Mangrove swamps are found in tropical and subtropical coastal regions which offer some shelter from the open sea - lagoons, bays and river estuaries. Alternative names for these regions aremangrove forest and mangal. Some coral reefs, too, bear mangrove swamps. At high tide, the areas are flooded with sea water. The global area of mangrove swamps is large, over 170 000 square km, covering three-quarters of tropical coastline. They are home for many animal species, and they afford some protection to the coastline against wave erosion, including tsunamis and storm surges. They also provide local human populations with many necessities, e.g. wood. Mangrove swamps are consequently habitats well worthy of study and preservation and the mangroves are very special plants.
Mangroves are not numerous. Out of the estimated 300 000 total species of flowering plants, there are only some 110 known to inhabit mangrove swamps; about half are swamp specialists growing exclusively in this environment. In particular locations, the species present may be counted in single figures; sometimes there is just one.
What makes life so difficult for plants in a mangrove swamp?
The stresses that this habitat poses are several. Firstly, there are physical forces - waves, gales. Secondly, there is lack of air in the mud. Thirdly, there is the problem of the salinity of the sea water. The salt (sodium chloride) concentration in sea water is 3% in round figures; for the vast majority of flowering plants, concentrations from 0.5% upwards are toxic. The first land plants emerged from the sea about 400 million years ago (late Silurian / early Devonian geological periods) and from these ancestors, flowering plants evolved on dry land with a fresh water supply. The mangroves are secondary returns to a saline habitat (some100 million years ago), just as marine mammals, e.g. dolphins, are secondary returns of mammals to the sea. In appearance, the evolutionary adaptation of the dolphin seems more extreme: it could be mistaken for a fish, whereas a mangrove tree still looks very much like - a tree. Numerous adaptive features which enable the mangroves to thrive in their extremely hostile habitat are not visibly apparent. The environmental stresses, and adaptive features that counteract these stresses, are discussed below. Not every mangrove species shows ALL the listed features, but all show some. A mangrove swamp is not a uniform environment but exhibits zonation; e.g. the lower down the shore, the greater the degree of inundation. Different species are adapted for different zones.
1. Physical forces
The water in most of the swamp undergoes much movement. In addition to the regular surge of the tides and the current down river estuaries, there is wave action, which is violent during storms, hurricanes and tsunamis, and storms themselves can dislodge trees. The mangroves are rooted in semiliquid mud or soft sand, which are not very stable substrates. The root systems of the mangroves are robust. In addition to a branching root system in the mud, typically with a wide spread, there may be numerous stilt roots (prop roots) growing from high up on the trunks to achieve stability. (Fig. 1).
Fig. 1. Mangroves showing stilt roots. Honduras, 2009. Photo by Tiiu Miller 2009.
An example is the red mangrove (Rhizophora mangle), named so for its reddish bark. This species grows lowest down on the shoreline in the zone of the greatest degree of inundation. Stabilization is enhanced by the root systems of
adjacent trees tangling together and affording mutual support. Indeed roots of adjacent trees can graft together to make a living unit.
2. Lack of oxygen for the roots
Plants are aerobic organisms, i.e. they need oxygen for respiration; at the biochemical
level, plant respiration is essentially identical with human respiration. Oxygen does dissolve in water, but only sparingly; the solubility decreases with rising temperature, and the swamps lie in the warm tropics. Accordingly, for even those parts of the roots that are merely submerged, the oxygen supply will be limited and the mud or sand at the bottom is practically anaerobic (oxygen-free). There are two ways in which mangrove roots are supplied with oxygen.
Firstly, where there are stilt roots, their upper parts which stand in the air at least periodically, bear minute aerating pores, lenticels, in their bark. The lenticels are areas of loosely packed cells, between which let air through and inside the stilt roots air channels lead into the submerged roots.
Secondly, some mangroves bear special aerating roots, pneumatophores. Most roots are positively gravitropic, i.e. grow downwards along the force of gravity. Pneumatophores are negatively gravitropic and grow upwards against the force of gravity. Inside, air channels again connect with the submerged root system. The pneumatophores of the black mangroves (Avicennia species), and the white mangrove (Laguncularia racemosa) are of a type called peg roots, which stick straight up from the water (Fig.2).
