20 VEGETATION IN CENTRAL AMERICA. 



kill seedlings of other plants. As the trees grow larger they are pro- 

 tected by a thickened bark which is a very poor conductor of heat and 

 not readily combustible. Nevertheless, the survival of the pines de- 

 pends on the chance of frequent fires which prevent the accumulation 

 of grass in large quantities. With grass enough to burn, even large 

 pines may be killed by fire and the pine forest driven back from areas 

 it has already occupied. (See PL VIII.) In this way a species of 

 wire-grass (Epicampes) is destroying forests of alders and pines on 

 the upper slopes of the Vulcan de Agua in Guatemala. Before the 

 access of fires this grass appears to have been confined to the crater 

 and to the very dry upper slopes, where the pine trees are small and 

 scattering. Now that the belts of humid forests lower down have 

 been broken by clearings the grass has the assistance of fire and is 

 destroying the trees with increasing rapidity. 



There are no springs or streams on the upper slopes of the volcano, 

 so that the grass is not pastured. Its long -wiry stems and leaves ac- 

 cumulate until there are quantities of fuel sufficient to kill large trees 

 and to drive back the forest for long distances at each conflagration. 

 The lower the grass comes the more luxuriant its growth and the 

 more destructive the next fire. This will continue as long as the grass 

 is ungrazed or care is not taken, to burn the grass every year in order 

 to prevent the accumulation of dangerous quantities of fuel. 



The roots of this grass are well protected from the fire by masses 

 of the closely packed stems. These tufts remain wet while everything 

 else is thoroughly dried. Except in rainy weather, no water can be 

 obtained from the extremely coarse and loose volcanic ashes and rocks 

 of which the upper parts of the mountain are composed. Weldenia 

 and other native plants show, striking adaptations for drought resist- 

 ance. Even the alder has a remarkably thick, cheesy bark, which 

 doubtless serves for the storage of additional supplies of water. 



BURIED ROOTS SHOW FORMER EXTENSION OF PINES. 



In eastern Guatemala the secondary character of supposedly prime- 

 val forests is shown by the fact that pine roots are often found in the 

 ground in districts from which living pines have been completely 

 driven out by the more luxuriant tropical types of vegetation. The 

 Indians dig up the pitchy roots of the extinct pines and use them 

 for torches. Such roots are found in the alluvial bottom lands of the 

 Polochic Valley, near Panzos, almost at sea level, and also in the 

 coffee district to the north of Senahu at altitudes of about 3,000 feet 

 and upward. Both localities are distant several miles from any living 

 forests of pine. 



It is easy to understand that pines were more abundant in this 

 region in former times than at present, because the native population 



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