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Introduction
Mangroves are composed of evergreen trees that inhabit intertidal zones of tropical and subtropical regions. These trees are halophytic plants that develop on muddy and anaerobic soils, with high incidence of tides, strong winds and high temperatures (Kathiresan & Bingham, 2001). The interaction of these factors often causes that mangroves vegetation shows a zonation relatively defined by the adaptations of the species that are distributed there, as well as by the geomorphology of the site (Hernández-Trejo, Priego-Santander, López-Portillo, & Isunza-Vera, 2006).
In Mexico, mangrove forests occupy a narrow strip on the coasts, characterized by a low specific richness, where the most important species are Rhizophora mangle L., Avicennia germinans (L.) L., Laguncularia racemosa (L.) C. F. Gaertn and Conocarpus erectus L. (López-Portillo & Ezcurra, 2002). Most of the mangroves in the country have been removed in favor of the development of fishing reserves, shrimp farming and oyster farming; paper production and tourism. In addition, with less impact, trunks have been used for construction, branches for firewood, and bark (obtaining tannins) for tanning leather (Comisión Nacional para el Conocimiento y Uso de la Biodiversidad [CONABIO], 2009).
The surface of estuaries in Veracruz represents 7.4 % of the national total, highlighting, at least, 11 relevant zones of lagoon bodies. Among these, the Landing System of Mandinga stands out due to its great territorial extension of more than 3 000 ha and due to the provision of multiple environmental, tourist and fishing services. Given the relevance and constant anthropogenic pressure, the mangrove of the Mandinga Lagoon is considered an ecosystem of biological importance with ecological rehabilitation needs. Therefore, there is a need to know the structure and composition of plant communities, as well as their benefits, to generate management and restoration plans (López-Portillo & Ezcurra, 2002). The restoration of the mangrove should be based on actions that ensure the permanence of the ecosystem along with its inherent goods and services (Carmona-Díaz, Morales-Mávil, & Rodríguez-Luna, 2004).
The objectives of this study were to identify the factors that control the vegetation distribution and structure at the Mandinga mangrove in Veracruz; the degree of dependence of the inhabitants towards the ecosystem; and the sources of habitat disturbance that prevent natural regeneration. The above will serve as a basis for the Mandinga mangrove restoration.
Materials and methods
Study area
The Lagoon System of Mandinga is composed of three main bodies of water: Laguna Larga, Laguna Redonda and Laguna Grande de Mandinga, all interconnected by estuaries; Laguna Grande is the furthest from the sea. The lagoon system is located in the eastern coastal plain of Mexico in the central part of Veracruz, between the parallels 19° 00’ - 19° 06´ NL and the meridians 96° 02´ - 96° 06´ WL (Figure 1). The predominant climate is Aw2(w) warm subhumid. Precipitation in rainy season is from 1 400 to 1 700 mm and in dry season from 125 to 200 mm. Temperature in dry season is between 18 and 27 °C and in rainy season between 22.5 and 33 °C. Average annual temperature is higher than 22 °C and the temperature of the coldest month is higher than 18 °C (García, 1990). The most abundant type of vegetation is the mangrove that is in the border of the lagoon, as well as in the propitiated (artificial) islands within the mangrove. The area occupied by the mangrove is approximately 428 ha. The types of soils that predominate are Vertisol (70 %), Regosol (15.2 %) and Gleysol (14.8 %) (Instituto Nacional de investigaciones Forestales y Agropecuarias [INIFAP] & Comisión Nacional para el Conocimiento y Uso de la Biodiversidad [CONABIO], 1995).
Characterization of the vegetation
The structure of vegetation was described based on the values of frequency, abundance and dominance of each of the tree species, which were obtained using the point-centered quarter method (Cottam & Curtis, 1956), modified for mangroves by Cintron and Schaeffer-Novelli (1983). This method does not involve the use of a specific area (sampling plots) and considers only the distance between trees, which is an advantage when studying areas are difficult to access, because it simplifies field work without affecting the reliability of the results (Agráz, Noriega, López, Flores, & Jiménez, 2006; Mitchell, 2007). There were 13 random points on transects of 25 to 250 m in length, depending on the size of the mangrove strip, of which 11 were located around the lagoon and two more on artificial islands within the mangrove (Figure 1); a total of 156 sampling points were obtained.
