"The experience of the nomadic farmer was that he would find fertile land only in the forest and it was natural for him to make clearings, piling smaller bushes around the greater trees to fell them by burning. The provided rich fertilizer for a season's growth but the land exposed to the elements failed to retain its fertility. So, after reaping a few crops, the nomadic farmer would penetrate ever deeper into the virgin forest ... The next stage in forest degradation is so-called orchard bush, with large trees widely scattered. Then comes a type of fringing forest, which in time will deteriorate into h. After that there is ever sparser vegetation and s, sometimes mobile but more often fixed; then follows the desert floor ..."