Crown Fire Occurrence Research Articles

Modeling Juvenile Stand Development and Fire Risk of Post-Fire Planted Forests under Variations in Thinning and Fuel Treatments Using FVS–FFE

In the past, the dry mixed conifer forests of California’s Sierra Nevada mountains experienced frequent low to mixed severity fires. However, due to fire suppression and past management, forest structure has changed, and the new fire regimes are characterized by large, high severity fires which kill a majority of the overstory trees. These new disturbance patterns require novel approaches to regenerate the forest as they are not adapted to large, high severity fires. We forecasted growth and fire behavior of young plantations for 100 years into the future using the Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE). In these simulations, we tested combinations of different fuel treatments (mastication only, mastication with prescribed burning, and no fuels treatments) with different overstory thinning intensities (residual densities of 370 SDI (stand density index), 495 SDI, 618 SDI (TPH), and no overstory thinning) on stand growth and potential fire behavior using analysis of variance. We compared growth and crowning index at the end of the simulation and the simulation age when the flame length, basal area mortality, and fire type reached low severity between fuel treatment, thinning intensity, and original management of stands (plantation with PCT [precommercial thinning], plantation without PCT, and natural regenerating stands). These comparisons are essential to identify which fuel treatment categories reduce fire risk. We found an overall pattern of decreasing crown fire occurrence and fire induced mortality across all simulations due to increasing canopy base height and decreasing canopy bulk density. In particular, stands with mastication and prescribed burning transitioned from crown fire types to surface fires 10 years earlier compared to mastication only or no fuel treatment. Furthermore, pre-commercially thinned stands transitioned from crown fire states to surface fires 10 years earlier in the simulations compared to un-thinned and naturally regenerating stands. Stands with mastication and burning went below 25% reference threshold of basal area mortality 11 and 17 years earlier before the mastication only and no fuel treatment, respectively. In addition, pre-commercially thinned stands went below 25% basal area mortality 9 and 5 years earlier in the simulation compared to un-thinned or naturally regenerated stands, respectively. Mastication with prescribed burning (MB) was the most effective treatment for quickly reducing fire behavior by consuming surface fuels, thus drastically lowing flame length (e.g., surface flame length of MB was 0.6 m compared to mastication only [1.3 m] and no treatment [1.4 m]). Furthermore, intensive thinning reduced risk of active crown fires spreading through the stand. Prioritizing prescribed burning, when possible, and thinning (both pre-commercially and from below) are the most effective ways to quickly improve fire resistance in mixed conifer plantations. Our results highlight the different stressors that post-fire planted forests experience and how different silvicultural treatments interact over time to reduce fire risk, which demonstrates the importance of treating stands early and the effectiveness of surface fuel treatments.

Improved logistic models of crown fire probability in Canadian conifer forests

Background Crown fires are an ecologically necessary but hazardous process in conifer forests. Prediction of their behaviour in Canada has largely depended on the Canadian Forest Fire Behaviour Prediction System, in which fire weather indices drive primarily fixed fuel type models. The Crown Fire Initiation and Spread (CFIS) system presents a more flexible approach to predicting crown fire occurrence than fixed fuel type models. Aims Using a multi-decadal database of experimental fires carried out in conifer plots (1960–2019, n = 113), our aim was to develop updated models based on the CFIS system approach, fitting crown fire occurrence models to fire environment variables using logistic regression. Methods We tested alternative fuel moisture estimates and compared various model forms using repeated cross-validation. In two-storeyed stands, crown fire occurrence was defined as the involvement of lower canopy stratum fuels. Key results Final models based on wind speed, fuel strata gap, litter moisture and surface fuel consumption predicted crowning events correctly in up to 92% of cases in training data (89% in cross-validation). Conclusions and implications These new models offer improved accuracy and flexibility that will help users assess how competing environmental factors interact under different fuel treatments and wildfire scenarios.