Trees with a diameter at breast height (DBH) ≥ 2.5 cm were taken as reference. The species to which they belonged, distance to the central point, distance to the lagoon, height and DBH were recorded for each tree; in R. mangle, the DBH was measured 30 cm above the wading roots. The height of the trees was obtained using a Haga gun. The relative importance value was estimated (Agráz et al., 2006) from the relative value of the structural variables (frequency, abundance and dominance).
Diagnosis of goods and services
To describe the dependence of the populations on the Mandinga mangrove and the factors that affect the ecosystem, 20 fishermen aged between 55 and 75 years old, from the communities surrounding the lagoon were interviewed. With the information gathered, the most relevant environmental goods and services for the inhabitants were identified according to the declared preference (Romo-Lozano, López-Upton, Vargas-Hernández, & Ávila-Angulo, 2017). The information was homologated with the nomenclature proposed by Sanjurjo and Welsh (2005), for the total economic value method.
Results and Discussion
Characterization of vegetation
The mangrove forest of the Mandinga Lagoon has an area of 460 ha (Moreno et al., 2002). The tree stratum is composed of A. germinans, L. racemosa and R. mangle, with abundant epiphytic bromeliads such as Tillandsia fasciculata Sw. and T. polystachia (L.) L. As scarce elements within the mangrove we found Pachira aquatica Aubl. and Annona glabra L., trees that develop in areas of lower salinity of the wetland, in addition to several species of Cyperus L. The herbaceous stratum is limited, due to the conditions of flooding and high salinity; however, some individuals of Batis maritima L., of the fern Acrostichum aureum L., developed, being more abundant in disturbed areas (clear), and of Cynodon dactylon (L.) Pers. in the periphery of the mangrove. In turn, in the most shallow and open portions, the herbaceous layer is more abundant and is composed of B. maritima, C. dactylon, Distichlis spicata (L.) Greene and Portulaca pilosa L., as representative elements.
Three associations were recorded in the mangrove: a) edge, dominated in the tree layer by R. mangle and A. germinans; b) basin or internal, dominated by A. germinans and L. racemosa; and c) island, with the presence of R. mangle, A. germinans and L. racemosa. These species are zoned in a gradient given by the level of flood or salinity, where each one is distributed in its physiological optimum: R. mangle has greater contact with water, A. germinans supports periodic floods and L. racemosa is located in solid ground (Figure 2). On the other hand, zoning is a multifactorial process that cannot be attributed exclusively to the physiological response, to flood or salinity; on the contrary, it could be a joint consequence coupled with the morphology of seedlings, where those with longer hypocotyls can be established in deeper water (López-Portillo & Ezcurra, 2002).
The density of the Mandinga mangrove was 721 trees·ha-1; A. germinans was the most abundant species with 384 trees·ha-1 (46.9 %), followed by R. mangle with 170 trees·ha-1 (26.7 %) and 167 trees·ha-1 of L. racemosa (26.5 %). The density is low compared to other areas of the country, since densities have been recorded in the lagoon of Cuyutlán (Téllez & Valdez, 2010) and Juluápan (Jiménez & González, 1996), both in the state of Colima, and in the Centla’s swamps, Tabasco (Corella et al., 2001), where contrary to that recorded in Mandinga, the most abundant and dominant species, regarding the basal area, was R. mangle.
Despite the low density of Mandinga mangrove trees, the basal area of the species per hectare is higher than that reported in the lagoon of Juluápan (Jiménez & González, 1996). Avicennia germinans had the highest value with 14.4 m2·ha-1 followed by L. racemosa and R. mangle with 2.3 m2·ha-1 and 1.5 m2·ha-1, respectively. The relationship between the basal area and the low density is indicative of the vegetation in a mature stage (Turner & Corlett, 1996), since the area of the stems is the reflection of a greater coverage in the canopy, which regulates and decreases the development of new individuals and saplings; therefore, lower densities were recorded.
