Patch Burning
Stephen Pyne. 2009. Patch Burning. The Wildland Fire Lessons Learned Center.
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Dr. Stephen J. Pyne is Regents Professor at Arizona State University. He is author of Awful Splendour: A Fire History of Canada (2007), Univ. British Columbia Press, Fire in America: A Cultural History of Wildland and Rural Fire (1982), Burning Bush: A Fire History of Australia (1991), World Fire: The Culture of Fire on Earth (1995), Vestal Fire: An Environmental History, Told Through Fire, of Europe and Europe’s Encounter with the World (1997), The Ice: A Journey to Antarctica (1986), and numerous other histories, memoirs, essays, and texts about fire.
This essay is one of three related essays about fire in the Midwest. The others are:
Missouri Compromise [here], and
People of the Prairie, People of the Fire [here]
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Full text [here]
Selected excerpts:
It is a biotic border that spans a continent, and it displays a continental sized roughness. In rude terms it traces the coarse shoreline between a sea of grass to the west and a land of mixed forests to the east, an edge sculpted into the ecological equivalent of bays, narrows, skerries, and estuaries, as climatic tides, the tectonic lurching of glaciers, and the sprawl of colonizing species have tugged and twisted, and here and there allowed grass or woods to mostly prevail. That textured shoreline holds a jumbled geography of incombustible wetlands and free-burning bottomlands, fire-flushed Barrens and fire-hardened forests, prairie peninsulas and prairie patches, oak mottes and woody copses, and landscapes latent with bits of them all, some extending over hundreds of miles.
It is a fractal frontier, patchy at every scale, with small patches within larger. And it is a frontier of fire, with each part checked or boosted by the ferocity and abundance of burning.
Cross Timbers
Even so, the Cross Timbers stand out. They proclaim a bold, woody headland, as distinctive as the White Cliffs of Dover, between the grassy sea that swells to the west and the humid forest that crowds the east. It is here that storm surges of fire, roaring over the long fetch of the Great Plains, whipped by the westerlies into whitecaps of flame, crash against the less combustible woods. The belt is long, stretching from the Edwards Plateau of Texas to the Flint Hills of Kansas; irregular and sinuous, roughly cruciform, historically varying from five to 30 miles wide, but at places spanning most of Oklahoma; and persistent, its 4.8 million hectares defying settlement’s attempts to log, plow, graze, or burn it into oblivion. Instead, it continues in Oklahoma to thicken with stubborn oaks – blackjack, shin, live, and post.
Re-Inventing the United States Forest Service: Evolution from Custodial Management, to Production Forestry, to Ecosystem Management
Doug MacCleery. 2008. Re-Inventing the United States Forest Service: Evolution from Custodial Management, to Production Forestry, to Ecosystem Management, IN Reinventing Forestry Agencies: Experiences of Institutional Restructuring in Asia and the Pacific, Edited by: Patrick Durst, Chris Brown, Jeremy Broadhead, Regan Suzuki, Robin Leslie and Akiko Inoguchi. Asia-Pacific Forestry Commission, FAO-UN, RAP Pub 2008/05.
Douglas W. MacCleery is Senior Policy Analyst, Forest Management Staff, USDA, Forest Service, Washington, DC
Full text [here]
Selected excerpts:
INTRODUCTION
Forest policy and institutional frameworks in all countries are fashioned according to their larger sociopolitical context, traditions and history. A major factor in shaping the historical sociopolitical context in the United States has been decentralization. At the time of their independence from England, the 13 original colonies entered the union as largely autonomous entities or “states” — and over time they have guarded this status jealously. In spite of this, over more recent decades, many policy and institutional functions have been centralized at national or federal levels. This trend has been slow at times — and has often been resisted by the states — with occasional attempts to reverse such centralization.
HISTORICAL CONTEXT LEADING TO THE ESTABLISHMENT OF THE NATIONAL CONSERVATION FRAMEWORK AND THE FOREST SERVICE
Throughout the nineteenth century, United States policy encouraged rapid settlement and economic development of its western territory. To accomplish this, a variety of approaches were developed, including transfer of federal (public domain) lands to individual farmers, ranchers and corporations, especially railroad companies that built transportation infrastructure.
