The belt of maximum precipitation on the eastern border of the Andean Cordillera in Peru lies between 4,000 and 10,000 feet. Judging by the temporary records of the expedition and especially by the types of forest growth, the heaviest rains occur around 8,000 feet. It is between these elevations that the densest part of the Peruvian montaña (forest) is found. The cold timber line is at 10,500 feet with exceptional extensions of a few species to 12,500 feet. In basins or deep secluded valleys near the mountain border, a dry timber line occurs at 3,000 feet with many variations in elevation due to the variable declivity and exposure of the slopes and degree of seclusion of the valleys. Elsewhere, the mountain forest passes without a break into the plains forest with change in type but with little change in density. The procumbent and suppressed trees of the cold timber line in regions of heavy winter snows are here absent, for the snows rarely reach below 14,000 feet and even at that elevation they are only light and temporary. The line of perpetual snow is at 15,000 feet. This permanent gap of several thousand feet vertical elevation between the zone of snow and the zone of forest permits the full extension of many pioneer forest species, which is to say, there is an irregular development of the cold timber line. It also permits the full use of the pasture belt above the timber ([Fig. 97]), hence permanent habitations exist but little below the snowline and a group of distinctive high-mountain folk enjoys a wide distribution. There is a seasonal migration here, but it is not wholesale; there are pastures snow-covered in the southern winter, but, instead of the complete winter burial of the Alpine meadows of our western mountains, we have here only a buried upper fringe. All the rest of the pasture belt is open for stock the year round.
This climatic distinction between the lofty grazing lands of the tropics and those of the temperate zones is far-reaching. Our mountain forests are not utilized from above but from below. Furthermore, the chief ways of communication lead around our forests, or, if through them, only for the purpose of putting one population group in closer touch with another. In the Peruvian Andes the largest population groups live above the forest, not below it or within it. It must be and is exploited from above.
Hence railways to the eastern valleys of Peru have two chief objects, (1) to get the plantation product to the dense populations above the forest and (2) to bring timber from the montaña to the treeless plateau. The mountain prospector is always near a habitation; the rubber prospector goes down into the forested valleys and plains far from habitations. The forest separates the navigable streams from the chief towns of the plateau; it does not lead down to rich and densely populated valley floors.
Students in eastern Peru should find it a little difficult to understand poetical allusions to silent and lonely highlands in contrast to the busy life of the valleys. To them Shelley’s description of the view from the Euganean Hills of northern Italy,
“Beneath is spread like a green sea
The waveless plain of Lombardy, ...
Islanded by cities fair,”
might well seem to refer to a world that is upside down.
There is much variation in the forest types between the mountains and the plains. At the top of the forest zone the warm sunny slopes have a forest cover; the shady slopes are treeless. At the lower edge of the grassland, only the shady slopes are forested (Fig. 53B). Cacti of arboreal size and form grow on the lofty mountains far above the limits of the true forest; they also appear at 3,000 feet in modified form, large, rank, soft-spined, and in dense stands on the semi-arid valley floors below the dry timber line. Large tracts between 8,000 and 10,000 feet are covered with a forest growth distributed by species—here a dense stand of one type of tree, there another. This is the most accessible part of the Peruvian forest and along the larger valleys it is utilized to some extent. The number of species is more limited, however, and the best timber trees are lower down. Though often referred to as jungle, the lowlier growths at the upper edge of the forest zone have no resemblance to the true jungle that crowds the lowland forest. They are merely an undergrowth, generally open, though in some places dense. They are nowhere more dense than many examples from New England or the West.
Where deep valleys occur near the border of the mountains there is a semi-arid climate below and a wet climate above, with a correspondingly greater number of species within short distances of each other. This is a far more varied forest than at the upper edge of the timber zone or down on the monotonous plains. It has a higher intrinsic value than any other. That part of it between the Pongo and Yavero (1,200 to 4,000 feet) is very beautiful, with little undergrowth except a light ground-cover of ferns. The trees are from 40 to 100 feet in height with an average diameter of about 15 inches. It would yield from 3,000 to 5,000 board feet per acre exclusive of the palms. There are very few vines suspended from the forest crown and the trunks run clear from 30 to 60 feet above the ground. Were there plenty of labor and a good transportation line, these stands would have high economic value. Among the most noteworthy trees are the soft white cedar, strong and light; the amarillo and the sumbayllo, very durable in water; the black nogal, and the black balsam, straight and easy to work; the heavy yunquero, which turns pink when dry; the chunta or black palm, so hard and straight and easy to split that wooden nails are made from it; and the rarer sandy matico, highly prized for dug-out canoes. Also from the chunta palm, hollow except for a few central fibers, easily removed, pipes are made to convey water. The cocobolo has a rich brown color and a glossy surface and is very rare, hence is much sought after for use in furniture making. Most of these woods take a brilliant polish and exhibit a richness and depth of color and a beauty of grain that are rare among our northern woods.