protected to preserve their ecosystem services (storm protection, nursery areas for fish, and blue C storage) (Donato et al., 2011). However, despite some studies indicating that coastal freshwater wetlands have a C storage capacity comparable to that of mangroves (95 to 670 MgC ha−1 with a median of 350 MgC ha−1) (Moreno-Casasola et al., 2017, Adame et al., 2013, Hernández et al., 2015, Marín-Muñiz et al., 2014), freshwater wetlands in Mexico are not protected. We acknowledge the importance of “blue C storage” but in this paper we focus on addressing a knowledge gap pertaining to the C storage of freshwater and/or forested coastal wetlands.
In Mexico, the payment for ecosystem services (PES) program that attempts to reduce deforestation promotes sustainable forestry activities, and derives policy actions to stop illegal logging (Correa López et al., 2014, Centro de Investigación para el Desarrollo, A.C, 2014). There are several types of PES projects and there is only one example involving C sequestration, in indigenous communities in Chiapas (Correa López et al., 2014). An important barrier for the implementation of the PES is the incomplete information on the capacity of wetland ecosystems to deliver particular ecosystem services (e.g. C storage) and the spatial extent of different vegetation communities needed to assign a monetary value to a particular service/ecosystem (Moreno-Casasola et al., 2010). To address this issue, we used the advanced remote sensing capabilities offered via Sentinel-2 data to accurately map different vegetation communities and their properties across large areas (Berger et al., 2012, ESA, 2015, ESA, 2019). From the 12 spectral bands Sentinel-2 provides, four are within the red-edge of the spectrum, which is suitable for measuring vegetation productivity (Berger et al., 2012, Zarco-Tejada et al., 2018, Delegido et al., 2011), and for differentiating among vegetative species and compositions (Immitzer et al., 2016, Laurin et al., 2016, Malenovský et al., 2012). In addition, we conducted field campaigns to determine above and below ground C stocks in mangroves, freshwater swamps, freshwater marshes, and flooded grasslands. Together, the remote sensing and the field campaign approaches, allowed determining the C storage of three coastal wetland regions in Mexico: i) Encrucijada located in Chiapas; ii) Pantanos de Centla and Laguna de Términos located in Tabasco and Campeche (subsequently Centla-Terminos), and iii) Boquilla de Oro and La Mancha located in Veracruz (subsequently Boquilla-Mancha).
Section snippets
Site description
Freshwater swamps in Mexico are mainly found in wetland complexes across the southeast coast of the Gulf of Mexico in the states of Tabasco and Campeche (Solórzano et al., 2018, Moreno-Casasola et al., 2012b, INE/SEMARNAP, 1997, INE, SEMARNAP, 2000), and along the Pacific coast e.g. in the state of Chiapas (Whigham et al., 1993). In Tabasco and Campeche, marsh ecosystems comprise large areas of freshwater wetlands (Solórzano et al., 2018, Moreno-Casasola et al., 2012b; whereas the states of
Carbon stocks
Aboveground biomass in locations with herbaceous vegetation ranged between 2.9 and 15.9 Mg ha−1 with grazed areas consistently having lower biomass than marshes (Log Vegetation: F3,99 = 136.49, p < 0.001; Table 3). Aboveground biomass also varied among regions (Log Regions: F3,99 = 100.49, p < 0.001) with the greatest biomass found at Centla-Términos. Forest biomass ranged from 97 Mg ha−1 for mangrove in Boquilla-Mancha to 247 Mg ha−1 for swamp forests in Centla-Términos. The C concentrations
Discussion
This study demonstrates that natural wetlands are important C stores in tropical Mexico. We report a total of 4,968 km2 of wetlands (sum of the four wetland types investigated) in the study regions and a 0.688 Gt C stock in the study regions alone. This highlights that the current estimate of 0.025 Gt C held in C dense wetlands (Page et al., 2011) underestimate the importance of natural wetlands as C stores in Mexico. Given that Mexico has other wetlands distributed across the Pacific (Sinaloa
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the Newton Fundation Institutional links programme grant number: 264737343 supported by the British council and CONACYT. J.H.S acknowledges ANID/FONDECYT/11200024 funding by the National Agency for Research and Development (ANID). Chloe Briwn acknowledges funding from the ENVISION doctoral training programme.
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