The next time you cross the northern bridge at MEMBG, be sure to observe two plants of Rhizophora mangle L., red mangrove (Family Rhizophoraceae), growing on water's edge upstream. Mangrove vegetation is a low, dense forest of small trees colonizing saltwater, tideland flats of tropical and subtropical coastlines. Throughout the world, only four dozen species of trees are capable of thriving with their roots in ocean water, potentially covered twice a day at high tide, and dealing with very low levels of soil oxygen for roots and toxic levels of sulfides. Although red mangrove--and likely most mangroves--can be grown in fresh water, at least one author has suggested that Rhizophora grows more rapidly in saltwater. At MEMBG, we established our plants in a muddy-bottomed fish tank with brackish water, then weaned on fresh water before transplanting to the stream.
Red mangrove is the most common and widespread tree species of all mangroves, and the pioneer species of sandy coastlines protected from heavy waves. The species occurs along the Pacific coast of the Americas from 28 degrees N latitude in Baja California and Sonora, Mexico, and Isla TiborŪn in the Gulf of California, to near the Panama-Colombia border and westward to the Gal·pagos Archipelago. Rhizophora mangle has become naturalized in the Hawaiian Islands during this century. On the eastern side of the hemisphere, R. mangle grows as far north as Bermuda (32 degrees20 minutesN) and South Florida, throughout the West Indies and coastal Central America to southern Brazil (25 degrees S). The same species appears again along the eastern Atlantic Ocean in tropical West Africa.
Rhizophora mangle is able to traverse broad ocean regions by producing large seedlings that float horizontally, undamaged by salinity. These seedlings can be washed up on sand flats, where they settle to establish new populations. The genus is one of a few exhibiting vivipary, germination of the embryo of a seed while the fruit is still attached to the mother plant. In this case, the hypocotyl develops as a long, stiff axis, sometimes exceeding a foot in length. Two other species in the genus Rhizophora occur in mangrove associations of the Neotropics, and three others occur in the Old World tropics--all viviparous, pioneer species.
In New World mangrove swamps, red mangrove forms a fringing band of trees on the outermost zone, facing the open sea. Black mangrove (Avicennia germinans) forms the next zone, and white mangrove (Laguncularia racemosa) and buttonwood (Conocarpus erecta) form the third zone, by land, because they are less tolerant of immersion in saltwater. The mangrove swamp community is established by red mangrove, which forms a tangle of aerial roots (prop roots, a type of adventitious root) that erupt through the bark, arising from the woody lower stems. The arching aerial roots provide firm anchorage and stability from tidal action, wind, and shifting sand. They also permit some oxygen to reach the oxygen-starved submerged roots. Aeration occurs through lenticels, blister-like structures that form on the outer root surface. By trapping silt and organic sediments, the tangled aerial roots create a buildup of a dense, mostly organic soil that provides a new environment for establishment of the other species.
When you observe our specimens closely, make note of the opposite leaf arrangement along the reddish stems of red mangrove. Also note its thick leaves, which are elliptical and have a smooth (entire) leaf margin. Growing tips never exhibit dormancy or bud scales; the shoot tips grow continuously and are evergreen, because their shoots always bear leaves. Stipules occur in pairs that enclose the younger leaf primordia, and also conceal branches that are already forming from axillary buds within the shoot tip (sylleptic branching). When the stipules fall away, this produces a prominent scar on the stem.
Because no mangrove tree species tolerates freezing air temperatures, we are keeping our fingers crossed here at MEMBG. Actually, our garden--located at 34 degrees N latitude and presumably too far north for Rhizophora--has not experienced a freezing temperature since February 1949. Wish us luck! We will let you know if this red mangrove survives its first winter, and if and when it flowers!
ARTHUR C. GIBSON