After 1850, the population grew rapidly (20 to 25 percent per decade) and settlement of the western territories accelerated. Concerns began to be voiced over some of the environmental and economic implications of rapid development, including: (1) accelerated deforestation (forests were being cleared for agriculture at the rate of almost 3 500 hectares per day); (2) massive wildfires due to logging and land clearing (wildfires annually razed 8 to 20 million hectares); (3) extensive areas of “cut-over” land or “stump lands” remained unstocked or poorly stocked with trees for decades (estimated at 32.5 million hectares in 1920); (4) significant soil erosion by wind and water in some places; and (5) major wildlife depletion due to commercial hunting and subsistence use (Trefethen 1975; Williams 1989; MacCleery 1992). It was gradually recognized that these conditions were jeopardizing future economic development, as well as being concerns in their own right.
Bushfires, Prescribed Burning, and Global Warming
Roger Underwood, David Packham, and Phil Cheney. 2008. Bushfires, Prescribed Burning, and Global Warming. Bushfire Front Inc. Occasional Paper No 1, April 2008 [here]
Roger Underwood is a former General Manager of the Department of Conservation and Land Management (CALM) in Western Australia, a regional and district manager, a research manager and bushfire specialist. Roger currently directs a consultancy practice with a focus on bushfire management and is Chairman of The Bushfire Front Inc.. He lives in Perth, Western Australia.
David Packham is Senior Research Fellow, School of Geography and Environmental Science, Monash University, Victoria.
Phil Cheney is Honorary Research Fellow, CSIRO, Canberra, ACT
Full text:
This is not a paper about climate change or the contentious aspects of the climate debate. Our interest is bushfire management. This is an activity into which the debate about climate change, in particular “global warming”, has intruded, with potentially damaging consequences.
Australia’s recent ratification of the Kyoto Treaty has been welcomed by people concerned about the spectre of global warming. However, the ratification was a political and symbolic action, and will have no immediate impact on the volume of carbon dioxide (CO2) in the atmosphere, and therefore will not influence any possible relationship between CO2 emissions and global temperatures.
However, the ratification could have an impact on Australian forests. Spurious arguments about the role of fire contributing to carbon dioxide emissions could be used to persuade governments and management agencies to cease or very much reduce prescribed burning under mild conditions.
Decades of research and experience has demonstrated that fuel reduction by prescribed burning under mild conditions is the only proven, practical method to enable safe and efficient control of high-intensity forest fires.
Two myths have emerged about climate change and bushfire management and are beginning to circulate in the media and to be adopted as fact by some scientists:
1. Because of global warming, Australia will be increasingly subject to uncontrollable holocaust-like “megafires”.
2. Fuel reduction by prescribed burning must cease because it releases carbon dioxide into the atmosphere, thus exacerbating global warming and the occurrence of megafires.
Both statements are incorrect. However they represent the sort of plausible-sounding assertions which, if repeated often enough, can take on a life of their own and lead eventually to damaging policy change.
Watershed Response to Western Juniper Control
Timothy L. Deboodt. 2008. Watershed Response to Western Juniper Control. Dissertation for the degree Doctor of Philosophy in Rangeland Ecology and Management, Oregon State University, presented on May 8, 2008.
Full text [here] (4.6 MB)
Abstract:
Western juniper (Juniperus occidentalis) encroachment has been associated with increased soil loss and reduced infiltration resulting in the loss of native herbaceous plant communities and the bird and animal species that rely on them. Hydrologically, however, change in water yield has been linked with the amount of annual precipitation a site received. Studies published in the 1970’s and 1980’s, suggest that a minimum 450 mm (18 inches) of annual precipitation was necessary before an increase in water yield manifested itself following vegetation manipulation.
In 1993, a paired watershed study was initiated in the Camp Creek drainage, a tributary of the Crooked River of central Oregon, to evaluate the impacts of cutting western juniper on the hydrologic function of those sites. The study involved a paired watershed approach using watersheds of approximately 110 hectares (270 acres) each to evaluate changes in a system’s water budget following the reduction of western juniper.
