80 THE ANDES OF SOUTHERN PERU 



all the surrounding hills and mountains are wrapped in cloud. 

 The greater number of hours of sunshine hastens the rate of 

 evaporation and still further increases the dryness. Under the 

 spur of much sunlight and of ample irrigation water from the 

 wetter hill slopes, the dry valley pockets produce huge crops of 

 fruit and cane. 



The influence of the local climate upon tree growth is striking. 

 Every few days, even in the relatively dry winter season, clouds 

 gather about the hills and there are local showers. The lower 

 limit of the zone of clouds is sharply marked and at both Santa 

 Ana and Echarati it is strikingly constant in elevation — about 

 five thousand feet above sea level. From the upper mountains 

 the forest descends, with only small patches of glade and prairie. 

 At the lower edge of the zone of cloud it stops abruptly on the 

 warmer and drier slopes that face the afternoon sun and continues 

 on the moister slopes that face the forenoon sun or that slope 

 away from the sun. 



But this is not the only response the vegetation makes. The 

 forest changes in character as well as in distribution. The forest 

 in the wet zone is dense and the undergrowth luxuriant. In the 

 selective slope forest below the zone of cloud the undergrowth is 

 commonly thin or wanting and the trees grow in rather even-aged 

 stands and by species. Finally, on the valley floor and the tribu- 

 tary fans, there is a distinct growth of scrub with bands of trees 

 along the water courses. Local tracts of coarse soil, or less rain 

 on account of a deep "hole" in a valley surrounded by steeper 

 and higher mountains, or a change in the valley trend that brings 

 it into less free communication with the prevailing winds, may 

 still further increase the dryness and bring in a true xerophytic 

 or drought-resisting vegetation. Cacti are common all through 

 the Santa Ana Valley and below Sahuayaco there is a patch of 

 tree cacti and similar forms several square miles in extent. Still 

 farther down and about half-way between Sahuayaco and Pabel- 

 lon are immense tracts of grass-covered mountain slopes (Fig. 

 53). These extend beyond Eosalina, the last of them terminating 

 near Abra Tocate (Fig. 15). The sudden interruption is due to a 



