National Geographic : 2007 Apr
Grasses depend on fire to sweep the prairie clean. Spring burns kill trees and scorch off last year's dry overburden, baring the ground to sun and rain. Within days, the black earth shows green again, as new shoots grow from roots unscathed by the flames. Within weeks, showy evening primroses (above) spread their petals, joining a wildflower pageant 650 species strong.