An artificial intelligence framework for predicting fire spread sustainability in semiarid shrublands

Background Fire behaviour simulation and prediction play a key role in supporting wildfire management and suppression activities. Aims Using machine-learning methods, the aim of this study was to predict the onset of fire propagation (go vs no-go) and type of fire behaviour (surface vs crown fire) in southern Australian semiarid shrublands. Methods Several machine-learning (ML) approaches were tested, including Support Vector Machine, Multinomial Naive Bayes and Multilayered Neural Networks, as was the use of augmented datasets developed with Generative Adversarial Networks (GAN) in classification of fire type. Key results Support Vector Machine was determined as the optimum machine learning classifier based on model overall accuracy against an independent evaluation dataset. This classifier correctly predicted fire spread sustainability and active crown fire propagation in 70 and 79% of the cases, respectively. The application of synthetically generated datasets in the Support Vector Machine model fitting process resulted in an improvement of model accuracy by 20% for the fire sustainability classification and 4% for the crown fire occurrence. Conclusions The selected ML modelling approach was shown to produce better results than logistic regression models when tested on independent datasets. Implications Artificial intelligence frameworks have a role in the development of predictive models of fire behaviour.

Assessing the effects of alternative fuel treatments to reduce wildfire exposure

Effective landscape-scale fuel management strategies are essential for reducing wildfire risk in Mediterranean fire-prone areas. In this study, the minimum travel time (MTT) fire-spread algorithm as implemented in FlamMap was applied to assess the potential of alternative fuel treatments for lowering wildfire losses in a 5,740-ha study area in eastern Sardinia, Italy. Twenty-seven wildfires at 10-m resolution were simulated considering three wind speeds (15, 18, and 21 km h−1) to compare fuel treatments: no treatment (NT), irrigated agroforestry areas with shrub clearing (T1), prescribed fire in eucalyptus stands (T2), and irrigated grasslands (T3). The simulations replicated a recent large wildfire that occurred in the study area (Orrì wildfire, 2019) and considered the weather and fuel moisture conditions associated with this event. The average wildfire exposure outputs (burned area, probability of burning, conditional flame length, potential crown fire occurrence, and surfaces withflame lengths above 2.5 m) decreased after fuel treatments, compared to no treatment. T1 was the most effective strategy in mitigating wildfire hazards and provided the most significant performance for several wildfire exposure indicators. Treating only 0.5% of the study area (~ 30 ha) resulted in a decrease in all wildfire exposure metrics to ~ 10% within the study area. In addition, the total surface characterized by high flame length (average > 2.5 m) was the lowest in the T1 treatment. This study can help land and fire managers optimize fuel treatment opportunities and wildfire risk mitigation strategies in Mediterranean areas.

Targeted grazing and mechanical thinning enhance forest stand resilience under a narrow range of wildfire scenarios

AbstractIncreasing wildfire activity has spurred ecological resilience‐based management that aims to reduce the vulnerability of forest stands to wildfire by reducing the probability of crown fire. Targeted grazing is increasingly being used to build forest resilience to wildfire, either on its own or in combination with treatments such as mechanical thinning; however, it is unclear how effective this method is at altering the probability of crown fire in forest stands. We use crown fire simulation models to quantify to what extent targeted grazing, mechanical thinning targeting the vertical fuel stratum, and a combination of both treatments alter eastern ponderosa pine savanna stand resilience to wildfire by modeling their relative impacts on the fuel stratum gap and subsequent crown fire occurrence under six different wildfire risk scenarios generated by altering wind and fuel moisture conditions. We then model changes in the probability of crown fire occurrence resulting from treatments across 75 field‐sampled sites in the Pine Ridge region of Nebraska relative to predicted crown fire occurrence when sites are left untreated. We find that mechanical (vertical) thinning has the potential to alter the probability of crown fire in ponderosa pine stands to a much greater extent than targeted grazing. Combining both approaches had a slightly higher probability of reducing crown fire risk across the greatest range of wildfire risk scenarios. Across 75 sample sites, targeted grazing was only predicted to prevent crown fire occurrence at two sites expected to experience crown fire under observed stand conditions across all six of our wildfire risk scenarios. In contrast, targeted grazing combined with mechanical thinning was predicted to prevent crown fire at approximately half of the sites expected to experience crown fire under observed conditions for mild and moderate wildfire risk scenarios. Thus, targeted grazing should be combined with mechanical thinning to best enhance forest resilience to wildfire. No combination of targeted grazing or mechanical thinning was able to alter the probability of crown fire under wildfire risk scenarios most conducive to wildfire, confirming that relying solely on vertical thinning and targeted grazing is unlikely to sufficiently enhance resilience of forest stands to future wildfire conditions.

