Prairie fires set by lightning or by people were common in this region, and most prairie plants survived because they are long-lived perennials with deep, extensive root systems that produce vigorous new shoots below the soil's surface, enabling them to endure fire, grazing, and drought. Prairies would not be here without the American Indian; in 1838, Joseph Nicollet wrote that the prairies watered by the Mississippi and the Missouri were the work of Indians who destroyed rich vegetation by fire to assure themselves of animal food, and that if the vast, shorn prairies were left untouched, forests would reappear over time. Today, conservationists set fires periodically to maintain and encourage prairie growth.