Re-evaluating historical fire regimes in west-side forests of the Pacific Northwest

Historical fire regimes and the 2020 Labor Day fires on the west side of Oregon and Washington with locations of large (>10,000 ha) and small fires (<10,000 ha), b) Reburns following the 1902 Yacolt Fire in 1902 in the western Washington Cascades, c) Reburns following the 1933 Tillamook Fire in the Oregon Coast Range.

Project Description

Fire regimes in west-side forests are more complex than previously thought, and critical uncertainties remain around historical fire regimes and the linkage between fire, human populations, and climate.

This project takes a regional look at historical fire activity in the west Cascades of Oregon and Washington by integrating spatial data on early 20th century fires and forest conditions with annual fire histories developed from fire-scarred trees. Specific objectives for this work include: 1) quantifying variability in pre-settlement fire frequency, fire severity, and fire’s influence on forest dynamics and stand development; 2) characterizing linkages between climate variability and fire at broad temporal and spatial scales.

Purpose and Scope

This project is developing quantitative knowledge about west-side fire history. New knowledge of the historical role of fire on stand, landscape, and regional forest dynamics will provide much needed context on variability and past climate linkages. This new knowledge will inform managers and policy makers on adaptation to future changes in fire activity and climate.

2020 fire perimeters and mapped extent of “stand-replacing fire” in 1902 and perimeters of known large westside fires.

Methods 

Researchers are using multiple sources of data including aerial photographs, historical stand inventory records, and current forest inventory data. In addition, researchers will collect and analyze cross sections from fire scarred trees to construct an annually precise multi-century, fire history spanning the west slopes of the Cascade Range in western Oregon and Washington.

Key Findings

1.) Large fires affected approximately 20 percent of the west-side in the late 18th and early 20th centuries, particularly in the western Cascades and Coast Range of Oregon, but small fires occurred in cooler and wetter parts of Washington as well. All of the largest fires in 2020 burned either within or in close proximity to large historical fires. Evidence of short interval reburns following large historical fires suggest that post-fire early seral conditions may be more prone to burning than old-growth forests.

2.) Preliminary findings indicate a far greater frequency of fire than existing literature suggests in relatively cool, moist environments, and emphasize the importance of multiple non-stand-replacing fires during the development of many old-growth forests. Results also indicate that frequent fire (<25 years) associated with indigenous burning occurred in remote parts of the interior western Cascades of Oregon.