Fig.2 Pneumatophores of black mangrove (Avicennia germinans) - roots growing upwards to absorb oxygen. Mangrove Lagoon Wildlife Sanctuary & Marine Reserve, St. Thomas, US Virgin Islands.Photo by Doug Moyer 2008.
The black mangroves grow higher up on the beach than the red, and the white mangroves higher up again. Some species have "knee" roots, which first grow up, then down, leaving a bend above the water; the reaction of these roots to gravity changes during growth.
3. Salt toxicity
In solution, the elements that make up salt exist separately as sodium ions with a positive electric charge, and chloride ions with a negative charge. Living cells are extremely complex systems and their constituents, e.g. proteins, also carry electric charges, on which their structure and function depend. The electrically charged salt ions interfere with the normal charges in cells.
One way of avoiding salt toxicity is to keep salt out. Red mangrove roots achieve a very considerable degree of salt exclusion, taking up water with a very low proportion of salt compared with the sea water.
Another adaptation is what the biologist calls compartmentation within the plant. A large proportion of the bulk of a mangrove is wood (xylem), which consists mainly of dead cells: fibres for strength, and conducting cells, hollow cylinders with strong woody walls laid end to end to make metres of pipework, Through these pipes moves xylem sap, distributing the water and minerals absorbed by the roots all through the plant.. This non-living compartment of course suffers no ill effects from salt ions present. Eventually, however, any salt carried in xylem sap finishes up in living cells, especially in leaves.
But there is also compartmentation within the living cells. One of the characteristics that distinguishes a mature plant cell from an animal cell is that most of the volume of the plant cell is taken up by a vacuole, like a bubble filled with sap, separated by a membrane from the truly living part of the cell, sometimes termed protoplasm. The vacuole is a storage area for nutrients (when we drink fruit juice, we drink mainly vacuolar sap) - and for waste. Mangroves pump salt that enters cells into vacuoles; mangrove leaf tissues tend to be succulent, with large vacuoles.
Finally, there is what one might call waste disposal. There is a limit how much salt a cell can store in its vacuole; but old leaves die and are shed, dumping their load of salt. Mangroves can also bear salt glands on leaves, which actively pump out a concentrated salt solution, using energy from respiration. The white mangrove is so named because its leaves glisten white with dried salt crystals, secreted from two knob-like glands at the leaf base. The black mangoves Avicenniaspecies) have tiny glands dotted over leaf surfaces, just about visible to the eye.
4. Lack of water - yes!
The salt in the mangrove swamps is also responsible for water stress. Salt stress and water stress are so closely interlinked, that salinity stress can be considered as a special case of water stress. Even the "direct toxic" effects involve interference with cell water at the submicroscopic level.
It may seem strange that a tree which stands in water should have problems with the water supply. But the water that bathes a mangrove tree is sea water, salt water; in tidal pools evaporation of water results in even higher salinity. In sea water, most flowering plants would find themselves in the same danger as sailors on the ocean when fresh water supplies run out: the water is physically there, but physiologically unusable.
Sodium chloride has a high affinity for water; many of us will have noted how table salt in an open container absorbs water from a humid atmosphere till it is a wet slush. A plant native to a non-saline habitat when placed in sea water would not just fail to take up water - it would actually be dehydrated, the salt drawing water OUT of the plant tissues.
In the preceding section we have seen that the mangroves avoid toxic effects of salt by excluding it altogether or/and pumping it out from the active, living protoplasm. They counteract the tendency of salt to draw water out by synthesizing high concentrations of so-called compatible solutes in the living protoplasm. These are organic chemicals, compatible with normal cell functioning, but capable of counterbalancing the high salt concentrations and drawing water in. Thus mangroves are able to extract water from sea water.
As in all plants, the sun lends the mangroves a helping hand. The upward movement of sap is energized directly by solar energy: the sun pulls water up plants - but that is another story.
