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Background Fire is an essential component in restoring and maintaining healthy forests. However, historic land use and decades of fire suppression has excluded fire from millions of forested hectares across much of the western United States, including Grand Canyon National Park (GCNP). Forest management at GCNP aims to reduce wildfire vulnerability by applying fire (both natural and planned ignitions) to remove forest vegetation and forest fuels and promote species diversity. However, the cost, complexity, and concerns associated with managing fire for resource benefit requires that fire managers utilize and implement locally relevant, science-based knowledge to strategically identify when and where to use fire to produce the greatest benefit. Observations from GCNP, topographic variation and weather were used to assess thresholds that affect burn severity and fire spread to identify conditions that may be beneficial or incompatible with multiple resource objectives. Results For ponderosa pine and mixed conifer forests, we developed burn severity and fire spread models by incorporating fire weather variables using gradient boosting machine learning on multi-day wildfires between 2000 and 2018. Elevation, wind, and vapor pressure deficit (VPD) were among the most influential across all models. Elevation was the most influential predictor for both the ponderosa pine severity and spread models. Wind and topographic roughness index were the most influential predictors in the mixed conifer spread and mixed conifer severity models, respectively. Using these models, prediction tables were generated to characterize expected burn severity and fire spread associated with common weather conditions, based on elevation, wind, and VPD. At lower elevations, a VPD threshold between low and moderate severity of 2.0kPa with winds between 8-10mph in both severity models was observed. Thresholds of VPD and wind decreased as elevation increased. In both forest types, average wind speeds greater than 4mph were found to be a threshold for fire spread. Greater fire spread was observed as winds and VPD increased. Conclusions Prediction tables can provide fire staff with a quick but comprehensive assessment of the relative likelihood of expected fire activity during changing weather conditions over the course of a fire event or season based on previous fire activity at GCNP.

Spatial patterns of high-severity wildfire in forests affect vegetation recovery pathways, watershed dynamics, and wildlife habitat across landscapes. Yet, less is known about contemporary trends in landscape patterns of high-severity burn patches or how differing federal fire management strategies have influenced such patterns. We assessed fires managed for ecological/resource benefit and fires that are fully suppressed and investigated: (1) whether spatial patterns of high-severity patches differed by management strategy, (2) whether spatial patterns were related to fire size and percent high-severity fire, and (3) temporal trends in spatial patterns. We examined high-severity spatial patterns within large fires using satellite-derived burn severity data from 735 fires that burned from 1984 to 2017 in Arizona and New Mexico, USA. We calculated a suite of spatial pattern metrics for each individual fire and developed a method to identify those which best explained variation among fires. Compared to managed fires, spatial pattern metrics in suppression fires showed greater patch homogeneity. All spatial pattern metrics showed significant relationships with fire size and percent high-severity fire for both management strategies. Mean annual spatiotemporal trends in suppression fires have moved toward smaller, more complex, fragmented patches since the early 2000s. Increases in fire size and proportion high-severity fire are driving more homogenous patches regardless of management type, with percent high-severity more strongly driving average temporal trends. Anticipated shifts in fire size and severity will likely result in larger, more contiguous, and simple-shaped patches of high-severity fire within southwestern conifer forests.

Much of the western United States is experiencing longer fire seasons with an increased frequency of high-severity fires and fire risk. Fire managers in the southwestern United States have increased efforts to reduce fire risk by managing more fires to meet resource objectives (e.g. thin forests, reduce hazardous fuel loads, and restore the landscape). However, little is known about the situational circumstances and decision space that inform the strategic response to wildland fire. Using generalized and time-to-event modeling techniques, we examined how fire management decisions are reached in a context informed by weather, burning conditions, and subsequent fire behavior. Modeling results captured daily containment probabilities along a gradient from limiting natural conditions to suppression invoked containment. Results inform fire management decisions, future research efforts, and the simulation of wildland fires with resource objectives.