The 30 year average annual precipitation for the area is 350 mm (13.75) and during the study period, annual precipitation ranged from 80 percent to 129 percent of average. In 2005, following 12 years of pretreatment monitoring in the 2 watersheds (Mays and Jensen) all post-European aged juniper (juniper < 140 years of age) were cut from the treatment watershed (Mays).
Analysis indicated that juniper reduction significantly increased late season spring flow by 225 percent (alpha > .05), increased days of recorded ground water by an average of 41 days (alpha > .05) and increased the relative availability of late season soil moisture at soil depths of .76 m (27 inches) (alpha > 0.1).
Australian Bushfire Management: a case study in wisdom versus folly
Roger Underwood. 2009. Australian Bushfire Management: a case study in wisdom versus folly
Roger Underwood is a renowned Australian forester with fifty years experience in bushfire management and bushfire science. He has worked as a firefighter, a district and regional manager, a research manager and senior government administrator. He is Chairman of The Bushfire Front, an independent professional group promoting best practice in bushfire management.
One man’s wisdom is another’s folly - Ralph Waldo Emerson
Many years ago, still a young man, I watched for the first time the grainy, flickering black and white film of the British infantry making their attack on the opening day of the Battle of the Somme. The stark and terrible footage shows the disciplined soldiers climbing from their trenches and, in line abreast, walking slowly across no-man’s land towards the enemy lines. They scarcely travel a few paces before the German machine gunners open up. They are mown down in their thousands. They are chaff before a wind of fire.
I can still remember being struck nerveless by these images, and later my anger when I realised what that calamitous carnage represented. It spoke of the deep incompetence of the Generals who devised this strategy of doom and then insisted upon its implementation. It spoke of front-line men led by people without front-line experience. It spoke of battle planners unable to think through the consequences of their plans, and who devalued human lives. It spoke of a devastating failure of the human imagination.
Worst of all, the strategies of the World War I Generals demonstrated that they had not studied, or that they had forgotten, the lessons of history. In the final year of the American Civil war, 50 years earlier, the Union army had been equipped for the first time with Springfield repeating rifles, replacing the single shot muskets they had previously used and still were being used by the Confederate army. The impact on Confederate soldiers attacking defenders armed with repeating rifles was identical to that later inflicted by machine guns on the Western Front. But it was a lesson unlearnt, of collective wisdom unregarded.
None of you will have any difficulty in seeing where this analogy is taking me.
The catastrophic bushfires in Victoria this year, and the other great fires of recent years in Victoria, New South Wales, the ACT and South Australia are dramatic expressions not just of killing forces unleashed, but of human folly. No less than the foolish strategies of the World War I Generals, these bushfires and their outcomes speak of incompetent leadership and of failed imaginations. Most unforgivable of all, they demonstrate the inability of people in powerful and influential positions to profit from the lessons of history and to heed the wisdom of experience.
Effect of low-temperature pyrolysis conditions on biochar for agricultural use
J. W. Gaskin, C. Steiner, K. Harris, K. C. Das, B. Bibens. 2008. Effect of low-temperature pyrolysis conditions on biochar for agricultural use. Transactions of the American Society of Agricultural and Biological Engineers, Vol. 51(6): 2061-2069.
Full text [here]
Selected excerpts:
Abstract:
The removal of crop residues for bio-energy production reduces the formation of soil organic carbon (SOC) and therefore can have negative impacts on soil fertility. Pyrolysis (thermoconversion of biomass under anaerobic conditions) generates liquid or gaseous fuels and a char (biochar) recalcitrant against decomposition. Biochar can be used to increase SOC and cycle nutrients back into agricultural fields. In this case, crop residues can be used as a potential energy source as well as to sequester carbon (C) and improve soil quality. To evaluate the agronomic potential of biochar, we analyzed biochar produced from poultry litter, peanut hulls, and pine chips produced at 400°C and 500°C with or without steam activation.
The C content of the biochar ranged from 40% in the poultry litter (PL) biochar to 78% in the pine chip (PC) biochar. The total and Mehlich I extractable nutrient concentrations in the biochar were strongly influenced by feedstock. Feedstock nutrients (P, K, Ca, Mg) were concentrated in the biochar and were significantly higher in the biochars produced at 500°C. A large proportion of N was conserved in the biochar, ranging from 27.4% in the PL biochar to 89.6% in the PC biochar. The amount of N conserved was inversely proportional to the feedstock N concentration. The cation exchange capacity was significantly higher in biochar produced at lower temperature. The results indicate that, depending on feedstock, some biochars have potential to serve as nutrient sources as well as sequester C.
