DESERT VEGETATION TX DENUDED AREAS. 15 



into a new region and as slowly dying out as the agricultural pos- 

 sibilities of the land became exhausted. Thus, if no time were lost 

 in the reoccupation of a region like that of Ocosingo as soon as re- 

 forestation had become complete and the humus-inhabiting organisms 

 had regained their natural distribution, the intervals between succes- 

 sive occupations are not likely to be less than two or three thousand 

 years, and might easily be greater. 



DESERT VEGETATION IN DENUDED AREAS. 



The Cahabon district of eastern Guatemala affords an excellent 

 example of an artificially cleared area surrounded by larger tracts of 

 forested country. The greater length of the dry season in Cahabon 

 is a fact familiarly recognized by residents and proved by the pres- 

 ence of cocoanut palms and other types of vegetation which do not 

 thrive in continuously humid regions away from the seacoast. The 

 traveler who visits Cahabon in the spring months may have even 

 more vivid evidence, for he may see day after day showers of rain 

 falling a few miles away before any come to Cahabon. 



The contrast with the neighboring forested districts is strength- 

 ened, no doubt, by the somewhat lower elevation and higher tempera- 

 ture of the Cahabon region. Yet forests would soon cover the whole 

 district if agriculture ceased, and would certainly affect the factors 

 of temperature and humidity by which the rainfall is controlled. 



A similar instance is found on the eastern slope of Costa Rica. If 

 the testimony of intelligent men who settled in this region a quarter 

 of a century ago is to be believed, the rainfall was then distributed 

 through the entire year, as it still is in adjacent districts where the 

 forests remain. But in the narrow belt of cleared lands along the 

 railroad that climbs the slope, the dry seasons have become longer and 

 longer. In recent years destructive droughts have occurred and 

 severe windstorms, also quite unknown before the forests were cut 

 away. 



That many localities in the Old World which once were occupied 

 by populous civilizations are now abandoned deserts has long been 

 known to historians and archaeologists. Some writers have held that 

 such changes could only be explained by supposing that denudation 

 has changed the climate and caused a general reduction of the rain- 

 fall. The facts observed in Central America show that denudation 

 may bring about artificial desert conditions, even where the total 

 rainfall continues to be sufficient for the growth of forests. It is only 

 necessary that the vegetation be cleared away and that the dry season 

 be lengthened. 



The exclusion of the more delicate moisture-loving types of plant 

 and animal life from denuded regions is sufficiently explained by the 



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