298 THE STUDY OF PLANT COMMUNITIES • Chapter X 



relative importance. Associated species other than grasses, often 

 called forbs, may be seasonally so conspicuous as to obscure the 

 grasses and, temporarily at least, to give the appearance of dom- 

 inance. 



Tropical Formations.— The truly tropical vegetation of North 

 America, which occurs only in southern Mexico and Central 

 America, probably includes as great a diversity of communities as 

 is usually found in temperate climates. The major controlling fac- 

 tor in this diversity is moisture, as affected by topography, expos- 

 ure, and seasonal distribution. Although numerous local studies of 

 the vegetation of American tropics have been made, it is only re- 

 cently that a comprehensive classification of the plant communities 

 has been attempted in the light of modern concepts. 15 



A misconceived but popular idea of tropical vegetation is un- 

 doubtedly one which can best be placed in the category of rain- 

 forest-in-its- jungle-form. But such tangled masses of vegetation 

 are found only on areas of disturbance and "True rain forest al- 

 ways gives the impression of the vault of cathedral aisles!' 15 It is 

 made up of many species of tall broad-leaved, evergreen trees in 

 several strata with the tallest sometimes rising ninety feet to the 

 lowest branch. Undergrowth is sparse, lianas are few, and epiphy- 

 tes are not abundant near the ground. Apparently, after disturb- 

 ance of any kind, such forests are replaced by a tangled jungle of 

 growth that is almost impenetrable. The rain forest is not wide- 

 spread because conditions for its development are by no means 

 everywhere available. It occurs where temperatures are fairly con- 

 stantly high, precipitation is plentiful (over two hundred inches in 

 some areas), and on good sites with proper drainage but with a 

 continuous supply of available water. 



It should be re-emphasized that not all tropical vegetation is 

 rain forest, and to this should be added that not all broad-leaved 

 evergreen forest is rain forest. The presence in the tropics of 

 mountains of sufficient height to have permanent snow on their 

 peaks insures altitudinal zonation similar to that of temperate re- 

 gions. These mountains may interrupt moisture-bearing winds and 

 so maintain desert conditions. Seasonal deciduous forests, pine for- 

 ests, and even tundra are to be found on their slopes. The major 

 variations in American tropical vegetation have been grouped into 