Historical and current fire regimes in ponderosa pine forests at Zion National Park, Utah: Restoration of pattern and process after a century of fire exclusion

Plant community resilience in frequent-fire forests of western North America has been compromised by over a century of fire exclusion. However, there are several western National Parks where fires have been reintroduced over the past several decades, including Zion National Park in southwest Utah. Here we reconstruct historical components of fire regimes in ponderosa pine dominated forests at Zion and compare these to recent fire frequency, current plant community and fuels structure, and potential fire behavior. Historical fires burned every 9–10 years on average up until 1879, when fires ceased contemporaneous with introduction of Euro-American livestock grazing and timber harvest in upland forests. Abundant tree regeneration occurred after fire exclusion, with tree density averaging 45 trees ha−1 in reconstructed 1880 forests versus 106 trees ha−1 today. Intervals between recent (since 1988) wildfires and prescribed fires in these same stands ranged from 7 to 13 years, similar to historical fire timing. Depending on whether plots had burned from zero to three times in recent fires, we found significant differences in canopy base heights (increased), duff and litter depths (decreased), and percent cover of grass and forbs (increased), but not tree density, tree basal area, shrub height, shrub cover, or woody fuels. Combined effects of recent fires on overstory and understory structure resulted in a significant difference in likelihood of crown fire occurrence, declining from a mean of 58% in plots with no fire since 1879 to 13% in plots with three fires since 1988. Significant effects were generally seen after two or three fires, suggesting it is the reintroduction of the fire regime and not just individual fire events that restore resiliency. Overall, effects of recent fires are building on the latent resiliency of ponderosa pine forests at Zion National Park, although questions remain about extent and future dynamics of oak and manzanita shrubfields that occupy similar environmental settings, along with a general lack of ponderosa pine regeneration across all plots.

Spruce Budworm (Choristoneura fumiferana Clem.) Defoliation Promotes Vertical Fuel Continuity in Ontario’s Boreal Mixedwood Forest

Spruce budworm, Choristoneura fumiferana (Clem.), defoliation has been shown to affect the occurrence of crown fire in Ontario, highlighting the need to better understand the driving factors of this effect on forest structure, including changes in fuel loading, type and position. Here, we investigate five boreal mixedwood sites within four zones that experienced different durations of continuous defoliation by spruce budworm in northeastern Ontario. Duration of defoliation had significant effects on vertical stand components, namely, host overstory to host understory crown overlap, host overstory and host understory crown to downed woody debris overlap, and downed woody debris height and quantity. Vertical stand components tended to increase with the duration of continuous defoliation, with the highest vertical fuel continuity occurring after 16 years of continuous defoliation. Such increases in the vertical spatial continuity of fuels may be a key reason for the greater percentage of area burned in those forests which have recently sustained a spruce budworm outbreak.

Potential climate change impacts on fire intensity and key wildfire suppression thresholds in Canada

Much research has been carried out on the potential impacts of climate change on forest fire activity in the boreal forest. Indeed, there is a general consensus that, while change will vary regionally across the vast extent of the boreal, in general the fire environment will become more conducive to fire. Land management agencies must consider ways to adapt to these new conditions. This paper examines the impact of that changed fire environment on overall wildfire suppression capability. We use multiple General Circulation Models and carbon emission pathways to generate future fire environment scenarios for Canada’s forested region. We then use these scenarios with the Canadian Forest Fire Behaviour Prediction System and spatial coverages of the current forest fuel composition across the landscape to examine potential variation in key fire behaviour outputs that influence whether fire management resources can effectively suppress fire. Specifically, we evaluate how the potential for crown fire occurrence and active growth of fires changes with the changing climate. We also examine future fire behaviour through the lens of operational fire intensity thresholds used to guide decisions about resources effectiveness. Results indicate that the proportion of days in fire seasons with the potential for unmanageable fire will increase across Canada’s forest, more than doubling in some regions in northern and eastern boreal forest.

