HABITS OF PINES AND OAKS. 19 



The storage of water by the forest on these very steep slopes is 

 largely limited to the wetting of the humus layer, which may be only 

 a few inches thick. If the rains cease for only two or three weeks 

 this slender supply of water is exhausted. The plants wilt and 

 shrivel up as though suffering from a prolonged drought. Slopes too 

 >teep for any rain to penetrate are often occupied by desert types of 

 vegetation, though immediately surrounded by the vegetation ap- 

 propriate to the general humidity of the region. 



The water of the torrents that gather from the forested slopes is 

 still clean, even after heavy rains, or is made onty slightly milky by 

 the washings of the leaves. Only the erosion of the streams loosens 

 any solid matter. After the forest has been cut and the dry brush 

 has been burned off the loose surface soil is left fully exposed. More 

 erosion can then take place in a single season than would be possible 

 in many centuries of undisturbed forest growth. 



DISTRIBUTION OF PINES AND OAKS DETERMINED BY CLEARING 



OF LAND. 



The presence of open forests of pines and oaks is not determined by 

 altitude. Pine forests occur under a wide variety of conditions and 

 at all elevations. They extend from near sea level in eastern Guate- 

 mala through the dry and elevated interior to the tops of the highest 

 volcanoes. The same may be said of the oaks, except that they do not 

 appear to ascend as high as the pines. Wherever there are grassy, 

 fire-ravaged tracts to be occupied, the pines and oaks appear to be 

 able to establish themselves if sufficient time is allowed. About Lake 

 Yzabal, in eastern Guatemala, there are pine forests very nearly at 

 the level of the sea. In the Palenque district of southern Mexico the 

 oaks descend in a similar manner. Oaks also grow close to the sea- 

 shore in the district of Guanacaste, on the Pacific side of Costa Rica, 

 as reported by Professor Pittier. 



The natural habitats of the several species of pines and of the very 

 numerous species of oaks of the Central American region are prob- 

 ably to be found on the dry exposed crags and summits of the moun- 

 tains. These are the only places where these slow-growing types 

 would be able to maintain footholds if they had to meet the compe- 

 tition of the tropical vegetation without the help of men and fires. It 

 is this competition of the more luxuriant tropical types of vegetation 

 that limits the spread of the pines and oaks in Central America, 

 rather than any particular requirements of altitudes, temperatures, 

 or other climatic conditions. 



Ability to resist fire is the characteristic that enables the pines to 

 establish themselves in open grass lands. Young pines with the 

 growing bud surrounded by many green needles can survive fires that 



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