Stuff happens: fires, hurricanes, volcanoes, floods, and earthquakes. On average, approximately 450,000 ha are burned in the United States annually, over 1 million ha are affected by hurricanes and over 20 million ha are affected by insects and pathogens (Dale et al. 2001). The economic cost to society is over 1 billion dollars/year in the United States (Dale et al. 2001). To most people, these events are catastrophes. They can kill people, destroy property, and they can be catastrophic for other organisms too. Wind uproots trees, fires burn dead wood, and floods erode streambanks. They can also be events that renew habitat for other species. Indeed biodiversity conservation depends on disturbance. Wind adds dead wood to a forest, fires open the tree canopy and initiate a new forest, and floods create a new seedbed for willows and cottonwoods. Disturbances to forests have occurred for as long as there have been forests. Animals living in forests have adapted to many of these disturbances and some species rely on disturbances to provide food, cover, and water for survival. Understanding characteristics of disturbances and how disturbances influence habitat elements in stands and over forests can provide information that forest managers can use to provide habitat for selected species or to aid in conserving biodiversity. Knowledge of natural disturbances can help when developing silvicultural systems that might meet the needs of forest-associated wildlife (Franklin et al. 2002).
There are several characteristics of disturbances that can be used to understand potential effects on forest development, forest function and the sizes, numbers and distribution of habitat elements: type of disturbance, size and pattern, frequency, and severity. Disturbance type is also important, with changes in habitat elements being quite different depending on the cause of the disturbance (e.g., fire vs. wind). Estimating these characteristics of natural disturbances can facilitate prediction of forest recovery and the subsequent development of vegetation structure.