After more than a century of low fire activity in the western United States, wildfires are now becoming more common. Reburns, which are areas burned in two or more fires, are also increasing. How fires interact over time is of interest ecologically as well as for management. Wildfires may act as fuel treatments, reducing subsequent fire severity, or they may increase subsequent fire severity by leaving high fuel loads behind. Our goal was to assess whether previous wildfire severity influenced subsequent fire severity across vegetation types and over time in the Southwest U.S. using remotely sensed fire severity data in 2275 fires that burned between 1984 and 2019. Points that reburned tended to be those that burned with lower severity initially. Shrublands burned predominantly at moderate to high severity in initial fires and in reburns. Pinyon-juniper-oak systems burned with mixed severity, and fire severity was consistent from fire to fire. In ponderosa pine and aspen-mixed conifer, fire severity tended to decrease with each fire. Initial and subsequent fire severity was lower in points that reburned after a short interval. These remotely sensed observations of reburn severity need verification through field work to understand specific effects caused by reburns in different ecosystems. However, in ponderosa pine and aspen-mixed conifer forests, it may be beneficial to consider wildfires as fuel treatments and work to maintain the fuel reduction effects they have on forested ecosystems.

Background Fire regimes are shifting in ponderosa pine (Pinus ponderosa Lawson & C. Lawson)-dominated forests, raising concern regarding future vegetation patterns and forest resilience, particularly within high-severity burn patches. The southwestern US has recently experienced a marked increase in large fires that produce large, high-severity patch interiors, with few surviving trees. These areas could be more susceptible for forest loss and conversions to alternative vegetation types than areas closer to the forest edge with more available seed sources. To better understand forest recovery, we surveyed ponderosa pine regeneration within edge and core areas (>200 m from edge) of high-severity patches in ten fires that burned between 1996 to 2008 across Arizona and New Mexico, USA. Specifically, we compared regeneration density, height, and canopy cover in patch edge and core areas and used generalized linear models to investigate the abiotic and biotic factors that contribute to ponderosa pine seedling establishment and density. Results High-severity burn-patch edge and core plots were not significantly different in seedling density, height, or canopy cover across fires. Seedling establishment was more likely at higher-elevation mesic sites and less likely when Gambel oak (Quercus gambelii Nutt.) was more abundant. Seedling density was negatively impacted by shrub, grass, and Gambel oak cover. Conclusions Regeneration density varied among fires but analysis of regeneration in aggregated edge and core plots showed that abundance of seed availability was not the sole factor that limited ponderosa pine regeneration, probably because of surviving tree refugia within high-severity burn patches. Furthermore, our findings emphasize that ponderosa pine regeneration in our study area was significantly impacted by xeric topographic environments and vegetation competition. Continued warm and dry conditions and increased wildfire activity may delay the natural recovery of ponderosa pine forests, underscoring the importance of restoration efforts in large, high-severity burn patches.

In the last three decades, over 4.1 million hectares have burned in Arizona and New Mexico and the largest fires in documented history have occurred in the past two decades. Changes in burn severity over time, however, have not been well documented in forest and woodland ecosystems in the southwestern US. Using remotely sensed burn severity data from 1621 fires (>404 ha), we assessed trends from 1984 to 2015 in Arizona and New Mexico in (1) number of fires and total area burned in all vegetation types; (2) area burned, area of high-severity, and percent of high-severity fire in all forest and woodland areas; and (3) area burned, area of high-severity, and percent of high-severity in seven different grouped forest and woodland vegetation types (Ecological Response Unit [ERU] Fire Regime Types). Number of fires and area burned increased across the Southwest regardless of vegetation type. The significant increasing trends held for area burned, area of high-severity, and percent of high-severity fire in all forest and woodland ecosystems. Area burned and area burned severely increased in all seven ERU Fire Regime Types while percent of high-severity fire increased in two ERUs: Mixed Conifer Frequent Fire and Mixed Conifer with Aspen/Spruce Fir. Managers must face the implications of increasing, uncharacteristic high-severity fire in many ecosystems as climate change and human pressures continue to affect fire regimes.

