The American prairie was shaped by fire initiated by lightning or by Native Americans, which burned off old trees, shrubs, and materials, recycled nutrients, and eased movement and hunting. Its climate brought long hot summers, cold winters, extreme temperature fluctuations, and seasonal droughts. Large herds disturbed the soil for seed-to-soil contact, spread seed, and added organic matter to the soil. Prairie plants adapted to fire, animal grazing, and weather extremes, with perennial plants developing deep root systems and growing points below the soil, and some bearing slender, thin leaves that bend in the wind while others have tough leathery leaves that ward off water loss.