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Wildfire Risk

Historical Wildfire Conditions in Oregon

Historical Wildfire Conditions in Oregon What were wildfire conditions in Oregon forests like historically?
  • Low-intensity fires were historically frequent in dry interior Oregon forests, and were key to maintaining wildfire resilience, forest structure and ecosystem health
  • Wildfires were typically much less frequent, but much more intense in western Oregon and coastal forests

Oregon's forests and their diversity are among the state's most remarkable features. Wildfire has always been part of these forests.

Ecologists estimate that prior to Euro-American settlement large, stand-replacing crown fires burned Pacific Northwest coastal forests every 200-500 years. Smaller surface fires revisited dry interior forests as often as every 4-20 years. West-side Cascade wildfire intervals and intensity fell somewhere in the range between.

Unmanaged ecosystems are used to establish these baselines, called historic range of variability (HRV). HRV is the range of variation in ecological conditions and processes (such as wildfire) that would occur in the absence of substantial agriculture or influence from mechanized equipment.

The HRV for wildfire is used to compare present conditions and assess change, and to help establish desired future conditions. The assumption is that ecosystems with most natural processes and conditions intact are more resilient to disturbance and able to sustainably produce goods and services that humans value.

Dry Oregon forests were characterized by frequent, low-intensity fires often ignited by lightning, but also by Native Americans. Most likely, historic surface wildfires were quite extensive, burning from late spring through summer until wetter weather arrived in the fall. These low-intensity fires:

  • reduced fuels
  • promoted regeneration of fire-tolerant ponderosa pine and Douglas-fir
  • maintained open, park-like forest stand structure of larger, fire resistant trees
  • cycled nutrients
  • decreased disease and insect impacts
  • provided habitats for wildlife species that favor open stands

In western Oregon ecosystems, historical fire intervals were so long that many of these forests may still be within their HRV for wildfire. However, in drier central, southern and northeastern Oregon, fire return intervals, fuel accumulations and other fire regime aspects are no longer within their historical range of variability.

Some dry forestlands in Oregon have missed as many as 7-10 fire intervals. In the absence of fires that periodically consumed them, wildfire fuels have increased dramatically in these forests.