Regarding the vertical structure, the average height of the canopy was 7.7 ± 3.2 m, with an upper stratum between 12 and 15 m high; A. germinans was the emergent species. The medium and higher density stratum is composed of A. germinans and L. racemosa with a height between 7 and 10 m and the low stratum is represented mainly by R. mangle with heights of less than 5 m.
According to Table 1, the species with the highest relative importance value was A. germinans, reducing its incidence towards extremes; that is, at the edge of the lagoon and towards the mainland. This is due to the fact that A. germinans is the most frequent species in the community and higher density, since the flood zone, the transition between the mainland and the edge of the lagoon, is the one with the greatest area. Laguncularia racemosa is the most affected in loss of plant cover because it is on the edge towards mainland, together with the fact that the strip between water and mainland, where R. mangle predominates, is very thin. This differs from that described in other areas of the country, where the dominant species is R. mangle or L. racemosa, but not A. germinans (Corella et al., 2001; Téllez & Valdez, 2010; Zaldívar, Herrera, Coronado, & Alonzo, 2004). Because the mangrove species are regionalized by certain levels of flood and salinity, the density of each of them is related to the topographic/hydrological (geomorphological) space available for their development (Hernández-Trejo et al., 2006).
Table1 Vegetation structural values of the Mandinga Lagoon, Veracruz.
| Species | Density (trees·ha-1) | Relative density (%) | Relative dominance (%) | Relative frequency (%) | RIV (%) |
|---|---|---|---|---|---|
| Rhizophora mangle | 170.1 | 26.7 | 12.7 | 29.0 | 22.8 |
| Avicennia germinans | 384.2 | 46.9 | 62.2 | 45.4 | 51.5 |
| Laguncularia racemosa | 167.1 | 26.5 | 25.1 | 25.6 | 25.7 |
RIV: Relative importance value
The islands located in Laguna Grande de Mandinga are propitious; the substrate used is a combination of oyster shells (stacked to serve as a basis for new oysters), mainly, and mud sludge, product of dredging by the opening of channels for boats. This has worked as a successful case of restoration for the Mandinga mangrove. These islands were generated near the mangrove edge 30 years ago, approximately, in the northeast-southeast and northwest-west of the lagoon. The vegetal association corresponds to island mangrove where the three species in question coexist.
The opening of a new space within the lagoon implies the rapid colonization and competition of mangrove species, since the ecological succession of the mangroves is determined by a relationship of tolerance; the species that support the high degree of salinity and flood are those that are established and distributed according to the limit of resistance to environmental factors. Therefore, there is no succession of floristic relay, since mangroves are pioneer trees and, at the same time, components of mature vegetation in these ecosystems (Febles, Novelo, & Batllori, 2009; Sánchez-Páez et al., 2000).
Diagnosis of goods and services
Direct (extractive and non-extractive) and indirect uses of the mangrove forest in the Laguna Grande of Mandinga were identified. Among the extractive uses we found self-consumption of wood of A. germinans and L. racemosa used for firewood and fences; construction of tools and homes; tourism infrastructure (docks); and elaboration of traps for shrimp fishing. The wood is not commercialized and there is no silvicultural management. With regard to R. mangle, its organs are used for medicinal purposes. Regarding non-extractive benefits, the inhabitants affiliated to the cooperative of boatmen of the Mandinga Lagoon carry out ecotourism tours, providing information on the ecological and economic importance of the ecosystem and mangrove and island, birds and fishing techniques are shown.
Among the indirect benefits; that is, goods and services that are obtained through the functions of the ecosystem (Sanjurjo & Welsh, 2005), one of the most notable is the fishery for self-consumption of the residents and sale to the tourism sector. This semi-controlled activity is carried out exclusively by the fishermen's cooperative, throughout the year, mainly from June to October, with the exception of the closed season that go from May to July for the case of crystal shrimp (Penaeus sp.), crab (Callinectes similis Williams) and oyster (Crassostrea virginica [Gmelin]). The closure season of sea bass (Centropomus parallelus Poey) is only for 15 days in May, during the breeding period. In addition to these species, seabream (Diplodus sp.), lebrancha (Mugil sp.) and catfish (Bagre sp.) are also fished.
