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Flammable Forest: Why today’s Sierra woods are quicker to burn

August 26, 2013

With the Rim Fire still sweeping out-of-control through Sierra woodlands, something tugged at our memory: Wasn’t this the same area that burned a couple of decades ago?

In fact, yes—the Lightning Complex Fire of 1987 (also called the Stanislaus Complex Fire) torched almost exactly the same terrain. So are we in some kind of vicious recursive wildfire loop, where the same areas of the Sierra will burn and burn again, until nothing is left but blackened rocks?

What we discovered is that a century ago, the land now being consumed by Rim Fire hosted a sparser canopy of huge, but widely spaced, old-growth conifers—a forest much more resistant to fire. By contrast, the conifers in today’s Sierra are often packed tight as the hair on a Rottweiler’s back, and burn with the intensity of an exploding meth lab.

What caused the transformation that made today’s forest more flammable?

For answers we sought out UC Berkeley professor of fire sciences Scott Stephens.

It turns out Stephens and his crew were surveying this region of the Sierra when they were chased out last week by the Rim Fire. Or rather, re-surveying it. They were following up on an inventory of the Stanislaus National Forest taken by the U.S. Forest Service in 1911; they had serendipitously found the documents detailing the study in the National Archives at San Francisco (technically, in San Bruno). 

“It’s really a huge dataset,” said Stephens. “We were able to survey the same areas right down to township and range. And the one thing that really jumped out at us was how different the forests looked then and now.”

Specifically, the 1911 forests were composed of giant conifers, mainly ponderosa pine, spaced at wide intervals. The forest canopy, Stephens estimates, was about 28 percent—far sparser than today’s forests, where the sun at times is blocked out by the thick if spindly stands of ponderosa pine, Jeffrey pine, sugar pine, Douglas fir and white fir.

“The 1911 forests were extremely resilient,” said Stephens. “I doubt a fire of the intensity of the Rim Fire could even get started in them. There wouldn’t be the fuels for it. If a fire did get kindled, it would just creep around and burn in a low-level mosaic pattern.”

Such modest fires generally are considered beneficial to woodlands, in that they seldom harm large trees, consume the dead fuels that could lead to larger conflagrations, eliminate destructive insects and pathogens, and return nutrients to the soil in the form of ash.

But the Sierra forests of 2013 are different, just as blazes such as Rim Fire are different. Much of the more than 150,000 acres that Rim Fire has incinerated thus far consist of those younger, more tightly packed conifers that serve to propel a fire.

How did we come to such a pass?

Clear-cut logging, decades of rigorous fire suppression and climate change are all culpable. But so is the post-fire strategy that government agencies typically apply to wildlands, says Stephens.

“In the case of the Lightning Complex Fire, the Forest Service went in and replanted with ponderosa pine,” he says. “The trees came back very well—maybe too well.  You had uniformly thick stands of pine, but the Forest Service never did any follow-up management (such as thinning). So you had these vast areas of steep terrain covered in highly combustible fuels. From that perspective, the Rim Fire is no surprise.”

Stephens isn’t condemning the Forest Service. Thinning large areas of woodland is an extremely expensive process, and federal agencies have been strapped for funds in recent years.

“But if you’re going to re-plant after a fire—especially in the Sierra—you’d better be prepared for a long-term commitment,” he says. “If you don’t manage the emerging woodlands, if you don’t try to incorporate resilience into the forest by thinning trees, you’re just setting yourself up for recurring wildfires. That’s what we’re seeing with the Rim Fire.”

But, Stephens adds, such cycles don’t necessarily end with a blasted moonscape. Rather, the former coniferous forests will likely revert to chaparral and grasslands, “and ultimately, to oak woodlands. And that, of course, isn’t completely undesirable.  Oak preservation is a primary concern in California. Oak woodlands support tremendous biological diversity. And they’re far more resistant to wildfire than coniferous forests, particularly second and third-growth ponderosa pine forests.” 

—Glen Martin

For an update on Rim Fire, see California Magazine’s report here.  For a look at how development is making wildfires more dangerous, see California Magazine’s report here.

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