Introduction:
… There are several lines of evidence that charcoal plays an important role in soil fertility. Charcoal has been identified as an important soil constituent in fertile Chernozems (Schmitdt et al., 1999) and in anthropogenic enriched dark soil (Terra Preta) found throughout the lowland portion of the Amazon Basin (Glaser et al., 2000). Research on tropical soils indicates that charcoal amendments can increase and sustain soil fertility (Steiner et al., 2007). The beneficial effects appear to be related to alterations in soil physical, chemical, and biological properties, such as reduced acidity (Topoliantz et al., 2005), increased cation exchange capacity (CEC) (Cheng et al., 2008; Liang et al., 2006), enhanced nitrogen (N) retention (Lehmann et al., 2003; Steiner et al., 2008b), increased microbiological activity (Steiner et al., 2008a), and increased mycorrhizal associations (Warnock et al., 2007). …
Charcoals produced from wildfire or traditional charcoal production may have different chemical and physical characteristics from pyrolytic biochars created under specific conditions for energy production. …
After forest fires, on average, only 3% of the N in the biomass is found in ash, which contains black carbon or biochar (Giardina et al., 2000). …
Studies of wildfire effects on biomass composition indicate that N begins to volatilize at 200°C, and above 500°C half of the N in organic matter is lost to the atmosphere. …
Conclusions:
Pyrolytic biochar has the potential to be used in agricultural production to sequester carbon and serve as a fertilizer. Although pyrolysis conditions are known to affect the chemical and physical characteristics of biochar, at the relatively low pyrolysis temperatures used in this study, feedstock characteristics had the greatest influence on key agricultural characteristics. Carbon concentrations in the biochars decreased with increasing mineral content of the feedstock. Little DC was leachable from the fresh biochar. A high proportion of the feedstock N was conserved in the biochar; however, the N may not be plant available. Nutrients such as P, K, and Ca are extractable with a weak double acid extractant and may be plant available.
The higher pyrolysis temperature increased nutrient concentrations, except for N, but decreased CEC. Recent literature has shown that natural long-term oxidation of biochar in the soil increases the amount of negative charges on the biochar surface (Cheng et al., 2008). Development and optimization of pyrolysis and post-production treatments to increase CEC or available nutrients is important in order to increase the immediate benefits of biochar applications in agriculture.
Nitrogen retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal
Christoph Steiner, Bruno Glaser, Wenceslau Geraldes Teixeira, Johannes Lehmann, Winfried E.H. Blum, and Wolfgang Zech. 2008. Nitrogen retention and plant uptake on a highly weathered central Amazonian Ferralsol amended with compost and charcoal. J. Plant Nutr. Soil Sci. 2008.
Full text [here]
Selected excerpts:
Abstract:
Leaching losses of N are a major limitation of crop production on permeable soils and under heavy rainfalls as in the humid tropics. We established a field trial in the central Amazon (near Manaus, Brazil) in order to study the influence of charcoal and compost on the retention of N. Fifteen months after organic-matter admixing (0–0.1 m soil depth), we added 15N-labeled (NH4)2SO4 (27.5 kg N ha–1 at 10 atom% excess). The tracer was measured in top soil (0–0.1 m) and plant samples taken at two successive sorghum (Sorghum bicolor L. Moench) harvests.
The N recovery in biomass was significantly higher when the soil contained compost (14.7% of applied N) in comparison to only mineral-fertilized plots (5.7%) due to significantly higher crop production during the first growth period. After the second harvest, the retention in soil was significantly higher in the charcoal-amended plots (15.6%) in comparison to only mineral-fertilized plots (9.7%) due to higher retention in soil. The total N recovery in soil, crop residues, and grains was significantly (p < 0.05) higher on compost (16.5%), charcoal (18.1%), and charcoal-plus-compost treatments (17.4%) in comparison to only mineral-fertilized plots (10.9%). Organic amendments increased the retention of applied fertilizer N. One process in this retention was found to be the recycling of N taken up by the crop. The relevance of immobilization, reduced N leaching, and gaseous losses as well as other potential processes for increasing N retention should be unraveled in future studies.
