Effects of Forest Management on Fire Behavior in the Southwest (adapted from Cram et al. 2006)

Abstract: Stand-replacement fires, particularly in ponderosa pine (Pinus ponderosa) forests, have replaced high-frequency, low-intensity historical fire regimes.  We examined whether forest stands treated recently using silvicultural practices would be (1) less susceptible to stand-replacing crownfires, and (2) more ecologically and functionally resilient compared to untreated stands following extreme wildland fire.  Reports detailing wildland fire behavior in treated stands remain largely anecdotal.  We compared fire severity indices, fireline intensity (btu/ft/s), stand characteristics including canopy bulk density (kg/m³), and post-fire recovery indices in silviculturally treated vs. untreated forest stands in New Mexico and Arizona.  Results indicated fire severity in pine-grassland forests was lowered when surface and aerial fuel loads were reduced.  Specifically, as density (stems/ac) and basal area (ft²/ac) decreased and mean tree diameter (in) increased, fire severity and fireline intensity decreased.  The more aggressive the treatment (i.e., where the canopy bulk density was reduced), the less susceptible forest stands were to crown fire.  However, mechanical treatments where slash was scattered rendered stands susceptible to near stand-replacement type damage when wildfire occurred within 4 years of treatment.  On our study sites, mechanical treatment followed by prescribed fire had the greatest impact toward mitigating fire severity (i.e., aerial and surface fuels were reduced).  Treated stands were also more ecologically and functionally resilient than untreated forest stands following wildland fire.

 

DISCUSSION

Fire Severity and Behavior

The fire behavior triangle states fuel, weather, and topography combine to determine fire behavior.  Research results indicated fire severity in middle elevation (about 6,400–9,100 ft) southwestern montane coniferous forests was lowered when the fuel leg (surface and aerial fuels) of the fire behavior triangle was reduced by silvicultural activities.  In particular, we observed that mechanical treatment followed by prescribed fire (including pile burning) had the greatest influence toward mitigating fire severity.  Specifically, as density and basal area decreased and mean tree diameter increased, fire severity decreased. 

Canopy bulk density (CBD) is known to be a limiting factor affecting crownfire initiation and propagation (Van Wagner 1977; Rothermel 1991).  An upper threshold in CBD (> 0.047 kg/m³, as calculated using FFE-FVS) was observed on mechanically treated stands that included broadcast fire or pile burning (slope was 0–10 %).  Two lop and scatter stands had CBDs below 0.046 kg/m³, and although they did not exhibit crownfire or torching, crowns were completely scorched.  A third lop and scatter stand with a CBD calculated at 0.084 kg/m³ experienced torching.  One untreated stand had a calculated CBD of 0.052 kg/m³.  This particular stand experienced a combination of crown scorch, torching, and in some cases crowning.  The remaining untreated stands had CBDs > 0.075 kg/m³ and all experienced near 100% crowning.  The idea of empirically derived CBD thresholds limiting or changing crownfire behavior is not new (Agee 1996).  Following a 1994 wildfire in the Wenatchee National Forest, Washington, CBDs from multiple sites were calculated and related to fire behavior; a CBD threshold of 0.10 kg/m³ (as calculated following Agee 1996) was reported with crownfire activity likely above the threshold and not below (Agee 1996).  Increased empirical data in combination with current theoretical modeling is necessary before specific threshold CBDs can be recommended as targets for use in fuel reduction planning and risk assessment.  Further, it is important to remember crownfire potential is not dependent on any one element of the fire behavior triangle, but rather from multiple combinations of fuel, weather, and topography (Scott and Reinhardt 2001).

  Silvicultural cutting prescriptions designed to reduce stand susceptibility to crownfire must consider resulting surface fuels following slash treatment, residual tree and stand characteristics, and slope and aspect.  Under extreme conditions created by drought, high winds, and suitable topographical conditions, we observed treated forest stands that, although suffering less severe crown and surface damage than adjacent untreated stands, were still subjected to near stand replacement type damage.  This was particularly evident on lop and scatter treatments completed 3–4 years before the Rodeo-Chediski fire.  However, this illustrates that even under extreme conditions fire severity can be mitigated by surface and aerial fuel reduction.  Furthermore, more recent and aggressive silvicultural treatments including prescribed fire may likely have resulted in still less surface and crown damage.  Silvicultural treatments can only be expected to change fire behavior within the limits of their prescription (Finney and Cohen 2003).  Spatial location and arrangement of fuel reduction treatments in relation to other treatments must also be considered if landscape scale fire hazard reduction is an objective (Finney 2001, 2003).

