SOS Forests

Forest Fires Degrade Soils

New findings by a team of Oregon forest scientists reveal that the Biscuit Fire (2002) not only incinerated 500,000 acres of forest, it also stripped soils clean off the landscape. Millions of tons of “sequestered” carbon were emitted by the Biscuit Fire, but more than that, the soil was sucked up into fire plumes and blown off the site, leaving a only a rubble of heavier stones.

The study, Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence by Bernard T. Bormann, Peter S. Homann, Robyn L. Darbyshire, and Brett A. Morrissette, is to be published in the peer-reviewed Canadian Journal of Forest Research [Can. J. For. Res. 38: 2771–2783 (2008)] in December. Extracts and a link to the full text may be found in the W.I.S.E. Colloquium: Forest and Fire Sciences [here].

The study was unique in that soil measurements were taken before the fire and the same plots remeasured after the fire. Most studies examine burned and unburned post-fire plots, and retrospective assumptions must be made regarding pre-fire conditions. In this case, however, the Biscuit Fire burned through a portion of a 150 ha Long-Term Ecosystem Productivity (LTEP) experiment (Bormann et al. 1994; Homann et al. 2008), and the forest scientists were able to examine soil changes in paired pre- and post-fire plots.

Bormann et al found that more than 10 tons per acre of carbon and between 450 to 620 pounds per acre of nitrogen were vaporized by the fire. Some 60% of soil carbon and 57% of soil nitrogen losses came from mineral soil horizons (below the duff and humus top layers). In addition they found that 127 megagrams (127,000 kilograms) of soil per hectare disappeared. The scientists conjectured:

An intriguing alternative explanation for most of the missing fine soil is transport via the massive smoke plume. The elevation of the smoke column and the spread of the plume provide a plausible convective erosion process for off-site transport of substantial material. Large plumes of smoke, some more than 1500 km long, were visible most days during the months of the fire from the NASA MODIS satellite (Fig. 9). Fine soil particles have been detected in smoke (Palmer 1981; Samsonov et al. 2005), and wind speeds near the soil surface — driven by extremely strong vortices resulting from fire-driven atmospheric convection (Palmer 1981; Banta et al. 1992) — can carry smoke to the lower stratosphere (Trentmann et al. 2006).

They called this an “alternative explanation” because their first thought was that post-fire water erosion carried the soil away. However, erosion box measurements accounted for only a third of the missing soil. The plume explanation was based on speculation because the plume contents and volume were not accurately measured (for obvious reasons).

Total soil carbon losses from the Biscuit Fire were estimated to be 9 teragrams (9 million metric tons). That does not include carbon emissions from the incinerated vegetation, which we estimate to be an additional 35 Tg. The sum (44 teragrams or million metric tons) is roughly equivalent to the carbon emissions of 9 million cars driven all year.

The fire was hot enough to melt heavy-duty aluminum tags on steel grid posts placed as part of the LTEP experiment. The scientists estimated that fire temperatures were hotter than 700 degrees C (1300 degrees F) based on kiln tests on similar tags. At those temperatures tree mortality was near total as was fine fuel consumption.

The result was a seared landscape, devoid of living organisms, charred beyond recognition, and cooked deep into the soil. Fine soils were blown away, seed banks destroyed, and the essential productivity of the site vastly depleted. Bormann et al concluded:

The intensity of wildfires and magnitude of losses of fine soils and soil C and N have additional implications for soil fertility and subsequent rates of plant production and C sequestration. Soil C losses lead to increased bulk density and reduced soil water-holding capacity, cation-exchange capacity, and sources of energy for microbial communities. To the extent that soil N, soil C, and soil structure control productivity, these changes should result in major declines that will last as long as it takes to return to prefire conditions.

That could take decades or perhaps centuries.

