Collapse: How Societies Choose to Fail or Survive

by Jared Diamond

The species is fast-growing. Its wood is excellent for timber and fuel. Its root nodules that fix nitrogen, and its copious leaf-fall, add both nitrogen and carbon to the soil. Hence casuarinas grown interspersed in active gardens increase the soil’s fertility, while casuarinas grown in abandoned gardens shorten the length of time that the site must be left fallow to recover its fertility before a new crop can be planted. The roots hold soil on steep slopes and thereby reduce erosion. New Guinea farmers claim that the trees somehow reduce garden infestation with a taro beetle, and experience suggests that they are right about that claim as they are about many others, though agronomists still haven’t figured out the basis of the tree’s claimed anti-beetle potency.