CHAPTER XIX 

 PORTO RICO 



General Conditions. — As in temperate climates the amount of 

 available moisture is the controlling factor in tropical tree dis- 

 tribution. This is the amount left in the soil after that evapo- 

 rated and that which is run off is deducted from the total annual 

 precipitation. For Porto Rico the latter varies from 169 to 21 

 inches. The trade winds are the dominant influence. Where 

 they blow directly of! the sea the rainfall is abundant but where 

 they must rise up over a mountain range the leeward side is 

 invariably dry. This accounts for the great variation in annual 

 precipitation. The south or leeward side is sheltered from the 

 trade -svinds by three ranges of mountains which comprise the 

 greater part of the islands. Hence it is only the northern slopes 

 of these mountains that are well enoughVatered to support vigor- 

 ous tree growth because the evaporation and runoflf are so great 

 in the tropics that the total annual precipitation must be at least 

 60 inches to support dense vegetation. Its distribution is a mat- 

 ter of secondary importance since many tree species are able to 

 endure a prolonged dry season by shedding their leaves. 



The great evaporation due to high temperatures has already 

 been referred to above; 76° F. is the annual mean for the whole 

 island. This means, of course, tiiat there is no cessation o! growth 

 thruout the year by reason of cold. 



The effect of the mountain ranges on the distribution of rain- 

 fall has already been described. To give a more definite picture 

 of the island it is merely necessary to add that only 20 per cent 

 is plains country while the remaining 80 per cent is made up of 

 mountains, ranging in elevation from a few hundred feet above 

 sea level to a maximum of 3400 feet. From the mountains flow 

 over 1300 named streams of which the Plata River is the largest 

 and longest. 



Unfortunately not enough is known about the tree distribution 

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