Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [176]
How long have New Guinea highlanders been practicing silviculture? The clues used by paleobotanists to reconstruct the vegetational history of the highlands have been basically similar to those I already discussed for Easter Island, the Maya area, Iceland, and Greenland in Chapters 2-8: analysis of swamp and lake cores for pollen identified down to the level of the plant species producing the pollen; presence of charcoal or carbonized particles resulting from fires (either natural or else lit by humans to clear forests); sediment accumulation suggesting erosion following forest clearance; and radiocarbon dating.
It turns out that New Guinea and Australia were first settled around 46,000 years ago by humans moving eastwards from Asia through Indonesia’s islands on rafts or canoes. At that time, New Guinea was still joined in a single landmass to Australia, where early human arrival is well attested at numerous sites. By 32,000 years ago, the appearance of charcoal from frequent fires, and an increase in pollen of non-forest tree species compared to forest tree species, at New Guinea highland sites hint that people were already visiting the sites, presumably to hunt and to gather forest pandanus nuts as they still do today. Signs of sustained forest clearance and the appearance of artificial drains within valley swamps by around 7,000 years ago suggest the origins of highland agriculture then. Forest pollen continues to decrease at the expense of non-forest pollen until around 1,200 years ago, when the first big surge in quantities of casuarina pollen appears almost simultaneously in two valleys 500 miles apart, the Baliem Valley in the west and the Wahgi Valley in the east. Today those are the broadest, most extensively deforested highland valleys, supporting the largest and densest human populations, and those same features were probably true of those two valleys 1,200 years ago.
If we take that casuarina pollen surge as a sign of the beginning of casuarina silviculture, why should it have arisen then, apparently independently in two separate areas of the highlands? Two or three factors were working together at that time to produce a wood crisis. One was the advance of deforestation, as the highland’s farming population increased from 7,000 years ago onwards. A second factor is associated with a thick layer of volcanic ashfall, termed the Ogowila tephra, which at just that time blanketed eastern New Guinea (including the Wahgi Valley) but wasn’t blown as far west as the Baliem Valley. That Ogowila tephra originated from an enormous eruption on Long Island off the coast of eastern New Guinea. When I visited Long Island in 1972, the island consisted of a ring of mountains 16 miles in diameter surrounding a huge hole filled by a crater lake, one of the largest lakes on any Pacific island. As discussed in Chapter 2, the nutrients carried in such an ashfall would have stimulated crop growth and thereby stimulated human population growth, in turn creating increased need for wood for timber and fuel, and increased rewards for discovering the virtues of casuarina silviculture. Finally, if one can extrapolate to New Guinea from the time record of El Niño events demonstrated for Peru, droughts and frost might have stressed highland societies then as a third factor.
To judge by an even bigger surge in casuarina pollen between 300 and 600 years ago, highlanders may then have expanded silviculture further under the stimulus of two other events: the Tibito tephra, an even bigger volcanic ashfall and boost to soil fertility and human population than the Ogowila tephra, also originating from Long Island and directly responsible for the hole filled by the modern lake that I saw; and possibly the arrival then of the Andean sweet potato in the New Guinea highlands, permitting crop yields several times those previously available with just New Guinean crops.