Blowdowns

• A tornado tracking through a forest may flatten large areas of timber, uprooting trees or snapping them off at the trunk. A 2007 tornado felled close to 14,000 acres of woods in northern Wisconsin. While powerful tornadoes may topple every standing tree in a given area, trees of shallow root systems or large, mature dimensions are often most vulnerable. In the eastern U.S., tornadoes, along with the downbursts associated with large thunderstorms and squall-belt winds called derechos, are among the most important agents of disturbance in the temperate forest.
  
  

Succession

• Even as tornadoes destroy individual trees, they set the stage for the proliferation of other species. Succession refers to the cycling of ecological communities; it may be thought of as the evolutionary track of a given landscape. In dense woods, the tornado's toppling of large trees opens the canopy, allowing more sunlight to reach the forest floor. This can spur growth of the saplings of shade-intolerant trees as well as a host of shrubs and herbaceous plants. In a study based at the Big Thicket National Preserve in eastern Texas, a tornado in 1983 encouraged the conversion of pine savanna to mixed pine-hardwood woodlands. In these situations, tornadoes act as intense, though localized, disturbance factors, shifting the course of succession from one track to another, or simply "rewinding" it.
  
  

Effects on Fauna

• The impact of tornadoes on wildlife is poorly understood, but animals likely respond to the habitat diversity rendered by the storm damage. In the immediate aftermath of a tornado in a forested area, species adapted to edge environments--the frontiers of habitat called ecotones--might take advantage of the new forest-clearing mosaic in the tornado track. Indeed, a 2003 assessment of tornado effects on avian diversity in the Ozark National Forest reported some forest-interior birds decreasing and some edge-habitat birds increasing in a storm-affected forest, though the pattern was not uniformly observed.
  
  

Tornado Habitat Modifications

• The physically destructive impact of a tornado in a woodland generates a variety of landscape features, which affect habitat type and availability in the ecosystem. A tree felled by a storm may become a "nurse log," offering in its slow decay a foothold for seedlings above the competition and shade of thick groundcover. So-called tip-up mounds also result from a toppled tree: the decomposition of its base and trunk creates a hummock bordering a hollow where the roots once nested. In moist environments, water often collects in the depression beside the upturned roots, providing habitat for salamanders and other aquatic organisms.