Mangroves make up extensive areas of coastal forests on the tropical and pan tropical zone of the planet. These ecosystems develop mainly where there are important deltas that flow into the sea and mud accumulations take place, as substratum and permanent variations of salinity. Therefore, the principal abiotic factors are: the continuous mixing of continental and marine waters, with variations in the salinity; the accumulation of mud on river banks and coastal strips, rain and atmospheric humidity and high temperatures with few variables (higher than 25ºC and with variations not lower than 5ºC), which is why cold weather is a restraint for their development.
Mangrove forests are generally found in places with protected coasts, associated to low energy waves and with abundant contributions of freshwater, which enable the deposition and accumulation of organic sediments, fine mud and waters with more than five percent values of salinity. Under these conditions and large extensions of tide, mangroves develop that can penetrate up to 60 km into terra firma from the sea. Mangrove forests mark a transition between sea and land, thus their great importance. By mangrove it is understood as a group of trees that have certain adaptabilities that allow them to survive and develop in permanently or temporarily inundated terrains and that are subject to the influence of the tides, with the intrusion of salt and fresh water.
In order to live under these aquatic and saline conditions, the plants that make up the mangroves have adaptations like:
• Tolerance to high levels of salinity, with structures that enable them to expel the salt or stop the entry of salt through the roots.
• Hanging roots in the shape of stilts, which give the tree stability in soft and unstable terrains.
• Roots called pneumatophores, which stick up out of the soil and allows them to breathe in inundated environments.
• Seedlings that germinate on the trees (propagules), which can float and move in the water, facilitating greater dispersion.
• Specialised structures to allow the entrance of oxygen and the exit of carbon dioxide, like lenticels.
• Presence of aeriforous tissue in the stems, which facilitates the circulation of water.
Cuba and its mangroves
Cuba, as an archipelago with an area of 110,922 Km², is formed by the Island of Cuba, long and narrow; the Isla de la Juventud and a great many cays and islets, which substantially increases the extension of the coasts and the importance of mangroves. Mangrove forests are an important part of its coastal wetlands, with significant functions, ecological as well as economic and strategic. The protecting role of mangroves in Cuba is of vital importance for the national economy and human wellbeing because of the ecosystem services they provide.
Cuban mangroves are located generally on biogenic, accumulative and swampy coasts with marshes, where the effect of the tides and the draining of fresh water determine their presence; and they constitute a valuable forest reserve, making up extensive masses of woodlands that rank, because of their extension, ninth in the world; they are among those with the greatest representation in the American continent and first among the Caribbean countries.
Mangrove forests in the Cuban archipelago create a valuable barrier protecting the coasts; they make up 5.1 percent of the country’s land area, which represents 20.1 percent of the current forest area, and are found in more than 50 percent of Cuba’s coasts. The protective function of mangroves has great importance in the face of tropical storms, hurricanes and swells, for economic installations and planted fields, as well as for coastal populations or which are close to the coasts, in the face of the consequences of the climate change of anthropogenic origin, like the possible scenarios of danger, vulnerability and risk of the coastal area, due to the rise of the mean sea level.
In Cuba, according to the distribution and abundance of mangrove forests, it is possible to differentiate at least three regions.
- The western region, from San Antonio to Bahía Honda, on the northern coast, and from the Bay of Pigs to Cabo Francés, on the south.
2. The central, from the Hicacos Peninsula to Nuevitas, on the north, and from Cabo Cruz to Casilda, on the south.
3. The eastern region, on the northern as well as the southern coasts.
The best hydroclimatic and ecological conditions for the development of mangroves are found in the western region. Those conditions start decreasing toward the eastern region, the latter with mainly abrasive coasts and an evident water stress to the south, which is why it is less apt for the development of mangroves.
Trees that make up our mangrove forests
Cuban mangrove forests are constituted fundamentally by four tree and shrub species, three of which are considered real mangroves because of the morphological and physiological adaptations they present for the saltwater environment where they develop. These species are: red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa) and buttonwood (Conocarpus erectus). The latter is considered a pseudo mangrove or peripheral species because it has the adaptations typical of the mangrove species. These species are easy to distinguish because of their characteristics.
