Fire is an important element in the natural maintenance of the Everglades. The majority of fires are caused by lightning strikes from thunderstorms during the wet season. Their effects are largely superficial, and serve to foster specific plant growth: sawgrass will burn above water, but the roots are preserved underneath. Fire in the sawgrass marshes serves to keep out larger bushes and trees, and releases nutrients from decaying plant matter more efficiently than decomposition.[43] Whereas in the wet season, dead plant matter and the tips of grasses and trees are burned, in the dry season the fire may be fed by organic peat and burn deeply, destroying root systems.[43] Fires are confined by existing water and rainfall. It takes approximately 225 years for one foot (.30 m) of peat to develop, but in some locations the peat is less dense than it should be for the 5,000 years of the Everglades' existence.[44] Scientists indicate fire as the cause; it is also cited as the reason for the black color of Everglades muck. Layers of charcoal have been detected in the peat in portions of the Everglades that indicate the region endured severe fires for years at a time, although this trend seems to have abated since the last occurrence in 940 BC.[44]