Managed wildfires, i.e., naturally ignited wildfires that are managed for resource benefits, have the potential to reduce fuel loads, minimize the effects of future wildfires, and restore critical natural processes across many forest landscapes. In the United States, the 2009 federal wildland fire policy guidance was designed to provide greater flexibility in the use of managed wildfires, but the effects of this policy on wildfires in the western US are not yet fully understood. Our goal was to compare managed and full suppression wildfires and to also analyze the differences between managed wildfires across space (Arizona/New Mexico and California) and time (before and after 2009) using four metrics for each wildfire: (1) distance to wilderness, (2) distance to the wildland–urban interface (WUI), (3) the percentage of area burned with high severity, and (4) the number of land management agencies. Across the study area, we found that managed wildfires were significantly closer to wilderness areas, were farther from the WUI, had a lower percentage of area that was burned at high severity, and had fewer agencies involved in managing the fire compared to full suppression wildfires. In California, managed wildfires occurred closer to wilderness and had a larger percentage of high-severity burn area compared to those in the southwest US (Arizona and New Mexico). Within each region, however, there were no significant geographic differences between managed wildfires before and after the implementation of the 2009 policy guidance. Despite the greater flexibility of the 2009 policy guidance, the basic geographic properties of managed wildfires in these two regions have not changed. As the climate warms and droughts intensify, the use of managed wildfires will need to expand during favorable weather conditions in order to address the threat of large and uncharacteristic wildfires to people and ecosystems.

As the effects of climate change accumulate and intensify, resource managers juggle existing goals and new mandates to operationalize adaptation. Fire managers contend with the direct effects of climate change on resources in addition to climate-induced disruptions to fire regimes and subsequent ecosystem effects. In systems stressed by warming and drying, increased fire activity amplifies the pace of change and scale of severe disturbance events, heightening the urgency for management action. Fire managers are asked to integrate information on climate impacts with their professional expertise to determine how to achieve management objectives in a changing climate with altered fire regimes. This is a difficult task, and managers need support as they incorporate climate adaptation into planning and operations. We present a list of adaptation strategies and approaches specific to fire and climate based on co-produced knowledge from a science–management partnership and pilot-tested in a two-day workshop with natural resource managers and regional stakeholders. This “menu” is a flexible and useful tool for fire managers who need to connect the dots between fire ecology, climate science, adaptation intent, and management implementation. It was created and tested as part of an adaptation framework used widely across the United States and should be applicable and useful in many fire-prone forest ecosystems.

The increasing incidence of wildfires across the southwestern United States (US) is altering the contemporary forest management template within historically frequent-fire conifer forests. An increasing fraction of southwestern conifer forests have recently burned, and many of these burned landscapes contain complex mosaics of surviving forest and severely burned patches without surviving conifer trees. These heterogeneous burned landscapes present unique social and ecological challenges. Severely burned patches can present numerous barriers to successful conifer regeneration, and often contain heavy downed fuels which have cascading effects on future fire behavior and conifer regeneration. Conversely, surviving forest patches are increasingly recognized for their value in postfire reforestation but often are overlooked from a management perspective. Here we present a decision-making framework for landscape-scale management of complex postfire landscapes that allows for adaptation to a warming climate and future fire. We focus specifically on historically frequent-fire forests of the southwestern US but make connections to other forest types and other regions. Our framework depends on a spatially-explicit assessment of the mosaic of conifer forest and severely burned patches in the postfire landscape, evaluates likely vegetation trajectories, and identifies critical decision points to direct vegetation change via manipulations of fuels and live vegetation. This framework includes detailed considerations for postfire fuels management (e.g., edge hardening within live forest patches and repeat burning) and for reforestation (e.g., balancing tradeoffs between intensive and extensive planting strategies, establishing patches of seed trees, spatial planning to optimize reforestation success, and improving nursery capacity). In a future of increasing fire activity in forests where repeated low- to moderate-severity fire is essential to ecosystem resilience, the decision-making framework developed here can easily be integrated with existing postfire management strategies to optimize allocation of limited resources and more actively manage burned landscapes.