The Rhizophora mangle species has stilt roots that allow them to anchor in flooded muddy soils. Pores and lenticels in those superficial roots facilitate the incorporation of nutrients and to carry out an exchange of gases. The germinated seedlings (propagules) are elongated and hanging, in the shape of a green- or brown-colour cigar on the tip; the tree’s bark goes from pink to almost red; the leaves are bright green and, on their reverse or lower part, small deep green points are observed. That species is found on the shoreline, bordering channels and lagoons, or in places where the inundation is permanent, favoured by the mixture of fresh water and saltwater.
The Avicennia germinans species has aerial roots called pneumatophores, which constitute one of its most outstanding characteristics for breathing in the aquatic environment. The leaves are pointy, the reverse a whitish colour and the bundle is dark green; on the reverse of the leaves the plant expels the salts, which is why it usually presents salt grains. Due to this, in some regions of America it is known as “salty mangle.” The fruits reach a length of 1.5 to 2 cm. The germinated seedlings (propagules) are ovoid, the colour of light green. The tree trunks are dark brown. That species holds the largest concentrations of salt; it establishes itself behind the strip of red mangrove, in higher places of the coastal plains, and can develop in places which are temporarily inundated.
The Laguncularia racemosa species also has pneumatophores, but it reaches a lesser height than the black mangroves and is characterised for having in its superior part a head-shaped widened surface. The leaves are oval shaped, with small orange or red glands at the base of the petiole, which are salt secretors. This species is not resistant to very salinized or inundated terrains and is located in the highest and driest areas of the bank. Due to the caloric power of its wood, it has been greatly used for making charcoal.
The Conocarpus erectus species is not considered a real mangrove since it does not have the modifications characteristic of the aquatic and saline mangrove species, as are the viviparous of the fruits and emissions of pneumatophores. Its most outstanding features are the small fruits in the shape of round purple or brown cones; it can be found in the driest, highest and faraway part of the mangrove strip, in the ecotones with other types of vegetation, like coastal woodlands, forests and swamp meadows. This species has also been greatly used for making charcoal.
In Cuba, depending on the ecological and hydrological conditions, forests which are considered high, if they are over 12 m high, can be developed and stunted or dwarf when they are not more three metres high, in places with high salinity values. These dwarf formations are more related, from the structural point of view, to woodlands, and in turn present differences in terms of height and density. Mangrove forests can be monospecific when they comprise one of the four species or also can be mixed, with two, three or four mangrove species.
Mangroves and climate change
One of humanity’s major concerns nowadays is the climate change of anthropogenic origin. The increase of the greenhouse effect, due to the concentration of gases in the atmosphere like carbon dioxide, methane and nitrous oxide, has unleashed important transformations on a planetary scale. The principal changes related to the increasing temperatures in the atmosphere and oceans include the rise in the mean sea level, as well as changes in the regimen of precipitations and in the frequency and intensity of extreme hydrometeorological events.
According to estimates by the 2001 Intergovernmental Panel on Climate Change, it is previewed that the rise in the mean sea level will be between 0.11 and 0.77 m between 1990 and 2100 due to the thermal expansion of the oceans and the loss of mass of the glaciers and polar caps. Even when the concentration of greenhouse effect gases were to become stabilised, the mean sea level would continue rising during hundreds of years. According to the 2007 Intergovernmental Panel on Climate change, with the new evidence and climate models an increase of 0.3 to 0.8 m is estimated for 2100. According to data from the Siboney tidal graphic station in Havana, the mean sea level in the country has risen in the last 40 years at a rate of 0.191 cm per year.
For Cuba, since it is an island, the rise in the mean sea level has been identified as the principal threat of climate change, which is why the coastal areas have been considered as the most vulnerable to global climate change and the mangroves as one of the ecosystems of greatest vulnerability.
The rise in the mean sea level for the Cuban archipelago could have negative consequences, among them the gradual decrease of the emerged surface, especially in low-lying areas with submerged wetlands and their possible displacement inland; the increase in coastal erosion, with the subsequent deterioration of beaches; the increase in salinity of the estuaries and the aquifers; and the alteration of the sedimentation patterns and the increase in the export of sediments to the platform and the ocean, to the detriment of the reefs.
