2 CHANGE OF VEGETATION ON SOUTH TEXAS PRAIRIES. 



which might have ventured out among the grass, and even for any 

 trees or forests against which the burning wind might blow. 



That such fires were evidently the cause of the former treeless 

 condition of the southwestern prairies is also shown by the fact that 

 trees are found in all situations which afford protection against fires. 

 Along beaches and on naked sand dunes, in grassless river bottoms 

 and abandoned channels of " slews," in deep swamps and in sterile 

 rocky places the forest has maintained footholds. Nor is there any 

 reason in the nature of the climate or the soil why trees should not 

 thrive over the vast areas of open prairie land. Trees of many kinds 

 have thriven well where planted in villages and about homesteads, in 

 addition to the natural spread of the w^oody vegetation as soon as 

 the fires cease. 



Those who are acquainted only with northern regions of hills and 

 valleys, heavy rains, and deep snows may find it difficult to believe 

 that the burning of grass can destroy or prevent the growth of forests 

 and keep vast regions in a treeless condition. One needs, perhaps, to 

 have the mind prepared by actual observation of the destruction of 

 forests by fires of grass. In humid countries dead grass is beaten 

 down and decays during the next summer season. Forest fires in 

 northern countries arise from accumulations of fallen leaves and 

 other debris, but in the warmer parts of the world these conditions 

 are generally reversed. The forests do not burn with their own 

 fuel, but may be invaded and driven back by the adjacent grass. In 

 this respect, as in several others, south Texas may be reckoned as a 

 part of the Tropics, in spite of the occasional " northers " of the 

 winter season, which carry the tem^Dcrature below the freezing point 

 and thus exclude all the tender tropical types of perennial plants. 



The traveler in Mexico and Central America encounters many 

 illustrations of the advantage enjoyed by grasses over other vegeta- 

 tion in the presence of fire. Large areas of land formerly cultivated 

 by the native Indians remain barren of everything except the coarse 

 grasses which afford the fuel of the fires that prevent the growth of 

 trees and the renewing of the soil. In regions not subject to such 

 fires the forest is rapidly renewed and the land can be cleared and 

 planted again at intervals of a few years. 



Even where the grass-grown land has not been cleared by man it 

 is possible for wild grasses to drive back adjacent forests with the 

 aid of fire. In this way a species of Avire grass (Epicampes) is de- 

 stroying 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 



[Cir. 14] 



C 



