Three other variables—island age, ash fallout, and dust fallout—had effects that we hadn’t anticipated, because we hadn’t been familiar with the scientific literature on the maintenance of soil fertility. Old islands that hadn’t experienced any volcanic activity for over a million years ended up more deforested than young, recently active volcanic islands. That’s because soil derived from fresh lava and ash contains nutrients that are necessary for plant growth, and that gradually become leached out by rain on older islands. One of the two main ways that those nutrients then become renewed on Pacific islands is by fallout of ash carried in the air from volcanic explosions. But the Pacific Ocean is divided by a line famous to geologists and known as the Andesite Line. In the Southwest Pacific on the Asian side of that line, volcanoes blow out ash that may be wind-carried for hundreds of miles and that maintains the fertility even of islands (like New Caledonia) that have no volcanoes of their own. In the central and eastern Pacific beyond the Andesite Line, the main aerial input of nutrients to renew soil fertility is instead in dust carried high in the atmosphere by winds from the steppes of Central Asia. Hence islands east of the Andesite Line, and far from Asia’s dust plume, ended up more deforested than islands within the Andesite Line or nearer to Asia.