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MANGROVE
(Rhizophora Mangle)

The mangrove family is large and widely scattered, but only one member has gained a foothold in the United States, and it occupies only limited areas in south Florida, at the delta of the Mississippi, and on the coast of Texas. The family’s fifteen genera are confined to the tropics, with a little overlapping on the temperate zones. The botanical name Rhizophora refers to the tree’s peculiar roots, and mangle is the Spanish for mangrove. This is one of the few trees in this country which are known by a single name. It is always called mangrove, and attains its best development in Florida.

The leaves hang two years, are from three to five inches long and one or two wide. Flowers are not showy, but they are nearly always present, blooming the year round, the yellow blossom about an inch in diameter. The fruit proper is about an inch long, but its habit of sprouting while still on the tree and sending down a long stem-like root, gives the impression that the fruit is several inches long, sometimes a foot.

It is not an easy matter to state the average size of mangrove trees. Peculiar habits of growth make measurements difficult. Neither is it easy to tell where a tree begins and where it ends. Mangrove thickets along some of the rivers of south Florida, within the influence of tide water, are strange forms of vegetation. If the foliage alone is considered from a little distance, it reminds one of a row of fig trees in Louisiana or California. The color and general appearance suggests fig trees. A nearer approach reveals beneath the crowns a mass of roots, stems, and limbs, joined with the ground beneath and the crowns above. In addition to these, there are many others that dangle from above, like rope ends, some nearly touching the ground, others several feet above. These are roots or limbs, by whichever name one cares to call them. They grow from overhead branches, and strike for the ground. When they touch the soil, they quickly anchor themselves, and become stems. They then look like slender poles set as props under the branches of an overladen fruit tree.

This strange habit of growth gives the tree its character. Most mangroves stand in water. They fringe the banks of rivers and bayous, extending the fringe as far as the water is shallow. Growth of that kind is generally from ten to twenty feet high, and the largest stems from an inch in diameter up to three or four; but these dimensions cannot be taken as limits to size. Sometimes the trees are sixty or seventy feet high, but those which stand in water seldom reach that size. Trees which have their beginning in the water sometimes end their days high and dry on the land.

The mangrove is a land builder. The sycamore and willow are land builders on a small scale, along northern water-courses, but mangrove excels them a hundred or a thousand fold where it grows on the low shores of Florida. The seed is prepared for land-building work before it drops from the tree. It sprouts a long, peculiar root—it looks like a very slender, big-ended cucumber—the large, heavy end down. This attains a length of several inches or a foot. When it drops from the branch, the end sticks in the mud and takes root, grows, and produces a tree. But generally it falls in water, and not on a mud bank. In that case it floats away, the heavy end down, the light end barely appearing on the surface. Winds and currents drive it about until the lower tip finally touches bottom in some shallow place. There it takes root, and unless circumstances are extremely adverse, it holds fast, finally becomes a tree, sends branches down from above to take root at the bottom of the water, and a clump is produced. The tangled mass of stems and roots catches driftwood and mud, resulting finally in a little island, and later the island is joined to the mainland. Thus the land is built. Many large flats in Florida owe their origin to this tree. When land is permanently above water, the mangrove loses, to some extent, its ability to send roots down from the limbs. Nature seldom does something for nothing, and since the mangrove’s aerial roots no longer serve a useful purpose in nature’s economy, they are dispensed with. Trunks then reach much larger size, and become timber instead of thickets. The accompanying picture shows a mangrove that no longer stands in water, and its habit of growth is changing.

Thickets of mangrove are useful, not only in building new land, but in protecting that already built. Frequently the force of waves is broken, which otherwise would destroy low shores. Tremendous seas, in time of storms, will roll over thickets of mangrove without uprooting them or breaking the stems. Again nature’s fine engineering is apparent. When men build lighthouses which must endure the shocks of waves, they have learned to construct them of open beams and lattice work. The wave passes through without delivering the full impact of the blow to the structure. No solid masonry will stand what a comparatively light open frame will endure without injury, because it allows the waves to pass on. A large wave may strike with a force of 6,000 pounds to the square foot. The mangrove thickets are like the open-framed lighthouse—they let the waves pass through and spend their force gradually beyond, but they hold the shore against washing.

Admirable and wonderful as is nature’s provision for protecting the land by a fringe of lattice work of branches and stems, the marvelous efficiency of the provision has been greatly increased in another way. Suppose, for illustration, that cottonwood instead of mangrove formed the protective thickets along stormy shores. The first hour of heavy seas would reduce the trees to fragments. The weak, brittle trunks and limbs would quickly break to pieces. But mangrove passes through storm after storm unharmed. It is scarcely believable that accident accounts for the fact that the best wood for the place is in the place; but it is probable, rather, that ages of development and natural selection gave to mangrove the qualities which make possible the accomplishment of its work. It is one of the strongest, and as far as available data may be depended upon, it is absolutely the most elastic wood in the United States. Shellbark hickory is rated high in both strength and elasticity; but mangrove rates higher. Sargent gives hickory’s measure of elasticity at 1,925,000 pounds per square inch; but mangrove’s is 2,333,000 pounds.

It is thus fitted in the highest manner to perform the work needed. It plants itself in the right place; develops stems which will endure most and suffer least; possesses enormous strength for resisting force, yet is so extremely elastic that the force of waves is exhausted upon the trunks and branches without flattening them upon the ground or crushing them. Few things of the vegetable world show more perfect adaptation to environment. The wood’s very heaviness seems to add one more quality fitting it for its place. When a trunk falls in the water, it does not float away as most trees would, but sinks like iron, lies on the bottom, helps to hold the forming island or bar in place, and in its death as in its life it is a land-builder. Its efficiency in that particular is increased by the fact that it is little affected by marine borers which, in the warm, brackish waters, usually destroy wood in a short time.

Mangrove is not important commercially, though it is used for a number of purposes. The wood weighs 72.4 pounds per cubic foot, takes good polish, though it is inclined to check in drying; it contains many small pores; medullary rays numerous and thin; color reddish-brown streaked with lighter brown. The principal use of the bark is for tanning and the trunks for piles. It is well fitted for fence posts, but not many have been used in the region where it grows. It rates high as fuel, but its great weight increases transportation charges if the haul is long.

Tanbark peelers in Florida have cut much of the large mangrove forest. They took the bark, and abandoned the trunks. There is no likelihood that the species will be exterminated. Much of the growth is practically inaccessible, and the trunks are too small to tempt bark peelers, and cordwood cutters find plenty of material more convenient.