Study: Erosion threatens area’s forests

Study: Erosion threatens area’s forests

19 October 2010

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USA —  A 30-foot sediment core sample taken from the bottom of upper Squaw Lake in the Applegate River watershed indicates soil erosion is a greater threat to local forests than wildfires.

That’s the conclusion of a six-page study reconstructing a 2,000-year history of forest fires in the Siskiyou Mountains by dating charcoal layers in the sediment core.

The study was released Monday by the Proceedings of the National Academy of Sciences in Washington, D.C.

Scientists Daniele Colombaroli and Daniel G. Gavin determined that wildfires are no more severe today than during the past two millennia but that soil erosion rates are now four times greater in that area. The core was taken in September 2007.

The reason, they concluded, is that logging and road-building have removed broad areas of low vegetation.

“After logging in the 1950s, sediment load was increased fourfold compared with that from the most severe presettlement fire,” the report said, referring to logging activity in the area in the 1950s and ’60s.

The lake is about five miles east of Applegate Dam in the Rogue River-Siskiyou National Forest.

Gavin is an assistant professor at the Department of Geography at the University of Oregon. Colombaroli, a former post-doctoral researcher in the department at UO, now teaches at the University of Bern, Switzerland.

The study comes in the aftermath of recent severe fires in the region, including the roughly half-million-acre Biscuit fire in 2002, which sparked intense debate over wildfire and forest management strategies. That fire burned in a mosaic pattern, scorching some areas while leaving islands of green forest inside the fire lines.

Based on the scientists’ reconstruction, it and other recent wildfires are no more severe in the region than those that occurred naturally during the past 2,000 years. In addition to charcoal, the scientists also studied pollen and other materials in the sediment.

Gavin and Colombaroli had sought to understand whether forests in this region have been subject to severe fire events before the 1800s, or whether light, low-intensity burns were typical.

They determined that the forest around the lake is less fire-resilient because of the increased erosion in the past six decades.

During periods of heavy rain, the exposed ground erodes much more rapidly, restricting the forest’s ability to recover from wildfires, they explained.

“Returning resiliency to the forested landscape, however achieved, will provide the conditions needed to move into a future with a more fire-conducive climate,” they concluded.

The goal was to better understand the precedence for severe fire in the low to mid-elevation forests with a mix of Douglas fir and pine, Gavin wrote in an e-mail to the Mail Tribune.

“In terms of how the science informs management: this study shows that even in forests that appear today to have ‘unnatural’ density due to fire suppression (as it does today around upper Squaw Lake), there have been times in the past with severe fire,” he wrote.

“We found that severe fire is linked to drought events throughout the 2000-year period so severe fire is not a new threat to the forests.”

The study is unique because it was able to reach back 2,000 years, he noted, adding that tree-ring records only go back some 400 years in the upper Squaw Lake area.

He also observed that the record revealed an “amazing episodic pattern” of frequent fire interspersed with periods of little fire.

“Known climate changes of the past appear to only partly explain this pattern, so whether it was humans or some unknown climatic changes of the past, it remains a mystery,” he wrote. “But it does indicate much greater variability through time than we have heretofore appreciated.”

However, he stressed that the particular findings in the study should be considered site specific.

“With data obtained on both the pattern of fire and erosion, it’s clear that severe fire is not a new threat to these forests, but what is new to these forests is the logging-related erosion levels and the current species composition,” he reiterated.

In addition to its own research, the scientists cited work done by other scientists, including a 2009 study whose authors included Dominick DellaSala, president and chief scientist at the National Center for Conservation Science & Policy in Ashland. Ashland area ecologist Dennis Odion was a co-author in that study.

Although their research focused on historic time, using satellite images, it also concluded that wildfires are no more severe now than in the past, DellaSala said.

“What’s interesting is the distant past ties together what has been happening in the recent past — we are seeing the same kind of pattern here,” said DellaSala after reading the research released Monday.

“We didn’t think fire regimes were outside the bounds of historic conditions,” he added. “This (study) goes beyond what we looked at but it also shows fire regime is quite episodic.”

The upper Squaw Lake research project was funded by the Swiss National Foundation Fellowship with additional support from the University of Oregon and the Oeschger Centre for Climate Change Research at the University of Bern, Switzerland.

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