High-severity fires in dry conifer forests of the United States Southwest have created large (>1000 ha) treeless areas that are unprecedented in the regional historical record. These fires have reset extensive portions of Southwestern ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. scopulorum Engelm.) forest landscapes. At least two recovery options following high-severity fire are emerging. One option is for post-fire successional pathways to move toward a return to the pre-fire forest type. Alternatively, an area may transition to persistent non-forested ecosystems. We studied regeneration patterns of ponderosa pine following eight fires in Arizona and New Mexico, USA, that burned in dry conifer forests dominated by ponderosa pine during a recent 18-year regional drought period, 1996 to 2013. Our a priori hypotheses were: 1) the most xeric areas within these severely burned dry conifer forests are least likely to regenerate to the pre-fire forest type due to persistent post-fire moisture stress; and 2) areas farther away from conifer seed sources have a lower likelihood of regeneration, even if these areas are climatically favorable for post-fire ponderosa pine establishment. We evaluated our hypotheses using empirical data and generalized linear mixed-effects models. We found that low-elevation, xeric sites are more limiting to conifer regeneration than higher-elevation mesic sites. Areas >150 m from a seed source are much less likely to have ponderosa pine regeneration. Spatial interpolations of modeled post-fire regeneration of ponderosa pine across the study landscapes indicate expansive areas with low likelihood of pine regeneration following high-severity fire. We discuss multiple post-fire successional pathways following high-severity fire, including potentially stable transitions to non-forest vegetation types that may represent long-term type conversions. These findings regarding landscape changes in Southwest forests in response to fires and post-fire regeneration patterns during early-stage climate warming contribute to the development of better-informed ecosystem management strategies for forest adaptation or mitigation under projected hotter droughts in this region.

Accurate estimates of canopy base height (CBH) and canopy bulk density (CBD) are critical inputs for many fire modeling and simulation programs. Commonly used LANDFIRE estimates of CBH and CBD are only available at 30 m resolution and have relatively low accuracy, and traditional field-based estimates can lead to inaccurate spatial estimates by assuming averages across spatial extents. Discrete-return airborne light detection and ranging (lidar) has become increasingly common in forestry applications, with demonstrated success in measuring canopy metrics and forest structure across broad landscapes and varied forest types. However, few studies have investigated the potential of airborne lidar for estimating these metrics in southwestern forests. This study developed an approach for estimating CBH and CBD using airborne lidar data and evaluated the accuracy of estimates at 20 m resolution. We also evaluated the predicted accuracy between managed and unmanaged forest stands. We employed a quantile-based method to derive CBH and utilized the Fire and Fuels Extension of the Forest Vegetation Simulator to estimate CBD from a lidar-derived tree list. Our results indicate that airborne lidar produced more accurate estimates of both CBH (R2 = 0.4; RMSE = 1.25 m) and CBD (R2 = 0.76; RMSE = 0.021 kg−3) as compared to LANDFIRE and other regional remote-sensing based estimates. We also report that airborne lidar is more accurate at estimating CBH in unmanaged stands versus managed stands, but CBD estimates maintain similar accuracy regardless of management history. Compared to methods developed in other regions, our approach resulted in relatively low R2 values, but our RMSE values were found to be similar or slightly improved. Our approach for deriving both metrics are conceptually and computationally simple making them especially valuable for use by land managers and ecologists alike. This study provides clear evidence that airborne lidar can be used to derive broad-scale, fine-resolution fuel data layers, which are commonly required in fire modeling and forest management activities and decision-making.