Ecological Implications

The ecological implications of different fire severities on natural processes are extensive and complex: wildlife behavior (Smith 2000), wildlife habitat (Smith 2000; Brown and Smith 2000), carbon release (Thornley and Cannell 2004), and watershed response (Campbell and others 1977).  These are just a few of many complex issues potentially affected by differing fire regimes, frequencies, and intensities.  As such, the following discussion seeks to stimulate thoughts about ecosystem responses as well as highlight one basic ecological implication (understory plant cover) above and below which rest many more.

Greater understory cover particularly that of grasses, in combination with less bare soil cover, suggests a difference in the relative ecological recovery between treated and untreated forest stands up to four years following wildland fire.  Further, because silviculturally treated stands experienced less severe fire damage and subsequently less loss of litter and herbaceous loading, these stands were less susceptible to soil loss and more conducive to residual plant growth and recovery.  This suggested difference in ecological condition may best be illustrated by the continued high percent of bare soil cover in untreated stands up to four years following wildland fire.  Extreme fireline intensities and long residual fire times can cause soil damage leading to loss of nutrient stores (Neary and others 1996), potential loss of viable seed (Miller 2000), change in microclimates (Raison 1979), and altered hydrologic soil behavior leading to rapid erosion events (DeBano and others 1996; Ice and others 2004).  This type of soil damage was most pronounced in the untreated study sites.

Management Implications

Functioning watersheds in forested landscapes are vital for flood and erosion control and to the sustainability of water supplies essential for stable societal operation.  Evidence of impaired watersheds in terms of erosion and sedimentation immediately following extreme wildfire events are obvious.  Natural resource managers of all perspectives (private, city, state, or federal) with stewardship charges for diverse landscapes and watersheds are increasingly concerned with minimizing the risk of large-scale crownfires in these systems.  Whereas in the past multiple resources such as wildlife habitat, wood products, and range condition were managed at the stand level, increasing pressure, particularly at the federal level, is being exerted to manage these ecological functions and renewable resources at the landscape level under the umbrella of sustainable watersheds.  Past and present research results suggest mechanical aerial fuel reduction (i.e., reduced canopy bulk density) followed by frequent prescribed fire is well suited as a management tool to restore and sustain entire watersheds and their ecological functions, particularly in pine-grassland forests.  Anywhere the fire has to drop to the surface is an area where some trees will survive and some fuel breaks and firefighters will stand a chance.  Not to be overlooked, the extra investment (i.e., prescribed fire and pile burning) required to reduce potential slash fuels and years of accumulated dead and down surface fuels is particularly important for maximizing desired watershed functions, and is evident when comparing lop and scatter vs. lop, pile, burn treatments or shelterwood or commercial harvest vs. harvest and burn treatments.  Although specific target prescriptions for density, diameter distribution, and basal area will depend on interactions with other management objectives (for example, stand regeneration strategies), as a general rule one can expect an inverse relationship between the degree of fuel reduction and the likelihood of crownfire initiation and propagation (within the limits of fuel moisture and wind as dictated by weather).  However, beyond recognition and agreement of a specific basal area, diameter, or density treatment, a considerable challenge remains in terms of landscape implementation.  Recent theoretical research and post fire analysis indicated that random placement of aerial fuel reduction treatments do little on the landscape scale to slow the rate of spread or change the overall behavior of a crownfire (Finney 2001, 2003; Graham 2003).  Spatial arrangement of fuel treatments or restoration prescriptions must be scientifically considered.

                In addition to reducing the threat of large-acreage crownfires across backcountry watersheds, wildland-urban interface zones and their respective watersheds must be considered as well in fire hazard planning.  Wildland-urban interface stakeholders have reason to thoughtfully consider the implications of this study when bearing in mind how to reduce the threat of high-intensity wildland fire within the interface.  Foremost within this group includes urban and rural community leaders and planners, land management agency personnel, as well as citizens and homeowners living within wildland-urban interface areas.  Within the wildland-urban interface where the priority is elimination of crownfire potential and reduced fire severity, specific prescriptions should consider (1) aggressive reduction in stem density and basal area while allowing for increased mean tree diameters, (2) reduced canopy bulk density and canopy continuity (i.e., via spatial arrangement of trees and their respective crowns), and (3) aggressive reduction of fine surface fuels.  Significantly, these treatments also serve numerous other benefits beyond simply reducing stand replacement fire potential.