Introduction:
The fertility of highly weathered Ferralsols in the tropics is low, and soil organic matter (SOM) plays a major role in sustaining soil productivity. Thus, long-term intensive use is not sustainable without nutrient inputs where SOM stocks are depleted (Tiessen et al., 1994). Due to low nutrient-retention capacity and high permeability of these soils, strong tropical rainfalls cause rapid leaching of mobile nutrients such as those applied with mineral N fertilizers (Hölscher et al., 1997a; Giardina et al., 2000; Renck and Lehmann, 2004). …
Only relatively small amounts of charcoal are produced by the traditional slash-and-burn technique. Charcoal represents only 1.7% of the preburn biomass if a forest is converted into cattle pasture (Fearnside et al., 2001). Producing charcoal for soil amelioration from aboveground biomass instead of converting it to CO2 through burning might be an alternative to slash-and-burn (Lehmann et al., 2002; Steiner et al., 2004b; Lehmann et al., 2006).
The existence of so-called “Terra Preta de Índio” (Indian black earth) suggests that a human-induced accumulation of SOM can be maintained over centuries (Sombroek et al., 1993). These soils are exceptionally fertile, and their productivity is most likely linked to an anthropogenic accumulation of P and Ca associated with bone apatite (Lima et al., 2002) and black C (BC) as charcoal (Glaser et al., 2001).
Soil respiration in Amazonian plantations treated with charcoal, and mineral or organic fertilisers
Christoph Steiner, Murilo Rodrigues de Arruda, Wenceslau G. Teixeira, and Wolfgang Zech. 2008. Soil respiration curves as soil fertility indicators in perennial central Amazonian plantations treated with charcoal, and mineral or organic fertilisers. Trop. Sci. (2008)
Full text [here]
Selected excerpts:
Abstract:
We assessed substrate-induced respiration and soil chemical properties in order to study the influence of charcoal, nitrogen and phosphorus fertilisation on two different perennial crops in a confounded factorial design on a highly weathered Amazonian upland soil. Each plantation tested three different factors in three different levels making up 27 treatment combinations. Whereas the banana plantation received mineral fertilisation in addition to charcoal applications (3rd factor), the guarana (Paullinia cupana) plantation was fertilised organically using chicken manure and bone meal as the corresponding factors.
Charcoal increased pH, total nitrogen, availability of sodium, zinc, manganese, copper and soil humidity, and decreased aluminium availability and acidity in the mineral-fertilised plantation only. This caused a signifi cant increase in basal respiration and microbial efficiency in terms of carbon dioxide release per microbial carbon in the soil. The microbial biomass, efficiency and population growth after substrate addition was significantly increased with increasing levels of organic fertiliser amendments. We conclude that charcoal is a valuable component especially in inorganic-fertilised agricultural systems.
Introduction:
Without continuous fertilisation, the extremely nutrient-poor Amazonian upland soils show no potential for agriculture beyond a tree-year lifespan of the forest litter mat, once biological nutrient cycles are interrupted by slash-and-burn (Tiessen et al. 1994). Slash-and-burn agriculture is a common practice in the tropics (Giardina et al. 2000; Goldammer 1993) and is considered to be sustainable if adequate (up to 20 years) fallow periods follow a short period of cultivation (Kleinman et al. 1995). Fertilisation is necessary for continuous cropping, but the strongly weathered soils of the tropics have a low nutrient-retention capacity and the intense tropical rains wash easily available and mobile nutrients rapidly into deep soil layers unavailable to most crop plants (Giardina et al. 2000; Hölscher et al. 1997; Renck and Lehmann 2004). …
The widespread existence of an anthropogenic fertile dark soil in the Amazon proves that human soil-manipulation can create permanently fertile soil (Woods and McCann 1999). The Amazonian dark earths (or ‘Terra Preta de Índio’) are found at pre-Columbian settlements throughout Amazonia in patches ranging in size from less than a hectare to many square kilometres (McCann et al. 2001). Today, and as assumed in the past, those soils are and were intensively cultivated by the native population. Their fertility is most likely to be linked to an anthropogenic accumulation of P and calcium (Ca) from bones (Lima et al. 2002), depositions of these and many other nutrients from a variety of human habitation activities (Woods 2003), and black carbon (C) as charcoal (Glaser et al. 2001b; Lima et al. 2002). Amazonian dark earths contain significantly more C, N, Ca and potassium (K) and up to 13.9 g kg-1 phosphorus (P2O Pentoxide 5) (almost 4 g kg-1 available P) (Lima et al. 2002), and cation exchange capacity, pH value and base saturation are signifi cantly higher than in the surrounding Oxisols (Glaser et al. 2000; Zech et al. 1990). Charcoal persists in the environment over centuries and is responsible for the stability of the Amazonian dark earth’s SOM (Glaser et al. 2001a).
