Even if the biomass was used in an optimal manner to produce electricity or make cellulosic ethanol, there would still be a net loss of carbon sequestration in forests of the Coast Range and the west side of the Cascade Mountains for at least 100 years, and probably much longer, the study showed.

The report was published earlier this year in the journal Ecological Applications. The lead author was Stephen Mitchell, who conducted the work as part of his doctoral thesis while at OSU, and is now at Duke University.

Two forest management objectives being debated in the context of federally managed landscapes in the US Pacific Northwest involve a perceived trade-off between fire restoration and carbon sequestration. The former strategy would reduce fuel (and therefore C) that has accumulated through a century of fire suppression and exclusion which has led to extreme fire risk in some areas. The latter strategy would manage forests for enhanced C sequestration as a method of reducing atmospheric CO2 and associated threats from global climate change.

—Mitchell et al. (2009)

The researchers explored the trade-off between the two strategies by employing a forest ecosystem simulation model, STANDCARB, to examine the effects of fuel reduction on fire severity and the resulting long-term carbon dynamics among three Pacific Northwest ecosystems: the east Cascades ponderosa pine forests, the west Cascades western hemlock–Douglas-fir forests, and the Coast Range western hemlock–Sitka spruce forests.

The simulations indicated that while fuel reduction treatments in these ecosystems consistently reduced fire severity, reducing the fraction by which C is lost in a wildfire requires the removal of a much greater amount of C, since most of the C stored in forest biomass (stem wood, branches, coarse woody debris) remains unconsumed even by high-severity wildfires.

For this reason, all of the fuel reduction treatments simulated for the west Cascades and Coast Range ecosystems as well as most of the treatments simulated for the east Cascades resulted in a reduced mean stand C storage. One suggested method of compensating for such losses in C storage is to utilize C harvested in fuel reduction treatments as biofuels. Our analysis indicates that this will not be an effective strategy in the west Cascades and Coast Range over the next 100 years.

We suggest that forest management plans aimed solely at ameliorating increases in atmospheric CO₂ should forgo fuel reduction treatments in these ecosystems, with the possible exception of some east Cascades ponderosa pine stands with uncharacteristic levels of understory fuel accumulation. Balancing a demand for maximal landscape C storage with the demand for reduced wildfire severity will likely require treatments to be applied strategically throughout the landscape rather than indiscriminately treating all stands.

——Mitchell et al. (2009)

Other findings included:

• On west side Cascade Range and Coast Range forests, which are wetter, the catastrophic fire return interval is already very long, and the additional levels of fuel accumulation have not been that unusual;

• A wide range of fire reduction approaches, such as salvage logging, understory removal, prescribed fire and other techniques, can effectively reduce fire severity if used properly;

• Considerable uncertainty exists in modeling of future fires, and some fuel reduction techniques, especially overstory thinning treatments, could potentially lead to an increase in fire severity.

The study authors concluded that fuel reduction may still make more sense in east-side Cascade Range and other similar forests, but that the west-side Cascades and Coast Range have little sensitivity to forest fuel reduction treatments—and might be best utilized for their high carbon sequestration capacities.

The study raises questions about how to maximize carbon sequestration in fast-growing forests and at the same time maximize protection against catastrophic fire.

It had been thought for some time that if you used biofuel treatments to produce energy, you could offset the carbon emissions from this process. That seems to make common sense and sounds great in theory, but when you actually go through the data it doesn’t work.

—Mark Harmon, holder of the Richardson Chair in the OSU Department of Forest Ecosystems and Society

Using biofuels to produce energy does not completely offset the need for other fossil fuels use and completely negate their input to the global carbon budget, the researchers found. At the absolute maximum, you might recover 90% of the energy, the study said.

That figure, however, assumes an optimal production of energy from biofuels that is probably not possible. By the time you include transportation, fuel for thinning and other energy expenditures, you are probably looking at a return of more like 60-65 percent. And if you try to produce cellulosic ethanol, the offset is more like 35 percent. If you take old, existing forests from these regions and turn them into almost anything else, you will have a net loss in carbon sequestration.

—Mark Harmon

Another recent OSU studied concluded that if forests of Oregon and northern California were managed exclusively for carbon sequestration, they could double the amount of sequestration in many areas and triple it in some.

The new study found that, in a Coast Range stand, if you removed solid woody biofuels for reduction of catastrophic fire risks and used those for fuel, it would take 169 years before such usage reached a break-even point in carbon sequestration. The study showed if the same material were used in even less efficient production of cellulosic ethanol, it would take 339 years.

The researchers did not consider in this analysis how global warming in coming years might affect the increase of catastrophic fire, Harmon said. However, “fire severity in many forests may be more a function of severe weather events rather than fuel accumulation,” the report authors wrote, and fuel reduction efforts may be of only limited effectiveness, even in a hotter future.

Part of what seems increasingly apparent is that we should consider using west side forests for their best role, which is carbon sequestration, and focus what fuel reduction efforts we make to protect people, towns and infrastructure. It’s almost impossible anyway to mechanically treat the immense areas that are involved and it’s hugely expensive. As a policy question we have to face issues of what approaches will pay off best and what values are most important.

—Mark Harmon

Resources

• Stephen R. Mitchell, Mark E. Harmon, Kari E. B. O’Connell (2009) Forest fuel reduction alters fire severity and long-term carbon storage in three Pacific Northwest ecosystems. Ecological Applications, 19 (3) pp. 643-655 doi: 10.1890/08-0501.1