National Geographic : 1996 Sep
How fires burn Frequent fires move swiftly across the forest floor, killing few large trees. When fire is rare, accumulated fuels explode into towering crown fires, and the thick floor burns long, hot, and deep, killing roots of grasses and trees. Frequency of fire Scientists measure frequency by scars on trees or ash lay ers inthe ground. Fire inter vals vary: short in ponderosa forests and grasslands, long in coastal Douglas fir forests. Inevitably, this year or next century, fire returns. Recovery from fire A healthy ponderosa forest recovers within three years, its roots intact (inset), its trees barely scarred. In unfit forest, fire kills big trees; soil no longer absorbs rain, and it erodes (insets bottom). Recovery may take centuries. Tallgrass prairie: Sur vives flames better than does invasive brush. Renewed by frequent large fires that can out run a horse. Boreal forest: Spruce, pine, and fir dominate the north to the subarc tic tree line. Large, kill ing fires recur every 25 to 150 years. Appalachian mixed forests: Conifers and deciduous trees mingle in shifting ratios set by climate and a mosaic of rare fires. Longleaf and loblolly: Southern pines in grassy, park-like stands. Mild surface fires clear debris every three to five years. I Wetlands: Saw grass needs flames to kill competition. Small patches burn to the waterline every 1 to 25 years.