Though in the literature the predictions on the mangrove scenario, based on the effects of the rise in the mean sea level, are very controversial, the ability to adapt to the changing conditions that the coasts generally present confirms the resilience that characterises the mangrove ecosystem. This work identifies the principal characteristics that could allow for the adaptation of the plant species that make up the Cuban mangroves to the conditions of the rising mean sea level as a consequence of climate change.
Mangroves, together with the coral reefs and seagrass, are the marine ecosystems of greatest importance in the Caribbean. They are very interrelated, fundamentally in their functions, due to the exchange of energy and organic matter, and make up the first protection barrier in the face of the rising mean sea level.
The great representation of the mangrove forests that occupy low-lying and swampy areas of the Cuban archipelago, as well as their great resilience – confirmed by their adaptation to the changes in conditions on the coasts -, constitute a possibility for the protection of the national territory in the face of the rising mean sea level.
The characteristics of the plant species that make up those mangroves are of great importance for the protection of the national territory. This is the case of the system of roots of the tree plant species that make up the mangrove forests, both the stilt roots as well as pneumatophores. This radicle system varies in the different tree species that constitute the mangroves. The roots of the mangrove swamp can be so tense that they create a barrier that, together with the mainly murky substratum, act as a “large sponge” capable of retaining water. This favours the capacity of the mangrove swamp strip to avoid inundations in the face of strong rains, hurricanes or tropical storms. It is known that the inundations toward inland have occurred in the places where the strip of mangrove swamps has been felled or weakened; and it is possible to cite several examples in the country that illustrate this consideration, as occurred on the southern coast of the province of Pinar del Río with the passage of Hurricane Ivan in 2005; or the inundations due to the effects of Hurricanes Gustav and Ike in 2008 in Playa Cajío and Playa Majana, on the southern coast of the province of Artemisa. Those inundations occurred mainly in the places where the mangrove forests had been destroyed or affected by changes in the hydrological regimen. The mangrove forests have sufficient resilience to recover after the passage of extreme meteorological events. It has been confirmed that six months after the passage of a hurricane through the Cuban archipelago, the mangroves had already recovered their tops and the regeneration is very high to replace the space of the downed or dead trees, but still standing, due to the effect of the winds.
The mangrove tree species generally occupy different sites on the mangrove strip where their development is more favourable, which is why the adaptation characteristics of each one of them varies, spatially, along the coastal strip.
The R. mangle species favourably develops by the shoreline, with 30 and 45 percent salinity values, and its morphological and physiological adaptations are a strategy for survival in the aquatic and saline environment: the stilt roots function for breathing and as a form of support, since they withstand the action of the waves, stopping erosion; the aeriferous tissue in the stalk and the roots allows for the circulation of the water and withstanding high osmotic pressures. The amount of propagules it produces facilitates greater possibilities of success and survival for this species. The stilt roots, moreover, act as retainers of the sediments that come from inland to the coast; on the other hand, the density of stilt roots is related to its resistance to the waves, so that, where there are greater swells one can observe a greater amount of roots per tree. In the places where the red mangrove strip is more protected from swells, as on the borders of lagoons and channels, or on river banks or deltas, the density of stilt roots is smaller. It can also be observed that along the shoreline, in addition to the greater density of roots, the trees tend to tilt toward the sea, while in the more protected places the trees are more erect. These adaptations of the species to changing environments leads to supposing the adaptability of the species to the rise in the mean sea level, an event that will gradually take place, allowing for that strip of vegetation to mitigate partly the negative effects of that climate change manifestation.
The red mangrove propagules float in the seawater, where they can conserve their viability for relatively long periods, and can be transferred by currents and the sea until they find the appropriate places for their establishment, which can be associated to low-lying places like sea meadows, where the propagules are retained; or toward the forests, where they are pushed by the tides. The establishment of the propagules in the areas with a rising sea level and the colonisation of the species in these places also can contribute to decreasing the level of inundation in these territories and to counteract the effects of the rise in the mean sea level. It is stated that the rise in sea level benefits the growth and development of the R. mangle, an affirmation that has been demonstrated in an experimental form.
