EXTINCTION OF GHASSES BY FOREST. 11 



spots, though their inhabitants may have reduced the surrounding 

 country to a desert. 



OCCUPATION OF CLEARED LANDS BY GRASSES. 



The usual system of corn culture involves the repeated burning 

 off of the woody growth and a resulting. exposure of the soil. This 

 causes a gradual deterioration of the crops of corn and a slower re- 

 newal of the woody vegetation. New clearings in the forest are soon 

 covered again with bushes, and can be cut, burned, and planted 

 again within a year or two. With each cutting the interval has to 

 be lengthened, until finally the land becomes thoroughly occupied 

 by coarse grasses which are not killed by fire. The Indians can then 

 make no further use of the land for agricultural purposes. 



Fires set to burn the brush off clearings are allowed to spread over 

 the adjacent tracts of abandoned land. These accidental burnings 

 give the grass an advantage over the woody vegetation, and the grassy 

 areas are gradually extended. The Indians know well enough that 

 the frequent burning over of the old cornfields keeps the land from 

 being used again, but their interest in the future is seldom strong 

 enough to lead to any precautions against the spreading of the fires. 



Reasons have been given in another place for believing that the 

 prairies of south Texas were kept in a treeless condition by fires in 

 the same way as the grass-land districts of Mexico and Central 

 America. In Texas the woody vegetation has been able to make rapid 

 advances in recent decades because the grass is grazed and the stock- 

 men burn the residue off every year, thus putting an end to destruc- 

 tive fires. a That frequent fires aid reforestation has also been ob- 

 served by Professor Pittier on savannas in the southern part of Costa 

 Rica, near the Pacific coast. The Indians did not make a practice 

 of burning the savannas, which were thus left for accidental fires at 

 long intervals, but the fires kindled every year by the civilized set- 

 tlers are not severe enough to kill the bushes and trees, and the woody 

 vegetation is now making rapid advances. 



EXTINCTION OF GRASSES BY FOREST. 



Under normal conditions of Indian agriculture, where burning is 

 left to chance, the occupation of the land by the grasses is likely to 

 continue as long as the proximity of agricultural natives insures the 

 access of fires at the necessary intervals. If a place is abandoned 

 and no fires are set for a sufficient period of years the grass is in- 

 vaded by woody vegetation and is finally overshadowed and killed. 



°Cook, O. F. Change of Vegetation on the South Texas Prairies. Circular 

 14, Bureau of Plant Industry, U. S. Dept. of Agriculture. 1908. 

 145 