Over the last several decades in forest and woodland ecosystems of the southwestern United States, wildfire size and severity have increased, thereby increasing the vulnerability of these systems to type conversions, invasive species, and other disturbances. A combination of land use history and climate change is widely thought to be contributing to the changing fire regimes. We examined climate-fire relationships in forest and woodland ecosystems from 1984 to 2015 in Arizona and New Mexico using 1) an expanded satellite-derived burn severity dataset that incorporates over one million additional burned hectares when compared to MTBS data, and 2) climate variables including temperature, precipitation, and vapor pressure deficit (VPD). Regional climate-fire relationships were assessed by correlating annual area burned, area burned at high and low severity, and percent high severity with fire season (May-August) and water-year (October-September) climate variables. We also analyzed relationships between climate and high-severity fire at the scale of the individual fire using a hurdle model. We found that increasing temperature and VPD and decreasing precipitation were associated with increasing area burned regionally, and that area burned at high severity had the strongest relationships with climate metrics. The relationship between climate and fire activity in the Southwest appears to be strengthening since 2000. VPD-fire correlations were consistently as strong as, or stronger than, temperature or precipitation variables alone, both regionally and at the scale of the individual fire. Notably, at the scale of the individual fire, temperature and precipitation were not significant predictors of fire activity. Thus, our results support the use of VPD as a more integrative climate metric to forecast fire activity. We suggest that the strong relationship between VPD and fire activity may be useful to assess the likelihood of high-severity fire occurrence through continued development of the high-severity fire threshold model we present. The link between increasing aridity and increasing wildfire activity suggests a future with more fire in Southwest forests and woodlands with projected warming, underscoring the urgency of restoration in dry forests to reduce the likelihood of uncharacteristic, large high-severity fires.

In 2009, new guidance for wildland fire management in the United States expanded the range of strategic options for managers working to reduce the threat of high-severity wildland fire, improve forest health and respond to a changing climate. Markedly, the new guidance provided greater flexibility to manage wildland fires to meet multiple resource objectives. We use Incident Status Summary reports to understand how wildland fire management strategies have differed across the western US in recent years and how management has changed since the 2009 Guidance for Implementation of Federal Wildland Fire Management Policy. When controlling for confounding variation, we found the 2009 Policy Guidance along with other concurrent advances in fire management motivated an estimated 27 to 73% increase in the number of fires managed with expanded strategic options, with only limited evidence of an increase in size or annual area burned. Fire weather captured a manager’s intent and allocation of fire management resources relative to burning conditions, where a manager’s desire and ability to suppress is either complemented by fire weather, at odds with fire weather, or put aside due to other priorities. We highlight opportunities to expand the use of strategic options in fire-adapted forests to improve fuel heterogeneity.

Fire shapes landscapes long after the flames are extinguished by leaving legacies of heterogeneous fuel mosaics, species composition patterns, and age classes. Fire perimeters have received little research attention, but their locations have implications for both landscape patterns and processes, including vegetation structure and subsequent disturbances. In this study, we focused on the role of previous wildfires and roads in limiting wildfire growth and influencing the pattern of fire at a regional scale. Using fire perimeter data from the U.S. Southwest, we asked (1) to what degree previous wildfires and roads limit the spread of subsequent fires, (2) what the temporal patterns are in fire perimeter limitations, in terms of time-since-fire and stability of patterns over time, and (3) whether limitations to fire spread differ across National Forests and topographic variables. We found strong evidence that previous fires and roads play a role in limiting subsequent fire progression. Of fires that spatially intersected previous wildfires, 8.7% of fire perimeters aligned only with previous wildfire perimeters. On average, 25.7% of fire perimeters aligned only with roads, compared to 11.6% when fires were randomly shifted, and road alignments tended to be on less steep slopes than wildfire alignments. More than 60% of fire perimeter alignments occurred when time since the previous fire was 5 years or less. Finally, results varied by National Forest; the Coconino and Kaibab National Forests, which have fairly flat terrain, had high percentages of fire-road alignments while the Gila National Forest, which contains a large amount of Wilderness, had the most fire-fire alignments. As more fires burn, fire interactions are likely to increase, and previous fire footprints may have more opportunity to act as fuel breaks or control points for subsequent fires.