Utilization of remote sensing techniques for the quantification of fire behavior in two pine stands

Quantification of field-scale fire behavior is necessary to improve the current scientific understanding of wildland fires and to develop and test relevant, physics-based models. In particular, detailed descriptions of individual fires are required, for which the available literature is limited. In this work, two such field-scale experiments, carried out in pine stands under mild conditions, are presented. A particular focus was placed on non-intrusive measurement, as the capabilities of advanced remote sensing techniques, along with more traditional approaches, are explored. A description of the fires is presented, with spread occurring predominantly in the surface fuels with intensities in the range of 200–4400kWm-1, and punctuated by isolated regions of crown fire. The occurrence of crown fire is investigated and linked to regions of greater canopy density, and it is found that the total fire intensity may increase locally to as much as 21,000kWm-1. The light winds do not appear to play a direct role in the changes in fire behavior, while fuel structure may be important. The measurements described herein provided a reasonable overall description of the fires, however, the current resolution (both spatial and temporal) falls short of definitively explaining some transitional aspects of the fire behavior, and future improvements are suggested.

The role of weather, past fire and topography in crown fire occurrence in eastern Australia

We analysed the influence of weather, time since fire (TSF) and topography on the occurrence of crown fire, as mapped from satellite imagery, in 23 of the largest wildfires in dry sclerophyll forests in eastern Australia from 2002 to 2013. Fires were analysed both individually and as groups. Fire weather was the most important predictor of crown consumption. TSF (a surrogate for fuel accumulation) had complex nonlinear effects that varied among fires. Crown fire likelihood was low up to 4 years post-fire, peaked at ~10 years post-fire and then declined. There was no clear indication that recent burning became more or less effective as fire weather became more severe. Steeper slope reduced crown fire likelihood, contrary to the assumptions of common fire behaviour equations. More exposed areas (ridges and plains) had higher crown fire likelihood. Our results suggest prescribed burning to maintain an average of 10 years’ TSF may actually increase crown fire likelihood, but burning much more frequently can be effective for risk reduction. Our results also suggest the effects of weather, TSF and slope are not adequately represented in the underlying equations of most fire behaviour models, potentially leading to poor prediction of fire spread and risk.

Predicted Crown Fire Risk Adds Incentive to Restore Open-Canopy Pine Barrens at the Wildland-Urban Interface

Thinning forest to restore fire-dependent, open-canopy pine barrens is important for promoting shrubland and young forest biodiversity and wildlife habitat and to mitigate chances for extreme fire behavior. Crown fire potential at the wildland-urban interface is a concern in northeastern (USA) pine barrens which contain extremely volatile fuels adjacent to densely populated areas. We used the Crown Fire Initiation and Spread simulator to model pitch pine (Pinus rigida) crown fire potential under simulated stand density reductions to help develop strategies for long-term management and conservation of pine barrens communities. Empirical data for model inputs were collected from pitch pine stands at the Albany Pine Bush (New York State, USA). Canopy structure, surface fuel moisture, and wind speed were used to predict crown fire occurrence, type (active vs. passive), and rate-of-spread. At winds > 20 km hr−1 and ≤ 9% fine fuel moisture, passive crown fire probabilities exceed 60%, and one stand near sensitive human infrastructure appears vulnerable to active crowning. Simulations suggest thinning to < 50 trees ha−1, 10–30% mean canopy cover, or about 5–10 m2 ha−1 basal area to minimize crown fire threat and restore ecologically high-quality pine barrens.

Landscape Scale Influences of Forest Area and Housing Density on House Loss in the 2009 Victorian Bushfires

Previous investigations into the factors associated with house loss in wildfires have focused on the house construction and its immediate environment (e.g. gardens). Here, we examine how nearby native forest and other houses can influence house loss. Specifically, we used a sample of 3500 houses affected by the Victorian bushfires of February 7th 2009 to explore how the amount of forest, proportion of forest burned by crown fire and the number of nearby houses affected house loss and how far from the house this influence was exerted. These fires were the most destructive in Australian history and so represent the extreme of fire risk. Using generalized linear modeling we found that the probability of house loss increased with forest extent and the proportion burnt by crown fire and this relationship was strongest for forest measured 1 km from the houses. Houses were more likely to be destroyed if there were other houses within 50 m and if they were on a slope. A model containing these variables predicted house loss with 72% accuracy. Our findings have three important implications: i) management to change the occurrence of crown fire will be effective in reducing house loss; ii) this management may be required up to 1 km away from houses in some situations (a much larger zone than is currently used); iii) high density of houses may increase risk of loss. Given the potentially large width of this management zone and the hazard from nearby houses, it may be more sensible to concentrate on modification of buildings to reduce their vulnerability.

