Decades of fire suppression and lack of indigenous fire ecology can contribute to dense forests with a lot of understory 'fuel' and many dead standing trees. When a forest fire eventually does occur, the increased fuel creates a crown fire, which destroys all vegetation and affects surface soil chemistry. Although such fires have been occurring sporadically for 100 million years, and are part of the natural ecological rhythm of forests, frequent and small 'natural' ground fires do prevent the accumulation of fuel and allow large, slow-growing vegetation (e.g. trees) to survive.