262 CAIN, DE OLIVEIRA CASTRO, PIRES, AND DA SILVA 



forest. In the Belem region Black et al. (1950) found 87 species of trees on 

 a 1 -hectare sample plot of terra firme rain forest and 60 species on a similar 

 plot of igapo rain forest, all 1 dm. or more in diameter. On a 2 -hectare sample 

 plot at Mucambo, which we will report on in some detail, Murga Pires found 

 173 species of trees. 



In January, 1956, the authors found it possible to visit Arapari Island, a 

 few hours by launch south of Belem, through the courtesy of the Brazilian 

 Malaria and Public Health services. The island lies approximately at 1 ° 40" 

 S. Lat. and 48° 30" W. Long., where it is part of the comphcated system 

 of rivers and channels of the Belem region southeast of Ilha do Marajo, 

 which is the principal island of the delta region of the Amazon. The rivers 

 and channels are subject to the influence of the Atlantic tides, and all of 

 Arapari island lies in the flood plain subject to the annual high-water periods. 



About 3^/3 million acres of the Amazon basin, the hylaea, are covered by 

 rain forest that is divided into two principal types. The mata da terra firme 

 is the non-flooded upland. The mata da varzea is essentially the flood plain. 

 It varies in width up to about 160 km. and in depth and duration of its an- 

 nual inundation. Some intermediate strips of land that are occasionally 

 but not regularly inundated are called restinga, and any small channel or 

 basin where the soil never dries out is called an igapo. Locally, as at Arapari 

 Island, shallow basins that stay wet from rain water are also called igapos. As 

 seen from the boat, Arapari Island shows no topographic relief. One sees an 

 even wall of forest with scattered emergent trees such as Ceiba pentandra. In- 

 ternally, however, the island is threaded by channels that carry tidal water 

 and the flood waters of the main rivers. There are numerous shallow, muddy 

 basins kept moist by the almost daily rains which accumulate to more than 

 100 inches a year. 



Along the muddy banks of rivers and channels that are subject to tides, 

 there occurs a very common community dominated by Aninga, Montrichardia 

 arborescens. This is a peculiar member of the Araceae that forms dense clans 

 by vegetative reproduction of stout rhizomes, with a pigmy forest of trunks 

 rising from a few to several feet, according to the fluctuation in tide levels, 

 topped by tufts of broad, arrow-shaped leaves. One afternoon a rather rough 

 sampling of this community was attempted from a rowboat. Ten units of this 

 shore-line community were examined for the presence of different species, 

 and for which coverage estimates were made. Each unit was approximately 

 30 m. (100 feet) long. Because the banks are typically steep, the community 

 usually has a depth of about 5 m., so it was possible to locate all species and, 

 when the identification was uncertain, collect them from the boat. The dom- 

 inance of the species was estimated according to six coverage classes: (*) 

 covering less than 1 per cent, (1) 1-5 per cent, (2) 6-25 per cent, (3) 26-50 

 per cent, (4) 51-75 per cent, and (5) 76-100 per cent. Table 1 shows the 



