22 VEGETATION IN CENTRAL AMERICA. 



throughout the Central American region. Differences of natural con- 

 ditions can influence only the factor of time, but could not prevent 

 an ultimate reforestation. How many centuries or millenniums 

 would be required to reclothe with forests such deserts as these of the 

 valley of Salama is a question in which many considerations of soils, 

 drainage, temperature, and rainfall would enter, but it seems super- 

 fluous to doubt that the result would be finally attained, if there were 

 no men to cut down the trees and kindle fires. 



SUMMARY. 



The distribution of grass lands and of open forests of pines and 

 oaks in Central America does not depend primarily upon natural 

 conditions, such as contours, elevations, soils, or rainfalls, but is 

 chiefly determined by previous occupations of the land by agricul- 

 tural natives. Pines and oaks represent stages of the process of re- 

 forestation; they descend even to sea level to occupy open country 

 where the fires exclude other less hardy vegetation. The former 

 existence of pines in regions now occupied by luxuriant tropical 

 forest is often shown by the persistence of the pine roots in the 

 ground. 



The driest and most sterile localities, those too forbidding for 

 human occupation, have retained their forest growth, scarcely ex- 

 cepting sheer precipices and exposures of bare rocks. The regions 

 which are now treeless and barren or covered onty with grass are 

 those naturally well suited for the growth of forests, for the forma- 

 tion of fertile soil, and for human occupation. Indications of pre- 

 historic agriculture are found in all the denuded areas, as well as in 

 other regions now covered with forests. 



Truly virgin forests seem not to exist in Central America. Relics 

 of ancient agricultural occupations seem nowhere to be lacking, even 

 in regions now entirely uninhabited, in dense forests as well as in 

 open desert regions. 



The abundance of rubber and other temporary types of trees and 

 the* absence of humus-inhabiting arthropods and forest palms enable 

 regions of recent reforestation to be distinguished from forests of 

 older growth. 



Facts of several different kinds thus support the conclusion that 

 the Central American region had a continuous forest covering before 

 the advent of agricultural man. If human interference were with- 

 drawn the normal growth of the vegetation would again cover the 

 Central American region with dense and continuous forests. 



Repeated clearing and burning of the woody vegetation for the 

 planting of corn allows the land to become overgrown with coarse 

 grasses. The burning over of the grass lands prevents the growth 



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