The continued threat of an invasive, annual brome (Bromus) species in the western United States has created the need for integrated approaches to postfire restoration. Additionally, the high germination rate, high seed production, and seed bank carryover of annual bromes points to the need to assay soil seed banks as part of monitoring programs. We sampled the soil seed bank to help assess the effectiveness of treatments utilizing the herbicide Plateau® (imazapic) and a perennial native seed mix to control annual Bromus species and enhance perennial native plant establishment following a wildfire in Zion National Park, Utah. This study is one of few that have monitored the effects of imazapic and native seeding on a soil seed bank community and the only one that we know of that has done so in a pinyon–juniper woodland. The study made use of untreated, replicated controls, which is not common for seed bank studies. One year posttreatment, Bromus was significantly reduced in plots sprayed with herbicide. By the second year posttreatment, the effects of imazapic were less evident and convergence with the controls was evident. Emergence of seeded species was low for the duration of the study. Dry conditions and possible interactions with imazapic probably contributed to the lack of emergence of seeded native species. The perennial grass sand dropseed outperformed the other species included in the seed mix. We also examined how the treatments affected the soil seed bank community as a whole. We found evidence that the herbicide was reducing several native annual forbs and one nonnative annual forb. However, overall effects on the community were not significant. The results of our study were similar to what others have found in that imazapic is effective in providing a short-term reduction in Bromus density, although it can impact emergence of nontarget species.

In 2009, the federal Joint Fire Science Program (JFSP) initiated a national network of boundary organizations,known as regional fire science consortia, to accelerate the awareness, understanding, and use of wildland firescience. Needs assessments conducted by consortia in eight regions of the United States are synthesized hereusing a case survey approach. Although regions used different methods based on their different ecosystems,geography, and demography, results showed striking similarities in how fire science is accessed and used,barriers to its use, and research information needed. Use of Internet-based information is universally high;however, in-person knowledge exchange is preferred. Obstacles to fire science application include lack of time,resources, and access to the most relevant information as well as communication barriers between scientists andmanagers. Findings show a clear need for boundary organizations to span fire science and management to (a)organize and consolidate fire science information through easily accessible websites and (b) strengthenrelationships between scientists and managers to facilitate production and communication of science relevant tomanagers’ concerns. This article contributes to boundary spanning theory by underscoring and documenting theadvantages of regionally focused boundary organizations in meeting user needs and building bridges betweenfire scientists and managers.Keywords: fire science, boundary organization, fire management, Joint Fire Science Program, technologytransfer

Climate changes and associated shifts in ecosystems and fire regimes present enormous challenges for the management of landscapes in the Southwestern US. A central question is whether management strategies can maintain or promote desired ecological conditions under projected future climates. We modeled wildfire and forest responses to climate changes and management activities using two ecosystem process models: FireBGCv2, simulated for the Jemez Mountains, New Mexico, and LANDIS-II, simulated for the Kaibab Plateau, Arizona. We modeled contemporary and two future climates—“Warm-Dry” (CCSM4 RCP 4.5) and “Hot-Arid” (HadGEM2ES RCP 8.5)—and four levels of management including fire suppression alone, a current treatment strategy, and two intensified treatment strategies. We found that Hot-Arid future climate resulted in a fundamental, persistent reorganization of ecosystems in both study areas, including biomass reduction, compositional shifts, and altered forest structure. Climate changes increased the potential for high-severity fire in the Jemez study area, but did not impact fire regime characteristics in the Kaibab. Intensified management treatments somewhat reduced wildfire frequency and severity; however, management strategies did not prevent the reorganization of forest ecosystems in either landscape. Our results suggest that novel approaches may be required to manage future forests for desired conditions.

