THE COMMUNITY 



482 



1926 it was thought tliat tliis level "remains 

 a fertile field for investigation from a 

 properly equipped aeroplane" (Allee, 

 1926a, p. 446). In fact, our knowledge of 

 forest stratification may be said to be 

 directly proportional to the distance of any 

 given layer from the ground, with the 

 subterranean and floor strata best known 

 and the canopy the least understood. 



Any of the first five strata may be sub- 

 stratified. This is increasingly true as one 

 proceeds from pioneer to climax communi- 

 ties. For example, the forest of Barro 

 Colorado Island in the Panama Canal Zone 

 has been shown to have eight strata: (1) 

 the air above the forest, (2) tree tops above 

 the main forest roof, 125 or more feet high, 

 (3) upper forest canopy, 75 to 100 feet 

 high, (4) lower tree tops (second story 

 trees or midforest), 40 to 60 feet high, (5) 

 small trees, 20 to 30 feet high, (6) higher 

 shrubs, 10 feet high, (7) forest floor, and 

 (8) subterranean, according to Allee 

 (1926a). The herbaceous stratum in such 

 forests is poorly developed. On the other 

 hand, the herbaceous layer is often well 

 developed in north temperate deciduous 

 forests. The "Big Woods" of Minnesota 

 (Daubenmire, 1936), dominated by sugar 

 maple and basswood, have, in addition to 

 the subterranean and floor strata, six de- 

 fined layers of vegetation, three of which 

 are herbaceous: (1) dominant tree, (2) 

 small tree, (3) tall shrub, (4) tall herb-low 

 shrub, (5) low herb, and (6) moss. 



The equatorial rain forest may have sev- 

 eral strata of trees with several developed 

 canopies, "forest piled upon forest" (Fig. 

 225). For example, the detailed physiog- 

 nomic study of British Guiana rain forest 

 by Davis and Richards (1934) shows four 

 tree strata: (1) a layer of occasional, scat- 

 tered trees with crowns up to 90 to 120 

 feet that forms the canopy, but is not a 

 closed stratum; (2) a layer 75 feet high; 

 (3) a layer at 45 feet; and (4) a lowei 

 layer at 20 feet. This organization is gen- 

 erally true of the American rain forest com- 

 munities, although the uppermost stratum 

 of trees may be absent (cf. Benoist, 1924, 

 for French Guiana rain forest). The true 

 rain forest having at least three tree strata, 

 is not so widespread as generally supposed, 

 and there are many other types of tropical 

 evergreen forest that may be labeled "rain 

 forest' in popular parlance (Beard, 1944). 



In the western hemisphere such typical rain 

 forest is fully developed in the Guianas, 

 from the Orinoco to the Amazon, in a strip 

 some 150 to 200 miles wide paralleling the 

 coast; in the Amazon basin; parts of low- 

 land Central America, Colombia, and 

 Ecuador; and in restricted areas of some of 

 the Lesser Antilles (Trinidad). Such a 

 forest is shown in diagram form in Figure 

 160. This same general stratification is not 



Fig. 160. Vertical stratification in an Ameri- 

 can rain forest community on Trinidad. (After 

 Beard.) 



restricted to a particular hemisphere, but 

 may be found in African forests, for ex- 

 ample, that are separated from the Amazo- 

 nian forest by thousands of miles in 

 longitude (Fig. 161). Furthermore, the 

 same general principle is applicable to 

 nontropical forests— for example, the Cana- 

 dian coniferous forest, separated from 

 tropical forest by thousands of miles in 

 latitude (Fig. 162), attesting to the uni- 

 versality of community organization. 



Epiphytes, especially in tropical forests, 

 may swell the mass of available habitat 

 space. Tree trunks with their ancillary 

 branches and vines form more or less 

 vertical highways between the several strata 

 of the forest and are extensively so used 

 by forest animals. Similarly, the floor, with 

 its vertical discontinuations such as tree 

 holes and "second floors" (p. 485), adds to 

 the complexity. 



Certain general principles may be noted. 

 The vegetation forms the primary biotic 

 gradient in terrestrial communities. Strati- 

 fication serves to increase the organic vol- 