Because of the integral role of reducing surface fuels in relation to changing fire behavior, managers should be careful not to mislead stakeholders that simply thinning trees without regard to detail will result in reduced fire behavior to a manageable level.  For example, as seen in the lop and scatter treatment, the simple rearrangement of the fuel loading (i.e., from aerial to surface fuels) technically reduced fire severity and intensity, but ecologically the end result was still complete stand mortality.  Within the interface, both aerial and surface fuel conditions must be addressed if the threat of crownfire danger is to be reduced, and fine surface fuels must constantly be maintained at manageable levels, particularly during the high-risk seasons.  Depending on site specific circumstances, frequent prescribed fire can often be an effective tool to keep surface fuel loads at a minimum. 

Strict adherence to reduced surface fuels within the scope of the wildland-urban interface should not be confused with the goals and objectives of managing watersheds at the landscape scale.  For example, following the recommendation by Cohen (2000) for a 131-ft fuel buffer surrounding dwellings as a proactive and effective approach to reducing risk of home loss in the face of an approaching canopy fire, some have naively questioned the need for further forest thinning and management.  Arguably, water supplies vital for urban and rural consumption are the most important of many natural resources directly tied to forested watersheds that without proactive rehabilitation management following nearly a century of fuel accumulation are at prime risk for extensive degradation in the event of a large-scale crownfire (Ice and others 2004; Kaufmann and others 1987).  Parallel arguments for proactive landscape management in the imminent path of wildfire can be made for habitat diversity (Waltz and others 2003), forage and rangeland condition (Scotter 1980), recreational use, riparian function (Rinne and Neary 1996), loss of carbon sinks (Kasischke and others 1995), wildfire costs (Lynch 2004), and so on.  Further, islands of structural survival in the midst of complete landscape consumption following wildfire are a short-term victory few would advertise as a successful means to an end.  The long-term setback in terms of lost natural resources outweighs short- term gains in structural protection, particularly when both are attainable via the same ideology, i.e., proactive management.

The objective of fuel reduction in the wildland-urban interface or within a watershed cannot be to “fireproof” the environment, but rather to reduce the likelihood of stand-replacement crownfire, i.e., change fire behavior.  In fact, it was attempts at fire proofing Western coniferous forests that largely led to the unsustainable conditions of today’s forest.  Furthermore, when forest canopies are opened up via mechanical means, fine understory fuels can be expected to increase.  The silver lining in increased fine surface fuels is the improved potential and efficiency to use back-burns ahead of a wildland-head fire, not to mention a key ecological role in the symbiotic relationship between fire and pine forests.  Backfires burning through fine surface fuels are more effective and efficient in burning out understory fuels as compared to a closed canopy forest with a deep, but compacted, litter understory.  Estimates of fireline intensity indicated that hand and dozer lines would have been effective containment techniques in treated stands.

The FFE-FVS is another useful tool that may be helpful to land managers and planners in the wildland-urban interface, particularly U.S. Forest Service personnel familiar with its capabilities.  FFE-FVS could be used to estimate existing (i.e., pre or post treatment) and future CBDs and thus provide insight on crownfire potential as well as how often aerial fuels will need to be treated to keep CBDs below crownfire hazard thresholds.  It is important to emphasize that CBD alone, without explicit knowledge of surface fuels, is unsuitable as an index to crownfire hazard because of the critical role surface fuels play in fire behavior (Van Wagner 1977).

                Implications of this study include increased public understanding of the ecological condition of today’s southwestern forests and the potential for long-term damage following crownfire, but more importantly the knowledge that proactive management can be successful in reducing crownfire potential and maintaining ecologically sustainability.  From the land manager’s perspective, published research data can be used to support proactive management in terms of public relations.  In addition, results can help managers assess the likelihood of a crownfire event in a specific stand, to understand how wildland fire behaves in treated vs. untreated stands, to understand how treated vs. untreated stands will respond following fire, and to understand what specific types of silvicultural treatments will best mitigate damage.

 

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