494 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



direction. Mangroves prefer brackish water but can tolerate a wide 

 range in salinity from virtually fresli to extremely salty water. Reef 

 corals cannot live long in water that falls below 70° Fahrenheit, while 

 mangroves thrive in much colder water. At Galapagos, for example, 

 the water temperature is frequently in the 50's, owing to the cold Hum- 

 boldt current, yet all three species of American mangrove are found 

 growing. On the other hand, a drop in air temperature to 25° Fahr- 

 enheit is almost certain to kill mangroves — while, of course, a drop 

 in air temperature per se has little or no effect on corals, unless fol- 

 lowed by a drop in water temperature. 



Although, obviously, the conditions under which corals and man- 

 groves can exist together are far from the optimum conditions for 

 either, both play important roles in creating tropical real estate — and, 

 in a sense, they often work together. Normally, the corals must first 

 do their work, however, producing conditions where mangroves can 

 grow. 



Young mangroves must have protection from waves and currents. 

 On beaches exposed to surf, or on bars and flats where currents are 

 strong, the seedlings cannot gain good footholds. They may root in 

 such areas and hang on for months but are almost sure to be killed or 

 swept away by wind, water, or shifting sands before reaching ma- 

 turity. If, however, a shore or a shoal is protected by a coral reef, 

 the mangrove seedlings can establish themselves. 



MANGROVES MAKE NEW LAND 



Thus mangroves may become established on the inside of a coral 

 reef, where relatively still water is found, sedimentation is rapid, and 

 debris accumulates. These, of course, are conditions that live corals 

 cannot tolerate ; so the mangroves take up where the corals leave off. 

 As the reef corals continue to gi'ow outward, toward the sea, the man- 

 groves grow along the rubble-strewn inner edge, trapping in their 

 great spreading roots the seaweed, sponges, shells, chunks of dead 

 coral, and other material that washes over the reef. By reducing the 

 flow of water, the mangroves aid further in the builduj) of sand, marl, 

 and other deposits. 



As the mangrove colony increases, broken limbs and roots from the 

 trees decay and form peat ; and the land may be further built up by 

 calcareous algae and the shells of mollusks and crustaceans that live 

 among the roots. Birds may roost in the trees and leave deposits of 

 guano. Other species of mangrove may become established in the 

 swamp, as well as other salt-marsh plants. As the land becomes 

 higher, coconuts may float ashore and take root, as well, perhaps, as 

 the seeds of the sea hibiscus, or mallow. In Indo-Pacific areas, screw- 

 pines and casuarinas are frequently found near the mangroves, while 

 in the Americas the seagrape is often present. 



