Natural Disturbance Regimes

Disturbance and change, particularly over long periods of time, are part of all ecosystems. Natural disturbances, from ice ages to droughts, can alter ecosystem characteristics. Some attributes of ecosystems depend on various types of disturbances—for example, some coniferous species depend on fire to open cones and clear ground cover for germination and growth of native species.

Understanding the roles that natural disturbances play in the evolution of ecosystems is key to determining how land use and management practices can improve ecosystem conditions. For example, an unprecedented epidemic of Southern Pine beetle currently is damaging many forests in the southeastern U.S. Understanding this pest and its disturbance patterns can assist in developing appropriate responses to restore ecological balance. The extensive acreages burned from wildfires in the western U.S. in recent years pose similar forest ecosystem challenges and opportunities for developing appropriate responses.

There have been few attempts to document regional or national natural disturbance regimes as indicators. The USDA Forest Service Forest Health Monitoring Programis an exception. Statistical data from the forest inventories conducted between 1979 and 1995 have been used to establish short-term baselines for natural disturbances such as climatic events, fire frequency, and insect and disease outbreaks. Several recent events proved to be outside the range of natural disturbance patterns in the 1979 to 1995 timeframe, including:

• El Niño from 1997 to 2000.
  
  
• Northeast ice storm in 1998.
  
  
• Spruce beetle outbreak in 1996, Spruce budworm outbreak in 1997, and a Southern Pine beetle outbreak in 2000.
  
  
• National acres burned in 2000 and the area burned in the West in 1996, 1998, and 2000.⁴³

Disturbance regimes can be changed by resource management. For example, in the two decades between 1980 and 1999, wildfires burned between 2 million and 7 million acres annually, down from a high of 52 million acres in 1930.⁴⁴ The decline is primarily due to fire suppression policies.⁴⁵ Wildfires in 2000, however, reached 8.4 million acres.⁴⁶