Fuel treatment effects on tree-based forest carbon storage and emissions under modeled wildfire scenarios
Matthew Hurteau and Malcolm North. 2009. Fuel treatment effects on tree-based forest carbon storage and emissions under modeled wildfire scenarios. Front Ecol Environ 2009; 7, doi:10.1890/080049
Full text [here]
Selected excerpts:
Abstract
Forests are viewed as a potential sink for carbon (C) that might otherwise contribute to climate change. It is unclear, however, how to manage forests with frequent fire regimes to maximize C storage while reducing C emissions from prescribed burns or wildfire. We modeled the effects of eight different fuel treatments on tree-based C storage and release over a century, with and without wildfire. Model runs show that, after a century of growth without wildfire, the control stored the most C. However, when wildfire was included in the model, the control had the largest total C emission and largest reduction in live-tree-based C stocks. In model runs including wildfire, the final amount of tree-based C sequestered was most affected by the stand structure initially produced by the different fuel treatments. In wildfire-prone forests, tree-based C stocks were best protected by fuel treatments that produced a low-density stand structure dominated by large, fire-resistant pines. …
As trees grow, C is sequestered, and these additional tons of C can be used to offset emissions in other sectors. In fire-prone forests, however, tree-based C storage may lead to large releases of C if trees are killed and partially consumed by a high-severity fire (Breshears and Allen 2002; Hurtt et al. 2002; Kashian et al. 2006; Hurteau et al. 2008). …
Beginning in the mid-1900s, US forested lands became a net sink for CO2, as a result of forest regrowth and fire suppression (Hurtt et al. 2002). Fire suppression has increased forest density and stand-replacement fire risk in forests that were historically characterized by frequent, low-severity fire regimes (McKelvey and Busse 1996). …
In fire-prone forests of the western US, there are three common management practices for reducing forest biomass and the risk of catastrophic fire: prescribed fire, mechanical thinning, and both treatments combined. …
Our objective was to model the amount of live- and dead-tree-based C stored and released over a century with and without wildfire in Sierra Nevada mixed-conifer forests, after fuel reduction treatments. Our hypotheses were:
(1) in the absence of wildfire, the no-fuels treatment alternative will store the most live- and dead-treebased C;
(2) with wildfire, treatments that develop and retain large trees will store the greatest amount of livetree C;
(3) pre-settlement forest structure will maximize tree-based C storage while minimizing C release during wildfire;
(4) with wildfire, prescribed fire treatments will have a lower total C release than unburned treatments; and
(5) reducing stand density and concentrating live tree C stocks in larger individuals will decrease the post-wildfire mortality, reducing the drop below the baseline.
Here, we use current CCAR FSP (2007) accounting methods to evaluate changes in C stocks using the Forest Vegetation Simulator (FVS) and track fire emissions using the Fire and Fuels Extension (FFE) of FVS (Crookston and Dixon 2005). Although FVS does not account for soil C, it is regionally calibrated, widely used by managers to model forest response to different treatments and disturbances, and one of the CCAR-approved models for establishing baselines. …
Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence
Bernard T. Bormann, Peter S. Homann, Robyn L. Darbyshire, and Brett A. Morrissette. 2008. Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence. Can. J. For. Res. 38: 2771–2783 (2008).