It is stunning to realize that the US Forest Service calls such fires “beneficial” to resources. The USFS has embarked on a program of Wildfires Use For Resource Benefit (WFU). They have not specified which resources benefit, or how, or quantified the alleged benefits. It is abundantly clear from this study that resources are seriously degraded by wildfire, at least by this fire. Soil, biological, air, and water resources were severely damaged and those damages will remain and continue for perhaps many human lifetimes.

The authors of this study point out that resource degradation is contrary to the legal mandate and mission of the USFS:

Any potential loss in productivity is relevant to the US National Forest Management Act of 1976, where the Secretary of Agriculture is required, ‘‘through research and continuous monitoring, to ensure that management systems will not produce substantial and permanent impairment of the productivity of the land’’. The US Endangered Species Act of 1973 is also relevant to the management of high-intensity fires, for example, in the case of the northern spotted owl that nests primarily in stands of large trees averaging only 32 large trees ha–1 (Hershey et al. 1998). When soils can no longer produce such trees, the area of suitable habitat that could redevelop after fire is also lessened.

It is hugely unlikely that spotted owls will ever reoccupy the Biscuit Burn. The area has been rendered unfit to grow large trees, and current USFS policies virtually guarantee that severe, catastrophic fires will revisit the area periodically.

There is no question that prevention of the kind of forest destruction inflicted by the Biscuit Fire is desperately needed before all our public forests are similarly destroyed. Current USFS policies of WFU and unrestrained forest incineration must be altered. Restoration forestry, which prepares forests to receive fire in a manner that protects, maintains, and perpetuates forests, must be mandated and implemented on a landscape scale as soon as possible. From Bormann et al:

Much of the recent debate has centered on the effects of post-wildfire management on tree regeneration, wildlife habitat, and future fire risk (Donato et al. 2006; Newton et al. 2006; Shatford et al. 2007; Thompson et al. 2007). In light of the first direct evidence of major effects of intense wildfire on soils — based on extensive and detailed pre- and post-fire soil sampling — we think that soil changes, especially the potential loss of soil productivity and greenhouse gas additions resulting from intense wildfire, deserve more consideration in this debate. In forests likely to be affected by future intense fire, preemptive reduction of intense-fire risks can be seen as a way to reduce losses of long-term productivity and lower additions of greenhouse gases. Preemptive strategies may include reducing fuels within stands but also improving fire-attack planning and preparation and changing the distribution of fuels across the landscape to reduce the size of future fires. Practices can include thinning and removing or redistributing residues and underburning.

In forests already affected by intense fire, amelioration to increase C sequestration, tree growth, and eventually late successional habitat should be strongly considered. Amelioration practices might include seeding or planting N2-fixing and other plants, fertilizing, and managing vegetation and fuels through time. To the extent that receipts from pre- and post-wildfire logging are the only means of paying for these practices, such logging should be balanced against other management objectives and concerns. Harvesting before and after fire to generate revenue, if done improperly, has the potential to harm soils, but this outcome needs to be weighed against the outcomes resulting from increased high-intensity fire and from not ameliorating after soils have been burned intensely.

This forest science paper is cutting edge and a breakthrough (we hope) from the typical dull and pointless pseudo-science we have been subjected to over the past two decades. It is late, but not too late, for the general public to realize that forest stewardship is preferable to forest incineration. The public must demand responsible forest stewardship, and particularly restoration forestry, from our public land management agencies.

This year we have (again) witnessed massive forest destruction by deliberate burning, from Idaho to California. Old-growth forests have been decimated in the South Barker, Rattle, Middle Fork, Iron, Siskiyou, Ukonom, Blue, Clover, and dozens of other fires. The resource degradation from fires of past years has been amply evident and continues. The situation is intolerable. The USFS MUST learn how to care for forests and MUST engage in forest stewardship right away. Resistance to stewardship is untenable and should serve as grounds for immediate dismissal of any who advocate or engage in forest destruction.

This paper quantifies some of the destruction inflicted by catatsrophic forest fires. Let us hope that the lessons learned are taken to heart.