Plant Migration: The Dynamics of Geographic Patterning in Seed Plant Species

by Jonathan D. Sauer

Overview

Using cases of plant migration documented by both historical and fossil evidence, Jonathan D. Sauer provides a landmark assessment of what is presently known, and not merely assumed, about the process.

Product Details

ISBN-13:	9780520068711
Publisher:	University of California Press
Publication date:	09/11/1991
Edition description:	First Edition
Pages:	298
Product dimensions:	6.00(w) x 9.00(h) x 0.80(d)

About the Author

Jonathan D. Sauer is Professor of Geography at the University of California, Los Angeles.

Read an Excerpt

Plant Migration

The Dynamics of Geographic Patterning in Seed Plant Species

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By Jonathan D. Sauer

University of California Press

Copyright © 1988 Jonathan D. Sauer
All right reserved.
ISBN: 0-520-06003-2

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Chapter One

Shoreline And Other Naturally Open Habitats

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 geomorphic-biotic 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 1960-1961 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 about 65 km wide, the only survivors being in very small patches on the leeward sides of the larger cays. Within the dead stands, the ground was still occupied by the old roots and there were a few seedlings. New seedlings were establishing mainly in new bare areas.

Great Barrier Reef, Queensland

(Hopley 1982; Stoddart 1980; Stoddart et al. 1978)

Queensland, Australia, has a much richer complement of mangroves than the neotropics, with 27 species belonging to many different angiosperm families at least locally dominant. All are sea dispersed and shared with the East Indies. With ocean swell shut off by the Great Barrier Reef and a tidal range of 3 to 6 m, mangrove swamps require only minimal topographic shelter and are very extensive.

Within a mangrove stand, species commonly show rather regular zonation, which has evoked deductive models of predictable, autogenic succession. However, where distribution patterns have actually been monitored over time, in some cases by detailed remapping over nearly 50 years, irregular and unpredictable changes were found. Rather than actively advancing by autogenic succession, the mangroves generally passively follow changes in reef morphology with coral growth and destruction.

Naturalized Mangroves, Hawaii

(Wester 1982)

Hawaii is better situated to receive drift from Alaska than from the tropical Pacific. It had no native mangroves, the nondormant seedlings evidently being unable to survive the circuitous voyage up the coast of Asia, across the northern Pacific, and back down to the tropics.

In 1902, the American Sugar Company introduced Rhizophora mangle from Florida to Molokai, Hawaii (fig. 1), where it was planted on the south coast in an attempt to stabilize sediment produced by erosion on the slopes of the island. Seedlings were transplanted from Molokai to Oahu and have since volunteered on the islands of Hawaii, Maui, Lanai, and Kauai, evidently spreading within the archipelago by natural drift. The neotropical button mangrove, Conocarpus erectus, was also planted in various places along the coast but has not naturalized.

In 1922, four Philippine mangrove species were introduced by the Hawaiian Sugar Planters Association and planted on mudflats around Oahu. Ceriops tagal promptly died; Rhizophora mucronata and Bruguiera parviflora survived for a few years. Bruguiera gymnorhiza has survived and reproduced in several sites on the Oahu coast, in spite of much exploitation of its flowers for leis.

Some of the Hawaiian mangrove colonies depend on artificially modified habitats. The prehistoric rock-walled fish ponds, built in the sea by the Polynesians, offer sheltered habitats for dense mangroves. Mangroves infest canals, including the Ala Wai behind Waikiki, where they must be periodically removed. Extensive stands occupy deltas below sugar mills, where sediment from cane washing is deposited. A 40-acre stand of Rhizophora mangle colonized such a delta in Pearl Harbor between 1951 and 1975. However, other colonies are established on shores naturally sheltered by reefs and embayments. Presumably, these would have been colonized prehistorically if viable seedlings had arrived.

Naturalization of Avicenna, California

(Moran 1980)

Several neotropical mangrove species range naturally up the Pacific coast of Mexico across the Tropic of Cancer. They extend above 29°N latitude in the Gulf of California, but none extend above 27°N on the Pacific side of Baja California.

In 1963, two of these species, Avicennia germinans and Lagunclaria racemosa, were planted experimentally at about 32.5°N latitude in California. The site is a salt marsh dominated by Spartina foliosa on Mission Bay in San Diego. It is administered as a nature reserve by the University of California, San Diego. Neither of these mangroves succeeded, although one Laguncularia survived for several years.

In 1968, seedlings of two Indo-Pacific mangroves were planted in the same marsh: Aegiciras corniculatum from tropical Queensland and Avicennia marina from 37°S latitude in New Zealand. The Aegiciras has survived and flowered but not reproduced. Avicennia marina established thousands of seedlings, now in the third generation; its success threatened the native biota of the marsh. Since 1976, university students have stopped Avicennia reproduction by removal of flowers and seedlings; in 1979 about 12,000 rooted seedlings were pulled up, but a few were left.

Comment

Normal dispersal of mangroves is entirely by ocean currents. Many species are viviparous, that is, seedlings germinate before release from the mother tree and continue to develop while floating. With light, water, and nutrients available, dormancy is pointless. Seedlings occasionally strike root in rock crevices, even on open shores. On loose substrates, they need quiet water. Even where there are only gentle waves, it is common to find dead and dying mangrove seedlings stranded in the drift zone, on the new island of Krakatau (discussed in the section on Fresh Lava and Pyroclastic Deposits), live seedlings of various mangrove species have repeatedly been stranded, but have failed to establish for lack of shelter.

Once established, mangroves are very resistant to storm and wave damage. The catastrophic destruction in the Belize cays (discussed above) is an extreme case. The same hurricane struck Grand Cayman Island less violently than Belize, but Grand Cayman had heavy surf and winds over 100 km/hr; these had little effect on the mangroves. Part of the distribution of mangroves is too close to the equator to be subject to any cyclonic storms. In some sites, mangroves survive in situ indefinitely, as shown by deep peat deposits. However, many populations are only temporary and are inevitably destroyed by gradual or catastrophic habitat changes.

Both the highly disjunct spatial pattern and the temporary survival of local populations imply strong natural selection for dispersibility and perhaps for long-range dispersibility. It is sometimes argued that plants do not evolve adaptations for long-range dispersal through natural selection but that long-range dispersal happens only occasionally and incidentally through adaptation for local dispersal. However, in the case of species obligately dispersed by ocean currents, there is complete elimination of any disseminules that do not complete the voyage to another shoreline. Seeds dropped at random in wind or by bird dispersal have some chance of survival, but seeds that sink at sea do not. Selection in mangroves would not favor long-range dispersal per se but rather long buoyancy, regardless of the rate of drift and distance covered. The more sites, near or far, that a seed or seedling can probe, sometimes by repeated stranding and refloating, the better chance of finding a safe place to take root. In the tropical Indo-Pacific region, which is the center of mangrove diversity, archipelagos of islands offer all degrees of spatial disjunction between suitable sites. Also, the seasonal reversals of monsoonal winds and ocean currents allow genetic feedback from successful distant colonies.

Not all mangroves are known to be capable of long ocean voyages, but various species have ranges spanning the Indian and western Pacific oceans; others span the Atlantic. That their dispersal capacities are not unlimited is shown by the lack of any pantropical species and by the absence of mangroves from Hawaii before the introductions discussed above.

Continues...

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