Full text [here]
Selected excerpts:
Abstract:
Direct evidence of the effects of intense wildfire on forest soil is rare because reliable prefire data are lacking. By chance, an established large-scale experiment was partially burned in the 2002 Biscuit fire in southwestern Oregon. About 200 grid points were sampled across seven burned and seven unburned stands before and after the fire. Fire-related soil changes — including losses of soil organic and inorganic matter — were so large that they became complicated to measure. The 51 Mg ha–1 of loose rocks on the soil surface after fire suggests erosion of 127 Mg ha–1 of fine mineral soil, some of which likely left in the fire plume. After accounting for structural changes and erosion with a comparable-layers approach, combined losses from the O horizon and mineral soil totaled 23 Mg C ha–1 and 690 kg N ha–1, of which 60% (C) and 57% (N) were lost from mineral horizons. Applying a fixed-depth calculation — commonly used in previous fire studies — that disregards structural changes and erosion led to underestimates of loss of nearly 50% for C and 25% for N. Although recent debate has centered on the effects of postwildfire forest management on wood, wildlife habitat, and fuels, this study indicates that more consideration should be given to the possible release of greenhouse gases and reduction of future forest productivity and CO2 uptake.
Introduction
Forest ecologists think of wildfire as an important natural process that regulates fuel accumulation and successional patterns across most western US forests (DeBano et al. 1998). Forest wildfires also have great societal consequences. Rural communities and firefighters are well aware of the dangers of high-intensity (high-temperature) large-scale fires. Given dry conditions and sufficient fuels, these fires can make their own weather, spread at alarming rates, and often become nearly unstoppable. The monetary and human costs of fighting such fires — loss of property, timber, wildlife habitat, water quality, C stocks, and other resource values, and remediation expenses — can be substantial (Neuenschwander et al. 2000; Dombeck 2001). The direct cost of fighting wildfires nationally in 2002 was $1.6 billion, with nearly a tenth of this budget spent on a single fire, reported on here, called the Biscuit complex fire, in southwestern Oregon, USA (Government Accounting Office 2004). …
Current understanding of the ecological effects of intense wildfire is severely limited by the lack of detailed knowledge of soils before wildfire. To date, only one study has reported effects of wildfire based on before and after soil sampling, but fire intensity was unknown (Murphy et al. 2006; Johnson et al. 2007). …
When the 2002 Biscuit fire serendipitously burned through part of a 150 ha Long-Term Ecosystem Productivity (LTEP) experiment (Bormann et al. 1994; Homann et al. 2008), we were given an opportunity to examine soil changes in paired pre- and post-wildfire samples. Because we had clear evidence that these plots had burned at high intensity, our objective was to determine the effects of high-intensity fire on the loss of soil C and N mass. Volumetric soil sampling, which has not been conducted in previous wildfire studies, allowed us to rigorously evaluate changes on a per-area basis. The parallel data from unburned stands provided important evidence on soil development that was not attributable to the fire. Having archived prefire samples also allowed us to reduce analytical biases. By combining these data, we provide an assessment of the potential effects of wildfire that is the most rigorous to date. Our assessment of soil physical and chemical changes provides unique information about the effects of high-intensity forest wildfire on soils and greenhouse-gas emissions, along with important implications for long-term productivity and future C sequestration. …
Results
The clearest effect of intense wildfire on our plots, which has also been widely noted across the Biscuit fire,2 was a substantial increase in the amount of near-surface rocks on the burned plots — there was 51 ± 8 M ha–1 (mean ± 95% CI) of loose rocks on the soil surface after the fire. (Fig. 5). …
The second notable effect of high-intensity fire was the major loss of soil organic matter at the soil surface that extended into the mineral soil, and corresponding losses of soil C and N. In the uppermost comparable layer 1 (the O horizon and mineral soil to 3.7 cm), soil C decreased by 19 ± 2 Mg ha–1 from the prefire sampling value in 1992 (Fig. 7 left side, Table 4 method 1). A small but significant (p <0.05) amount of C (2.5 Mg ha–1) was lost from the two deepest layers combined (4 and 5). If all C in the prefire O horizon (9 ± 1 Mg ha–1) was combusted, then 60% of the soil C loss came from mineral soil layers.
