What the Northern Rockies Can Teach Us About Wildfires

Sunday - November 5 2023, 19:02 UTC - 1 year ago

The Big Burn of 1910 was a major fire disaster in the Northern Rocky Mountains of Idaho and Montana, and recent fires suggest that the climate is changing further. To study how and why ecosystems changed in the past, paleoecologists tracked how often forest fires occurred across the Northern and Southern Rockies over the past 2,500 years. The research suggests that high-elevation, or subalpine, forests of the Northern Rockies can withstand even the most extreme fire seasons, while those of the Southern Rockies are becoming increasingly vulnerable.

Strong winds blew across mountain slopes after a record-setting warm, dry summer. Small fires began to blow up into huge conflagrations. Towns in crisis scrambled to escape as fires bore down.

This could describe any number of recent events, in places as disparate as Colorado, California, Canada and Hawaii. But this fire disaster happened over 110 years ago in the Northern Rocky Mountains of Idaho and Montana.

The "Big Burn" of 1910 still holds the record for the largest fire season in the Northern Rockies. Hundreds of fires burned over 3 million acres – roughly the size of Connecticut – most in just two days. The fires destroyed towns, killed 86 people and galvanized public policies committed to putting out every fire.

Today, as the climate warms, fire seasons like in 1910 are becoming more likely. The 2020 fire season was an example. But are extreme fire seasons like these really that unusual in the context of history? And, when fire activity begins to surpass anything experienced in thousands of years – as research suggests is happening in the Southern Rockies – what will happen to the forests? .

As paleoecologists, we study how and why ecosystems changed in the past. In a multiyear project, highlighted in two new publications, we tracked how often forest fires occurred in high-elevation forests in the Rocky Mountains over the past 2,500 years, how those fires varied with the climate and how they affected ecosystems. This long view provides both hopeful and concerning lessons for making sense of today’s extreme fire events and impacts on forests.

Lakes record history going back millennia .

When a high-elevation forest burns, fires consume tree needles and small branches, killing most trees and lofting charcoal in the air. Some of that charcoal lands on lakes and sinks to the bottom, where it is preserved in layers as sediment accumulates.

After the fire, trees regrow and also leave evidence of their existence in the form of pollen grains that fall on the lake and sink to the bottom.

By extracting a tube of those lake sediments, like a straw pushed into a layer cake from above, we were able to measure the amounts of charcoal and pollen in each layer and reconstruct the history of fire and forest recovery around a dozen lakes across the footprint of the 1910 fires.

Lessons from Rockies’ long history with fire .

The lake sediments revealed that high-elevation, or subalpine, forests in the Northern Rockies in Montana and Idaho have consistently bounced back after fires, even during periods of drier climate and more frequent burning than we saw in the 20th century.

High-elevation forests only burn about once every 100 to 250 or more years on average. We found that the amount of burning in subalpine forests of the Northern Rockies over the 20th and 21st centuries remained within the bounds of what those forests experienced over the previous 2,500 years. Even today, the Northern Rockies show resilience to wildfires, including early signs of recovery after extensive fires in 2017.

But similar research in high-elevation forests of the Southern Rockies in Colorado and Wyoming tells a different story.

The record-searing fire of 2012 in the San Juan National Forest provides a good case in point. That fire and other recent megafires have left large swaths of forest without trees.