Environmental services are also indirect values of great relevance; however, at present, a fully formed market cannot be recognized. With respect to these services, it has been determined that carbon content in soils with mangrove vegetation is higher than in soils from other types of vegetation, thus releasing less carbon than the rest of the forest ecosystems (Moreno, Guerrero, Gutiérrez, Ortiz, & Palma, 2002; Valdés et al., 2011). On the other hand, when filtering wastewater, the mangrove has the capacity to reduce the load of organic matter, which it uses in its natural processes; in addition, it removes contaminants and promotes the movement of nutrients (Foroughbakhch, Céspedes, Alvarado, Núñez, & Badii, 2004). The mangrove also controls soil erosion and acts as a protective barrier for coasts or populations, reducing the speed of wind and tides in the case of hurricanes and northerly winds (Sanjurjo & Welsh, 2005). In addition to this, mangroves are feeding, refuge, breeding and nesting areas for a large number of animal species, both aquatic and terrestrial (Sanjurjo & Welsh, 2005).
Table 2 shows the disturbance factors in the Mandinga mangrove. Although the Mandinga Lagoon System is a source of wealth and wellbeing for society, the mangrove is under constant disturbance due to intensive and extractive activities such as fishing, shrimp farming, tourism and livestock, apparently with greater profits, leaving aside the potential as a provider of indirect goods and services (Moreno et al., 2002). The constant disturbance that destroys the Mandinga mangrove is basically due to economic activities outside a territorial ordering scheme, such as: human, tourist, hotel, industrial, oil and fishing settlements (Botello, 2005; Guzmán, Villanueva, & Botello, 2005; Olguín, Hernández, & Sánchez, 2007; Ponce & Botello, 2005). These activities cause severe damage to both the vegetation and the lagoon ecosystem, mainly due to the degradation of sand dunes, silting, the discharge of wastewater and the alteration of the water flow.
Table 2 Disturbance factors of the Mandinga mangrove in Veracruz.
| Effect of disturbance | Type of disturbance | Disturbance factor |
|---|---|---|
| Conversion | Land use change | a) Urban development |
| b) Agricultural expansion | ||
| c) Livestock | ||
| Degradation | Pollution | a) Wastewater discharge |
| b) Dredging | ||
| c) Introduction of exotic species | ||
| d) Dumping of oils and fuel produced by boats | ||
| e) Use of agrochemicals and fertilizers from agriculture | ||
| f) Heavy metals in sediments and water (Cd, Cu, Cr, Ni, Pb and Zn) | ||
| g) Oil spill | ||
| Tourism | a) Pollution by solid waste | |
| Alteration of hydrological flows | a) Opening of channels and roads | |
| Harvesting | a) Timber harvesting for different uses |
The strip of arboreal vegetation goes from 30 to 400 m from the coastal body to the mainland and corresponds exclusively to the mangrove, since the transition to tropical sub-deciduous forest was removed as a whole, due to the change of land use by agriculture and intensive livestock. Laguncularia racemosa was the most affected species because it develops in the outermost area of the mangrove.
Conclusions
The strip of the Mandinga mangrove reaches a maximum of 400 m from the lagoon to the mainland; however, the average thickness of the strip is less than 100 m and there is low tree density. Because the inhabitants of the lagoon depend directly and indirectly on the mangrove species, it is necessary to improve the conditions of the wetland in order to conserve and even increase the benefits that the ecosystem provides. The restoration of the Mandinga mangrove should consider the formation of mangrove islands using shells and mud from dredging as has already been done; reforestation based on zoning, taking into account the microclimatic requirements of each species and geomorphology; in addition, it should promote the conservation of the surrounding vegetation of the mangrove forest, such as the tropical sub-deciduous forest and the coastal dunes.
