Fig. 94—Cloud belt at 11,000 feet in the Apurimac Canyon near Incahuasi. For a regional diagram and a climatic cross-section see Figs. [32] and [33].
Fig. 95—The tropical forest near Pabellon on the slopes of the Urubamba Valley. Elevation 3,000 feet (915 m.).
The plains forest northeast of the mountains is in the zone of moderate rainfall where there is one long dry season and one long wet season. When it is dry the daytime temperatures rise rapidly to such high levels that the relative humidity of the air falls below 50 per cent (Fig. 110). The effect on the vegetation is so marked that many plants pass into a distinctly wilted condition. On clear days the rapid fall in the relative humidity is astonishing. By contrast the air on the mountain border heats more slowly and has a higher relative humidity, because clouds form almost constantly in the ascending air currents and reflect and absorb a large part of the heat of the sun’s rays. It is striking to find large tracts of cane and bamboo on the sand bars and on wet shady hillslopes in the slope belt, and to pass out of them in going to the plains with which we generally associate a swamp vegetation. They exist on the plains, but only in favored, that is to say wet, spots. Larger and more typical tracts grow farther north where the heavier rains of the Amazon basin fall.
The floods of the wet tropical season also have a restricting influence upon the tropical forest. They deliver such vast quantities of water to the low-gradient lowland streams that the plains rivers double, even treble, their width and huge pools and even temporary lakes form in the shallow depressions back of the natural levees. Of trees in the flooded areas there are only those few species that can grow standing in water several months each year. There are also cane and bamboo, ferns in unlimited numbers, and a dense growth of jungle. These are the haunts of the peccary, the red forest deer, and the jungle cat. Except along the narrow and tortuous animal trails the country is quite impassable. Thus for the sturdiest and most useful forest growth the one-wet-one-dry season zone of the plains has alternately too much and too little water. The rubber tree is most tolerant toward these conditions. Some of the best stands of rubber trees in Amazonia are in the southwestern part of the basin of eastern Peru and Bolivia, where there is the most typical development of the habitat marked by the seasonal alternation of floods and high temperatures.
When tropical agriculture is extended to the plains the long dry season will be found greatly to favor it. The southwestern quadrant of the Amazon basin, above referred to, is the best agricultural area within it. The northern limits of the tract are only a little beyond the Pongo. Thence northward the climate becomes wetter. Indeed the best tracts of all extend from Bolivia only a little way into southeastern Peru, and are coincident with the patchy grasslands that are there interspersed with belts of woodland and forest. Sugar-cane is favored by a climate that permits rapid growth with a heavy rainfall and a dry season is required for quality and for the harvest. Rice and a multitude of vegetable crops are also well suited to this type of climate. Even corn can be grown in large quantities.
At the present time tropical agriculture is almost wholly confined to the mountain valleys. The reasons are not wholly climatic, as the above enumeration of the advantages of the plains suggests. The consuming centers are on the plateau toward the west and limitation to mule pack transport always makes distance in a rough country a very serious problem. The valleys combine with the advantage of a short haul a climate astonishingly like the one just described. In fact it is even more extreme in its seasonal contrasts. The explanation is dependent upon precisely the same principles we have hitherto employed. The front range of the Andes and the course of the Urubamba run parallel for some distance. Further, the front range is in many places somewhat higher than the mountain spurs and knobs directly behind it. Even when these relations are reversed the front range still acts as a barrier to the rains for all the deep valleys behind it whose courses are not directly toward the plains. Thus, one of the largest valleys in Peru, the Urubamba, drops to 3,400 feet at Santa Ana and to 2,000 feet at Rosalina, well within the eastern scarp of the Andes. The mountains immediately about it are from 6,000 to 10,000 feet high. The result is a deep semi-arid pocket with only a patchy forest (Fig. 54, p. [79]).[25] In places the degree of seclusion from the wind is so great that the scrub, cacti, and irrigation remind one strongly of the desert on the border of an oasis, only here the transition is toward forests instead of barren wastes. The dense forest, or montaña, grows in the zone of clouds and maximum precipitation between 4,000 and 10,000 feet. At the lower limit it descends a thousand feet farther on shady slopes than it does on sunny slopes. The continuous forest is so closely restricted to the cloud belt that in [99] the two limits may be seen in one photograph. All these sharply defined limits and contrasts are due to the fact that the broad valley, discharging through a narrow and remote gorge, is really to leeward of all the mountains around it. It is like a real desert basin except in a lesser degree of exclusion from the rains. If it were narrow and small the rains formed on the surrounding heights would be carried over into it. Rain on the hills and sunshine in the valley is actually the day-by-day weather of the dry season. In the wet season the sky is overcast, the rains are general, though lighter in the valley pocket, and plants there have then their season of most rapid growth. The dry season brings plants to maturity and is the time of harvest. Hence sugar and cacao plantations on a large scale, hence a varied life in a restricted area, hence a distinct geographic province unique in South America.