Canopy fuel characteristics in relation to crown fire potential in pine stands: analysis, modelling and classification

Crown fire occurrence and subsequent crown fire behaviour are strongly dependent on canopy fuel characteristics, especially canopy fuel load (CFL), canopy bulk density (CBD) and canopy base height (CBH). Therefore, quantification of such variables is required for the appropriate selection of silvicultural treatments aimed at reducing susceptibility to crown fire. Data from the IV Spanish National Forest Inventory and individual tree biomass dry weight equations were used to estimate the canopy fuel characteristics of four representative types of pine stands in north-western Spain. Probability of crown fire initiation and crown fire rate of spread were simulated by using the mean surface fuel load observed for each type of pine in this area and assuming different burning conditions. The results indicate that a 22.13 % of the sample plots analysed showed a rather high potential for active crown fire spread under moderate burning conditions, and this value increases to 69.27 % under extreme burning conditions. Equations relating the canopy fuel characteristics to common stand variables (stand density, basal area and dominant height) were fitted simultaneously for each pine, and weighting factors for heteroscedasticity were included. The models explained more than 93.90, 74.70 and 69.42 % of the observed variability in CFL, CBD and CBH, respectively. Basal area was the most important variable for estimating CFL and CBD while dominant height explained most of the observed variability in CBH. The use of the fitted equations together with existing dynamic growth models and fire management decision support systems will enable assessment of the crown fire potential associated with different silvicultural alternatives used in these types of pine stands.

Climatic variability regulates the occurrence and extent of large fires in the subalpine forests of the Canadian Rockies

Drought occurrence and wildfire activity in subalpine forests in Kootenay National Park (KNP) of the Canadian Rockies are studied by focusing on the interannual and multi-decadal variations in climate patterns of the Pacific Ocean. The question addressed is whether broad-scale climate patterns can regulate both large fire occurrence and fire-free periods causing fuel buildups. This study compared years of large wildfire outbreaks for the subalpine forests of KNP during the last three centuries using indices of climate patterns of the Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO). PDO and ENSO were correlated with drought and fire occurrence in KNP. A positive PDO–positive ENSO combination created extreme drought conducive to crown fires, indicating that the occurrence of high-severity wildfires in these generally moist/cool forests is strongly determined by climatic anomalies. Large fire activity is chiefly modulated by PDO compared to ENSO, because a negative PDO phase greatly decreased fire activity in the mid-twentieth century. Although this fire-free period is seemingly matched with a fire-suppression period, it may be attributable to a negative PDO, which increased precipitation in the region. This mid-century fire gap contributed to the accumulation of old forests serving as loaded fuels within the landscape and ultimately led to occurrences of crown fires as the PDO shifted to the positive phase. Thus, in addition to the fundamental importance of interannual variations in the Pacific that initiates a current-year severe drought, multi-decadal scale climate variability also influences the extent and severity of subsequent fires by modulating pre-fire landscape conditions. For fire management in subalpine areas of the Rockies, although drought occurrence is a primary concern and there are still uncertainties in the detailed changes in fire risk through the successional process, it is worth paying attention to fuel-loads of older forests in the landscape, which may lead to extensive large fires. Because the climatic teleconnection pattern is one of the main drivers of crown fire occurrence in the region in terms of creating current summer drought, and also for constructing a landscape structure prone to larger fires, more of a focus on short to long-term variations in the climate for wildfire management is needed in high-elevation forested landscapes.