Soil N losses were also large, 547 ± 79 kg ha–1. No significant subsurface soil N losses were seen, but an increase of 40 ± 32 kg ha–1 was observed in layer 3. If all N in the prefire O horizon (226 ± 21 kg ha–1) was volatilized, then 57% of the soil N loss came from mineral soil layers. …
Discussion
Comparison to other studies
Our estimated loss of 23 Mg C ha–1 from organic and minerals soil layers is higher than most previous estimates. Our losses of 500 to 700 kg N ha–1 fell in the upper range of reported values. Comparing our results with others is challenging, however, given the variety of assumptions, sampling methods and depths, and analyses used. Uncertainties with these studies come from multiple sources and cloud our knowledge of the effects of intense fire on soils. Because of the lack of opportunities to directly measure soil changes before and after intense wildfire, researchers have had to rely on estimates obtained in retrospective studies or extrapolated from laboratory and lower-intensity prescribed fire studies. …
We cannot rule out a bias in retrospective studies because they assume that unburned areas can be used to represent the preburn conditions (Baird et al. 1999). Inherent differences between burned and unburned areas with respect to moisture, site conditions, and burn history can influence soil properties, as has been demonstrated for part of the Biscuit fire (Thompson et al. 2007). Different site histories confound interpretations and may lead to incorrect conclusions about soil dynamics (Yanai et al. 2003). …
Several mechanisms may explain the loose surface rocks after fire (Fig. 3): postfire erosion of fines, small-scale resorting of soil constituents, and atmospheric losses during the fire. Most of the soil organic matter in the O horizon was burned, and the products of combustion, including CO2 and volatilized nutrients, were exported as gas or smoke particles, leaving behind over seven times more rocks above the mineral soil surface. Losses of fine mineral soil from upper mineral soil layers are usually attributed to postfire waterdriven erosion, and our erosion-box estimates support this explanation to a point. Water-driven erosion for the 2003–2004 water year on burned soils in erosion boxes, placed across a range of slopes, averaged 57 m3 ha–1 compared with 0 m3 ha–1 on unburned soils. Below a 15 ha catchment with two burned LTEP stands, only a tiny fraction of the estimated 850 m3 of moving sediment (extrapolated from the erosion-box data) appeared in the ditches along logging roads. The export of the 127 Mg ha–1 of missing soil, estimated by the difference between pre- and post-fire soil sampling (Fig. 6) and extrapolated to this catchment, would be about 1900 m3 (assuming a sediment bulk density of 1 Mg m–3). The complex microtopography — partly created by windthrows, downed logs, tree trunks, and needles cast off after the fire — appeared to capture much of the moving soil. The fire also created soil voids — where decayed stumps and roots burned deeply into the soil — that filled over time. This vertical sorting mechanism does not appear to be responsible for the increase in the amount of surface rocks because we observed no corresponding drop in the
rock concentration at deeper soil layers (Fig. 5).
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). The possibility that a substantial mass of fine particles, including mineral soil, was transported high into the atmosphere raises questions about the effects of intense fire on radiation interception, water-droplet nuclei, and off-site terrestrial and ocean fertilization.
Implications of intense-fire-induced soil changes on climate, forest productivity, and management decisions
Many previous estimates of fire contributions to greenhouse gasses (e.g., Crutzen and Andreae 1990) are based on biomass combustion alone and fail to consider mineral soil losses. Although Campbell et al. (2007) considered soil C losses from the entire Biscuit fire, a concern about the lack of prefire soils data in their estimates is expressed in the range of their C-emission estimates, 0.7 to 1.2 Tg C for the portion of the fire with vegetation damage classes similar to those of our plots. If we extrapolate our results to this area of the Biscuit fire, the resulting soil C loss would be about 1.6 Tg and N loss about 45 Gg. Mineral soil (<4 mm) particulate losses (Fig. 6), extrapolated to the same area, sum to nearly 9 Tg.
Our soil C loss is greater than the high end of the estimates of Campbell et al. (2007); this discrepancy may be related to bias from their unburned controls or to our small sample of the Biscuit fire area. To the extent that our estimates might apply more broadly to other intense fires, climate models may need to be recalibrated to account for effects of intense fire, including fire-induced greenhouse gases and emissions of particulates.
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. …
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.
Much of the recent debate has centered on the effects of postwildfire 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 greenhousegas 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.