However, there are works that propose that since these ecosystems are so specialised, a small variation in the tide regimen or the hydrological conditions can be lethal. The observations made in the mangroves of diverse sites of the Cuban archipelago have confirmed their ability to adapt to gradual changes; they only die if the changes are brusque, like the closing of a cannel or construction of a road that prevents the exchange of waters.
The A. germinans species is located generally behind the first strip of red mangroves and can make up mono dominating forests or can associate with the other mangrove species, constituting mixed forests. This species is the one that can withstand the greatest concentrations of salinity in its environment (up to 60 percent) and has two adaptations that allow it to survive under those extreme conditions, like the capacity to let saltwater in through the roots and excrete the salt through the leaves, as well as incorporating specialised structures in the roots for the exchange of gases known as pneumatophores. The black mangrove roots, compared to the red, are superficial and vulnerable in the face of the battering of swells and heavy seas, which is why this species generally is not located along the shoreline. However, behind the strip of red mangroves, or in places where due to the conditions of salinity it occupies the first line of vegetation in channels and lagoons, the roots of the black mangrove, represented by their pneumatophores, make up a protective barrier in the face of the rise of the sea level. The height of the pneumatophores, which deserve a mention apart, varies with the level the high tide reaches in different places. The pneumatophores constitute cylindrical structures that cover the soil of the mangroves like a carpet and their size is linked to the height this mangrove can reach in the water, since the maximum height almost never gets to cover them completely. This capacity of elongation of the pneumatophores of black mangrove, according to the level of inundation of the forest’s floor, will prevent the trees from asphyxiating; the rise in the level of inundation occurs gradually, which enables the adaptation of the plants to those changes and, therefore, the elongation of the pneumatophores according to the rising water level, which can also contribute to minimising the rise of the mean sea level toward inland. To this is added that the black mangrove roots, together with the pneumatophores, can make up a dense layer to retain part of the water.
Like the red mangrove, this species is viviparous, since the fruits germinate before falling to the forest floor, which is why they are considered propagules. These propagules have the shape of an oblong to elliptic capsule, are light green and slightly pubescent. The fructification occurs in the early rainy period, which is why the propagules are distributed with the movement of the waters on the forest floor until their establishment. The distribution of the propagules by the waters facilitates their implantation in the interior, allowing for the establishment of the species and its protective function.
The L. racemosaes species, considered a pioneer species in perturbed sites, is generally located on the posterior strip to that occupied by the black mangrove, though it also can form mono specific populations bordering rivers and lagoons, is less tolerant to salinity than the black mangrove and can develop together with the red mangrove. The radical system of that species is not very deep, with roots coming out of the trunk in a radial shape and producing projections with negative geotropism (pneumatophores), whose height depends on the height of the water column. These roots also can create a dense layer with protective functions.
Another aspect to take into consideration is related to the greater protective capacity of the strip of mangrove forest according to the height and structural development of the mangrove, as well as the width of the strip of vegetation of the coast toward terra firma. Bacon stated that that the predictions of the effects of the rise in the mean sea level over the mangroves have been assessed in a very simplistic way, because it has been assumed that the structure and distribution of the species is uniform on the coastal strip. However, the diversity of types of mangrove forests, their distribution, plant composition, their state of health and the level of affectations, anthropic as well as natural, imply the response in the face of the rise in the mean sea level. Which is why the changes in dominance of species, the migration inland and increased area in the short term will depend on the characteristics of each particular site.
In some coastal sites the strip of mangrove forest has suffered affectations, like their bulldozing to increase the agricultural area, felling and extraction of wood, the development of urbanisations, the construction of roads and the damming of rivers, with the subsequent decrease in the flow of water, nutrients and energy, among other actions. This has led to their reduction, weakening or decrease of the baldachin’s height, with an impact on its function of protecting the coasts in general and especially in the face of the consequences of the rise in the mean sea level caused by climate change.
It is urgent, therefore, that the strip of coastal wetlands be conserved, among them the mangrove forests, through the implementation of programmes and actions of adaptation like the ecological restoration and the creation of awareness in the coastal communities geared at guaranteeing their protection and care. (2013)
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