Effect of Pinyon–Juniper Tree Cover on the Soil Seed Bank

As pinyon–juniper (specifically, Pinus monophylla and Juniperus osteosperma) woodlands in the western United States increase in distribution and density, understory growth declines and the occurrence of crown fires increases, leaving mountainsides open to both soil erosion and invasion by exotic species. We examined if the loss in understory cover that occurred with increasing tree cover was reflected in the density and diversity of the seed bank. Seed banks in stands with low, medium, and high tree cover were measured in late October for 2 yr. Multivariate analyses indicated that cover and diversity of standing vegetation changed as tree cover increased. However, the seed bank did not differ in overall seed density or species diversity because seeds of the 13 species that comprised 86% of the seed bank occurred in similar density across the tree-cover groups. Sixty-three percent of the species that were in the seed bank were absent from the vegetation (mostly annual forbs). In addition, 49% of the species that occurred in the standing vegetation were not in the seed bank (mostly perennial forbs and shrubs). Only Artemisia tridentata, Bromus tectorum, and Collinsia parviflora displayed positive Spearman rank correlations between percent cover in the vegetation and density in the seed bank. Thus, much of the standing vegetation was not represented in the seed bank, and the few species that dominated the seed bank occurred across varying covers of pinyon–juniper. Conforme los bosques de “Pinyon–juniper” (específicamente, Pinus monophylla Torr. & Frém. y Juniperus osteosperma (Torr.) Little) del oeste de Estados Unidos de América se incrementan en distribución y densidad, el crecimiento del estrato herbáceo disminuye y la ocurrencia de incendios aumenta, dejando las laderas de las montañas expuestas a la erosión del suelo y la invasión de especies exóticas. Examinamos si la pérdida de cobertura herbácea, que ocurre al aumentar la cobertura de los árboles, se reflejó en la densidad y diversidad del banco de semillas del suelo. Durante 2 años, a finales de octubre, se midió el banco de semillas en poblaciones con cobertura de árboles baja, media, y alta. Los análisis multivariados indicaron que la cobertura y diversidad de la población vegetal cambió al incrementarse la cobertura de árboles. Sin embargo, el banco de semillas del suelo no difirió en la densidad y diversidad general de especies, porque las semillas de las 13 especies que constituyeron el 86% del banco de semillas, ocurrieron en densidad similar en los tres niveles de cobertura de árboles. El 63% de las especies que formaban el banco de semilla no estaban presentes en la vegetación (principalmente hierbas anuales). Además, 49% de les especies presentes en la vegetación no estaban en el banco de semilla (principalmente hierbas perennes y arbustos). Solo Artemisia tridentata Nutt., Bromus tectorum L. y Collinsia parviflora Lindl. Mostraron correlaciones de rango Spearman positivas entre el porcentaje de cobertura en la vegetación y la densidad en el banco de semilla. Por lo tanto, buena parte de la vegetación no estuvo representada en el banco de semillas, y las pocas especies que dominaron el banco de semillas ocurrieron en los diferentes niveles de cobertura de “Pinyon–juniper”.

Cruz, M.G., M.E. Alexander, and R.H Wakimoto. 2004. Modeling the likelihood of crown fire occurrence in conifer forest stands. For. Sci. 50(5):640–658.

Cruz, M.G., M.E. Alexander, and R.H Wakimoto. 2004. Modeling the likelihood of crown fire occurrence in conifer forest stands. For. Sci. 50(5):640–658.

Canopy Fuel Characteristics and Fuel Load in Young Black Pine Trees

ABSTRACTCrown structure and fuel loading are important factors affecting crown fire occurrence and behavior. This paper reports on the canopy fuel characteristics and regression models to determine Canopy Fuel Loading (CFL) of young black pine (Pinus nigra Arnold) trees. Equations were based on the data from 11 destructively sampled black pine plantation trees. This data was used to develop best equations fit with ordinary least squares procedures that estimate available and total canopy fuel load for young black pine plantation trees. Model fits for available and total canopy fuel load were generally good. Results indicated that available and total canopy fuel load could be easily predicted using diameter at breast height (D), variable. The resulting equations were able to account for 94% of the observed variation in the total canopy fuel load. Average tree canopy fuel load was 8.9 kg. On average 41% of canopy fuel load was allocated to the needle, 13% to the fine branch, 20% to the medium branch, 18% to thick branch and 8% to the very thick branch. Vertical distribution of canopy fuels was variable. Most foliage was found on the main branches of the medium portion of the canopy. The lower and upper portions of the crown had relatively less foliage. The results of the study allowed also for the comparisons of linear and nonlinear equations of biomass estimation commonly used in literature.

