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Plant Migration

University of California Press

Mangrove Swamps

By definition, mangroves are trees rooted in substrates that are flooded by seawater, either constantly or periodically. Mangroves belong to many different angiosperm families, some of which also include upland genera, but all the mangrove genera are tightly confined to their saline swamp habitat. Most mangrove genera are tropical, with a minority ranging into the subtropics. Members of the genus Avicennia, a tree belonging to the Verbena family, range farthest into temperate regions. Within their latitudinal limits, mangroves have very broad edaphic and climatic tolerances. Because they are able to draw water molecules and nutrients from the sea, they are remarkably indifferent to substrate, growing in rock crevices and many kinds of sand and mud. Where there is good circulation of seawater, they are also remarkably indifferent to rainfall, growing adjacent to rain forests, deserts, and everything between. Nevertheless, suitable mangrove habitats are extremely discontinuous because of the need of shelter from wave action for seedling establishment.

Within a mangrove forest, species patterns often show striking zonation, which has often been interpreted to be the result of autogenic succession: a pioneer fringe advances seaward, traps sediment, builds up land, and prepares the way for advance of the next zone. However, case histories show the dynamics are usually more complex.

River Deltas, Southern Gulf of Mexico

(Thom 1967; West et al. 1969)

The great uncontrolled rivers of Tabasco and Campeche have provided a dynamic habitat complex in which mangrove distribution patterns are continually changing. In historical time, active river mouths have switched between some distributary channels that fan out over the deltas. During floods, active distributaries build ever higher and wider natural levees along their channels. The sand they pour into the Gulf of Mexico is carried by longshore currents to be laid up by waves as a series of accretion beach ridges parallel to the shore. The constant loading of additional sediment causes regional subsidence of the whole delta. When a distributary is becoming inactive, the old channel is slowly filled with fine sediment deposited by general floods during the winter rains. During the dry season, saline water invades abandoned stream channels and lagoons, and penetration increases during the years as subsidence goes on. Water levels are less affected by the trivial lunar tides than by changes in wind direction, the highest levels coming during coincidence of maximum river discharge and strong north winds during winter. These northerly storms, called nortes, also cause severe beach erosion. After the sand supply has been diverted by a shift in a river mouth, huge areas of beach ridge systems near the abandoned mouth are eventually lost to the sea.

The dominant mangrove species in this region are Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa ; all produce nondormant seedlings capable of long flotation. The mangroves do not colonize natural levees being built by active distributaries; there the first colonists are marsh plants, such as Phragmites, Spartina, and Typna spp. When a distributary is being abandoned and becoming saline, Rhizophora seedlings floating in from the sea usuallv colonize the banks. The tangle of stilt roots of the Rhizophora fringe catches debris and sediment, and the mangroves add organic detritus of their own. As the channel fills and narrows, the Rhizophora fringe widens by establishment of new seedlings, while Avicennia and Laguncularia seedlings join the older Rhizophora in the rear, forming a mixed mangrove forest. As long as peat formation under the mixed mangrove forest compensates for subsidence, the species may persist in situ indefinitely unless the forest is destroyed by retreat of the seashore or is overrun by a new active stream channel.

A different geomorphicbiotic sequence begins on the back side of the natural levees, which slope gradually to mudflats along seasonally saline lagoons. As the mudflats are built up by sedimentation during floods, they are first colonized by Spartina and Batis maritima, a prostrate perennial halophyte. Here Avicennia is usually the pioneer mangrove, with the other

two joining in as sedimentation proceeds. When the distributary becomes inactive and sedimentation is shut off, subsidence will cause the mangrove fringe, with Avicennia in the lead, to move toward the crest of the sinking levee. Lagoons enlarged by subsidence eventually have enough fetch for generation of waves, which undercut the mangroves and wash them away.

Thom (1967) postulated a variety of other geomorphic sequences in this deltaic complex. His general conclusion was that in certain situations, mangroves trap sediment and lay down peat so that vegetationally controlled, autogenic succession may temporarily prevail, but that over the long run habitat changes controlled by the mangroves are overwhelmed by a grand geomorphic cycle controlled by the rivers and the sea.

Coral Cays, Belize

(Stoddart 1962, 1963, 1969)

Mangroves occupy a totally different geomorphic setting on the other side of the Yucatan peninsula, where they grow on coral cays with no river or terrestrial sediments. The cays lie on the barrier reef and on atoll-like structures beyond. The cays most exposed to wind and surf have beach ridges built of coarse coral debris, which are colonized by species discussed in the next chapter. On these exposed cays, mangroves are confined to a fringe on the leeward shore. On more sheltered cays, typically shoals with no dry land, mangroves occupy the whole area. Rhizophora mangle is generally strongly dominant, particularly on the outer margins of the mangrove forest, joined by Avicennia germinans and Conocarpus erectus on slightly higher areas. Laguncularia racemosa occurs infrequently.

Autogenic succession, with mangroves advancing as they trap sediment and lay down peat, cannot be a general process in these cays. Some of the mangrove cays apparently have permanent shorelines with rock foundations rising from fairly deep water. There, mature mangroves grow to the edge with no seedling colonization beyond. Other unconsolidated cays are gradually migrating shoreward; whether they are constant in area, increasing, or disappearing probably depends on the reef flat topography, sand supply, waves, and currents, not on the vegetation.

Catastrophic changes in mangrove patterns were documented by Stoddart's (1962, 1963, 1969) surveys before and after Hurricane Hattie in 1961. His 19601961 maps of the cays show vegetation patterns that may have been in approximate equilibrium. Most of the cays had not had a major hurricane strike for 30 years; the southernmost part of the surveyed area may have had hurricane damage 15 years before. The center of the 1961 cyclone passed directly over the cays. Sustained wind speeds were estimated at 250

km/hr with gusts to 320 km/hr. Atmospheric pressure was so low that over an area about 100 km in diameter, the sea rose about 5 m above normal level. Huge waves were superimposed on this storm surge.

The storm broke branches and stripped leaves from mangroves over a wide swath. Stoddart's 1965 resurvey found mangroves were generally dead within a swath...