5. Multiple stresses; consequences for overall productivity
The presence of several stresses of course makes it harder for plants to survive than if there were a single one. Moreover the environmental conditions are very far from constant. The stilt roots and pneumatophores must be able to survive under water (high tide) and in air (low tide), two very different conditions. In a heavy rainstorm or when a river estuary floods, the plants higher up the shore are suddenly subjected to more or less fresh water. In tidal pools, evaporation can concentrate the salt to several times the sea water level and rainwater can dilute it greatly. The mangroves' adaptive systems obviously can cope with all that change. For some species at least, the variation in salinity has become a necessity - they need occasional flushings with fresh water to survive.
The adaptations have to be paid for in terms of productivity. Growth, measured as the net synthesis of plant material, is low in mangrove swamps, especially considering that they enjoy warm temperatures and high levels of sunlight all the year round. Austin (1989) quoted the primary net dry matter production in tonnes per hectare per year as 6 - 8 for mangrove swamps. For comparison, the values for both temperate forest and tropical savannah grassland were 10 - 20, and for tropical rain forest 30 - 40. The mangroves must use up a larger proportion of their photosynthetic products in respiration to generate energy for survival, leaving less for growth. Salt glands have a very high rate of energy consumption. Stilt roots and pneumatophores need a large investment of organic material and energy for their growth and maintenance, but do not contribute to net synthesis of organic material.
6. Survival of the next generation
A flowering plant's fertilized egg develops into an embryo within the ovary of the flower. In most flowering plants there comes a stage when the embryo stops growing, dries out, and its metabolic activity comes to a virtual standstill: it becomes dormant ("sleeping"). A seed contains the embryo wrapped in a protective seed coat of parent tissue and supplied with a nutrient store by the parent plant. When ripe, it is released, sometimes within a fruit. Under favourable conditions, a seed eventually germinates, i.e. it imbibes water, the embryo swells, bursts the seed coat and puts out its first root. Dormancy lets seeds survive unfavourable environmental conditions (cold winter, dry season); seeds may remain dormant in soil for years before germinating.
Mangroves, growing in a permanently warm moist habitat, do not produce dormant seeds. Their seed do not dehydrate but once fully formed, germinate immediately, either when shed, or in some species even while still attached to the plant. The latter species are described as viviparous, "live [young] bearing", for the embryo just grows until a sizeable plantlet is is dropped off. Fig. 3
Fig. 3. Seedlings of red mangrove (Rhizophora mangle) hanging on tree. The visible part is root and stem; brownish seeds, enclosing top of seedling, can be seen in places. Photo by Jaro Nemcok 2007 http://nemcok.sk/copyright.htm)
shows red mangrove seedlings hanging from the trees; these consist mainly of a stout axis of stem plus root about 20 cm long; the leaves are still within the seed. Falling, such a seedling plants its stout root in the mud. In some species, seedlings float on their sides on the water surface for a while, resulting in dispersal further from the parent plant; gradually the root tip gains weight and the seedling straightens, root down. Some seedlings live practically under water for several years, before they are tall enough to escape prolonged submersion. Seedlings that get stranded sideways on mud are stated to grow out roots in such directions as to pull themselves upright.
(a)What should be called a mangrove?
Sometimes this name is reserved for plants of the family Rhizophoraceae, with the largest number of mangrove swamp species. Some botanists keep it for the genus Rhizophora, or even for one species, Rhizophora mangle, the red mangrove. Others distinguish between "true mangrove" and "mangrove associate" species. Such restriction suits botanical specialists concerned with plant classification. But for this article, aimed at a general readership, it seemed most convenient to use the popular terminology of "mangrove" for all plants specialized for the swamp habitat.
(b) What is a halophyte?
In the literature, mangroves are often referred to as halophytes. This is a general term meaning "salt plant" applicable to any plant, in any region, that can grow at salt concentrations of 0.5% or more.
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Data on net dry matter production from:
Austin, R.B. (1989). Prospects for improving crop production in stressful environments. In Plants under Stress. Society for Experimental Biology Seminar Series 39, 234-48. Cambridge University Press.