Climate change

A warming climate is having significant effects in western Oregon forests, and will likely continue to do so for decades to come. Though numerous climate-induced effects have been reported, e.g., drought stress and Douglas-fir “decline spiral,” and other effects such as insect outbreaks may appear more often in the future, the most immediate effects are due to changes in fire regimes. More frequent large fires, including large areas of very severe fire have drastically reduced the amount of old growth within the footprint of many fires (e.g., Beachie Fire in the Opal Creek Wilderness, and in the Clackamas drainage from the Riverside Fire.)

It has taken time to recognize, but relatively small changes in temperature are causing big changes in fire frequency, size and intensity. The reality of atmospheric physics is that the aridity of the atmosphere increases disproportionately more than any given temperature increase. This leads to much drier fuels that are better connected, both vertically within a forest and over large landscapes. In addition, a warming climate is lengthening fire seasons with longer intervals between precipitation events. These changes together are creating a significantly more flammable forest.

The effects of climate change are playing out across landscapes that are very different from historical conditions. Compared to the environment prior to arrival of European-Americans, northwest landscapes are heavily developed, fragmented and full of invasive species. If our region were in historical conditions, it may be much more resilient to a period of increased fire. However, the liquidation of old growth on private lands, and the 45-year period of clearcutting on federal lands, has drastically reduced the resilience and connectivity of our old forests and amplified the effects of large disturbances. Twenty-five years ago, there was a shortage of diverse, young post-fire forest in the Cascades. Now the opposite is true: there is an abundance of snag-rich, post-fire young forest and the area of essentially irreplaceable old growth is small and steadily decreasing.

To put it in perspective, wildfires have burned nearly 1/3 of the forested area in the Willamette National Forest over the seven-year period from 2017-2023 (approximately 470,000 acres burned and 1,500,000 acres of forested land). Approximately the same proportion burned is also found in the Mt. Hood National Forest. There is high variability of fire effects within these burned areas, but the majority of the fire area was burned at moderate to high severity. This is a massive increase in the rate of severe fire compared to estimates of historical averages, and if this rate continues will result in a predominantly snag-dense, young forest subject to regular reburns.

No one knows what the future climate will be, but climate projections from the Intergovernmental Panel on Climate Change have so far been relatively conservative compared to actual changes in climate, and projections show continued warming for several decades under almost any scenario. Dr. Larry O’Neill, Oregon State University Associate Professor and State Climatologist, recently stated (Explore Oregon Podcast April 24, 2023) that the climate of Corvallis in 30-40 years will likely resemble the climate of present-day Sacramento. Ultimately, vegetation and disturbance cycles will adjust to match the new climate, meaning that future vegetation communities will likely look quite different. We are in the early stages of the creative destruction that will usher in a new forest.

The consequences for future old-growth forests are highly uncertain, but it is likely that we will see a continued conversion to a much younger forest where old-growth remnants are restricted to a few lucky locales. The old growth of the future in the western Cascades will likely more closely resemble older dry mixed-conifer forests with scattered fire-resistant large trees and varied young post-fire understories. Iconic old-growth forests that developed for long periods without major disturbance will be rare indeed.