46 WEST 



grows in solid stands; individual plants often reach heights of 80 to 100 feet. At low tide the 

 gnarled prop roots of this tree, four to ten feet high, are completely exposed, rising from a 

 surface of soft, brackish ooze, into which a heavy man would sink to his knees. At high tide, 

 however, the Rhizophora swamp can be penetrated along a maze of tidal estuaries and creeks 

 in small boats or canoes. 



Although the Rhizophora is the dominant mangrove component, it is best developed along 

 the edge of tidal channels where there is complete flooding by the tide twice each 24-hour pe- 

 riod. Inland, there exist round patches of low growth, four to six feet high, composed of 

 dwarfed Rhizophora with stunted prop roots, mixed with black mangrove ( Avicennia nitida ) and 

 other brackish water swamp plants. These patches of low growth occupy ground that is higher 

 and drier than the channel banks. They are flooded usually by semimonthly spring tides only. 

 The soil is peat-like, soggy, and quaking underfoot; a heavy man sinks to his ankles in the top 

 muck. Often in the center of these patches are found hammocks of the naidi palm, a plant 

 characteristic of the fresh-water swamp, or in some instances even clumps of equatorial rain- 

 forest. The centers of these patches are the highest, driest, and firmest parts of the mangrove 

 swamp, but they are extremely difficult to reach. 



The curious occurrence of such patches of low growth on higher, drier ground is typical 

 of other mangrove areas. For example, along the coast of Malaya such areas are called 

 "byiaks" and are readily noticed on aerial photographs. 5 The formation of such features is 

 probably due to the slow rise of land through the decay of mangrove vegetation. The underlying 

 peat contains remains of large Rhizophora roots and trunks. 



Another curious feature characterizes the mangrove swamps of the Pacific coast of 

 Colombia. Within the swamp back from the seacoast for a distance of one-half to one mile, 

 one frequently encounters along tidal channels small areas of sandy material which rise slightly 

 above the general level of the swamp muck. These sandy "islands" are locally called "firmes." 

 They are sites of human habitation within the mangrove swamp, fresh water is usually found at 

 a depth of three to four feet below the surface, and coconut palms, patches of maize and other 

 crops are grown. These peculiar sandy "islands" appear to be remnants of old beach ridges 

 that have been largely destroyed with the general seaward advance of the coast. 



THE FRESH-WATER SWAMP 



The fresh-water swamp affords greater ease of travel and better possibilities for food 

 supply than the mangrove. The prop roots of red mangrove and the soft brackish muck are 

 absent. Yet the daily flooding by river overflow makes most of the swamp penetrable only by 

 canoe, during high tide. During the past 25 years natives from up-river have been utilizing the 

 flooded banks of the rivers within this zone for growing rice. This development has lessened 

 the problem of obtaining food within the coastal area. 



Although malaria occurs throughout the Pacific coast, the Anopheles mosquito prevails in 

 the fresh-water swamp zone. Pools of standing fresh water, left by the retreating tide, form 

 excellent breeding places for these insects. At dusk and at dawn swarms of mosquitos and other 

 biting insects (mainly black flies and gnats) occur around every hut and canoe to pester unpro- 

 tected occupants to distraction. Smaller numbers of mosquitoes breed in the mangrove zone 

 (chiefly in small bodies of rainwater that collect in epiphytic plants on the trunks of large trees) 

 and in the beach area (principally in the shallow wells dug into the sand). Curiously, few mos- 

 quitos are encountered along the middle and upper courses of the streams that flow across the 

 narrow coastal plain. 



MANGROVE AS A GEOLOGIC AGENT 



It is commonly stated that red mangrove (Rhizophora) is an important agent in the sea- 

 ward advance of coastlines. It is supposed that Rhizophora is the pioneer colonizer of 



^Stamp, L. D., "The Aerial Survey of the Irrawaddy Delta Forests," Journal of Ecology , Vol. 13, 

 pp. 262-276, 1925. 



