Fig. 54—Climatic cross-section from the crest of the Cordillera Vilcapampa down the eastern mountain valleys to the tropical plains.
At Santa Ana this feature is developed in an amazingly clear manner, and it is also combined with the dry timber line and with productivity in a way I have never seen equaled elsewhere. The diagram will explain the relation. It will be seen that the front range of the mountains is high enough to shut off a great deal of rainfall. The lower hills and ridges just within the front range are relatively dry. The deep valleys are much drier. Each broad expansion of a deep valley is therefore a dry pocket. Into it the sun pours even when 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 tributary 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 Pabellon are immense tracts of grass-covered mountain slopes ([Fig. 53]). These extend beyond Rosalina, the last of them terminating near Abra Tocate (Fig. 15). The sudden interruption is due to a turn in the valley giving freer access to the up-valley winds that sweep through the pass at Pongo de Mainique.