Assessing the Effectiveness of Landscape Fuel Treatments on Fire Growth and Behavior

Abstract This article presents a methodology for assessing the effectiveness of landscape fuel treatments on fire growth and behavior in southern Utah. Treatment areas were selected by fire managers from the Bureau of Land Management (BLM) based on the threat of fire to communities and the need for range and wildlife improvement. A fire density grid was derived from the BLM's fire start layer to identify historically high ignition areas. FireFamily Plus was used to summarize and analyze historical weather and calculate seasonal severity and percentile reports. Information from FireFamily was used in FARSITE and FlamMap to model pre- and posttreatment effects on fire growth, spotting, fireline intensity, surface flame length, and the occurrence of crown fire. This procedure provides managers with a quantitative measure of treatment effectiveness as well as spatial output that can be used for analyzing fuel treatment effectiveness, burn plan development, National Environmental Policy Act (NEPA) documentation, public education, etc.

Modeling the Likelihood of Crown Fire Occurrence in Conifer Forest Stands

Abstract The unknowns in wildland fire phenomenology lead to a simplified empirical model approach for predicting the onset of crown fires in live coniferous forests on level terrain. Model parameterization is based on a data set (n = 71) generated from conducting outdoor experimentalfires covering a significant portion of the spectrum of burning conditions associated with the initiation of crown fires. A logistic model is developed to predict the likelihood of crown fire occurrence based on three fire environment variables, namely the 10-m open wind speed, fuel stratagap (equivalent to live crown base height in some stands), estimated moisture content of fine dead fuels, and one fire-behavior descriptor–surface fuel consumption. The model correctly predicts 85% of the cases in the data set used in its development, and the receiver operatingcharacteristic statistic is 0.94. The model is evaluated for its sensitivity to its inputs, and its behavior is compared with other models used in decision support systems to operationally predict crown fire initiation. The results of a limited test of the model against two independent experimentalfire data sets for distinctly different fuel complexes is encouraging. FOR. SCI. 50(5):640–658.

DISTURBANCE AND STAND DYNAMICS IN PONDEROSA PINE FORESTS IN ROCKY MOUNTAIN NATIONAL PARK, USA

Fire is thought to be the dominant disturbance agent in pure ponderosa pine (Pinus ponderosa Laws.) forests, but fire severity and disturbances other than fire, and the effects of these events over time, have rarely been analyzed. We first created systematic criteria to (1) identify the causes of tree regeneration and mortality events and (2) classify the severity of these events. These criteria were then applied to understand the effect of events on ponderosa pine forests in Rocky Mountain National Park (RMNP), Colorado.For each of nine intensive study plots sampled in the pure ponderosa pine zone of RMNP, we mapped and dated live and dead trees and the spatial extent of fire and non‐fire events using dendrochronology. Events were identified using evidence of disturbance agents, such as blue‐stain fungus in the case of mountain pine beetles, fire scars, and climatic reconstructions. Disturbance severity was classified based on amounts and spatial distribution of regeneration, mortality, and survivors. We studied the temporal and spatial relationships between events and tree age structures to determine historical and contemporary stand dynamics.We identified 103 events among the nine study plots; 97% of these events were fire. High‐severity events were 7% of events, 2% of events were mixed‐severity, and the other 70% were low‐severity. The severity of 20% of events could not be determined. All but one of the high‐severity events were fire. In seven plots, at least one crown fire occurred within the last 300 years. In RMNP, pre‐EuroAmerican settlement crown fires led to dense, even‐aged stands. Subsequent surface fires killed some trees and facilitated the regeneration of others, leading to less dense, uneven‐aged stands after ∼200–300 years. Climate may be the cause or co‐cause of one or two regeneration pulses within the last 300 years, but most regeneration and mortality is associated with fire. The historical occurrence of crown fires, as well as surface fires, in pure ponderosa pine forests in RMNP indicates that the fire regime is more variable than previously thought. Suppression of crown fires, though not completely possible, would move the fire regime outside its historical range of natural variability.Corresponding Editor: D. R. Strong