cook: vegetation in southern peru 291 



forests in inaccessible places 



Trees are often found growing under very unfavorable natural 

 conditions, in places that are too steep, rocky, or isolated to 

 be cleared for cultivation or for providing fuel. In the lower 

 Urubamba Valley it was observed that the driest and rockiest 

 hillsides in the vicinity of Santa Ana are covered with forests 

 of huillca (Piptadenia) and other tropical trees, while the smooth- 

 er and more fertile lands on either side have no trees, but a 

 heavier growth of grass. 



Tree seedlings often appear in grass lands, but are killed 

 when fires sweep over them. Hence, the forests are confined 

 to the rocky slopes as long as the adjacent grass lands are visited 

 by fire. Grazing reduces the danger of fire, and this assists 

 in reforestation; but the forests themselves may burn after 

 sufficiently long periods of drought. In the lower Urubamba 

 Valley, at altitudes of from 4000 to 8000 feet, the forests have 

 been burned on many slopes altogether too steep for cultivation. 

 This not only kills the trees but often has the effect of loosening 

 the soil and rocks, causing destructive landslides. 



PAUCITY OF THE HUMUS FAUNA 



Another indication of the more complete denudation of this 

 region in former times is the paucity of the humus fauna, com- 

 prising the insects, millipeds, centipeds, and other small ani- 

 mals that live normally in the upper layers of the soil. These 

 creatures become very abundant under conditions that afford 

 permanent moisture in the soil, but are killed when the land is 

 burned over or parched by severe drought. In southern Peru 

 the humus-inhabiting animals are everywhere extremely scarce, 

 and often lacking altogether. The number of species is very 

 small, as well as the number of individuals. Of millipeds only 

 three orders are represented, Merocheta, Anocheta, and Diplo- 

 cheta; in many localities only Merocheta, and most of these of 

 Antarctic types rather than tropical. The three orders of very 

 primitive arthropods, Symphyla, Rhabdura, and Dicellura, are 

 present, but were nowhere found in abundance, even in places 



