On a sun-parched plateau, high in South Africa's rugged Cederberg mountains, Boltwin Tamboer harvests rooibos tea in much the same way his forefathers would have done. Watched over by a cave adorned with 6,000-year-old depictions of elephants and winged medicine men, he slices through a fistful of the hardy shrub with a deft flick of his sick...