We investigate interactions between successive naturally occurring fires, and assess to what extent the environments in which fires burn influence these interactions. Using mapped fire perimeters and satellite-based estimates of post-fire effects (referred to hereafter as fire severity) for 19 fires burning relatively freely over a 31-year period, we demonstrate that fire as a landscape process can exhibit self-limiting characteristics in an upper elevation Sierra Nevada mixed conifer forest. We use the term ‘self-limiting’ to refer to recurring fire as a process over time (that is, fire regime) consuming fuel and ultimately constraining the spatial extent and lessening fire-induced effects of subsequent fires. When the amount of time between successive adjacent fires is under 9 years, and when fire weather is not extreme (burning index

Recent research has concluded that forest wildfires in the western United States are becoming larger and more frequent. A more significant question may be whether the ecosystem impacts of wildfire are also increasing. We show that a large area (approximately 120000 km2) of California and western Nevada experienced a notable increase in the extent of forest stand-replacing (“high severity”) fire between 1984 and 2006. High severity forest fire is closely linked to forest fragmentation, wildlife habitat availability, erosion rates and sedimentation, post-fire seedling recruitment, carbon sequestration, and various other ecosystem properties and processes. Mean and maximum fire size, and the area burned annually have also all risen substantially since the beginning of the 1980s, and are now at or above values from the decades preceding the 1940s, when fire suppression became national policy. These trends are occurring in concert with a regional rise in temperature and a long-term increase in annual precipitation. A close examination of the climate–fire relationship and other evidence suggests that forest fuels are no longer limiting fire occurrence and behavior across much of the study region. We conclude that current trends in forest fire severity necessitate a re-examination of the implications of all-out fire suppression and its ecological impacts.

Over a century of fire exclusion in frequent-fire ponderosa pine and dry mixed conifer forests has resulted in increased tree densities, heavy surface fuel accumulations and an increase inlate successional,fire-intolerant trees. Grand Canyon National Park uses prescribed fires and wildfires to reduce fire hazard and restore ecosystem processes. Research isneededtodeterminepost-firevegetationresponsethusenablingfutureforestsuccessionpredictions.Ourstudyfocussed on the effects of burn entry and burn severity on species composition and regeneration in two forest types: ponderosa pine with white fir encroachment and dry mixed conifer. We found no difference in tree composition and structure in a single, low-severity burn compared with unburned areas in the white fir encroachment forest type. We found no white fir seedlings or saplings in a second-entry, low-severity burn in the white fir encroachment forest type. Second-entry burns were effective in reducing white fir densities in the white fir encroachment forest type. There was significant aspen regeneration following high-severity fire in the dry mixed conifer forest type. This research suggests that repeated entries and an increase in burn severity may be necessary for prescribed fire or wildfire to be effective in meeting management objectives. Additional keywords: aspen, burn entry, burn severity, mixed conifer, ponderosa pine, regeneration.

Seeding following high-severity wildfires is motivated by the goals of increasing vegetative cover and decreasing bare soil in order to minimise soil erosion and exotic plant invasions. We compared the ground cover and vegetation response of seeded versus non-seeded areas located in the Warm Fire in northern Arizona, where post-fire seeding treatments with Italian ryegrass (Lolium perenne spp. multiflorum (L.)) were conducted in 4000 ha of high-severity burned areas. Over the course of the study, we observed no significant difference between seeded and non-seeded plots in percentage of bare soil, total vegetative cover or exotic plant cover. However, there were significant differences in plant community composition as revealed by PERMANOVA and Indicator Species Analysis. Two years post-fire there were significantly fewer ponderosa pine seedlings, and the cover of annual and biennial forbs was significantly lower in plots that were seeded with Italian ryegrass. In the third year, the cover of native bunch grasses was significantly lower in seeded plots. The differences we observed may be due to differences in pre-existing vegetation composition because of the geographic separation of the plots across the landscape. Our results illustrate the ineffectiveness of post-fire seeding in achieving the goals of increasing vegetative cover and decreasing the invasion of non-native plants, and we suggest that alternative post-fire remediation should be considered in the future.

Fire is a primary change agent in many terrestrial ecosystems. Appreciation is growing for the essential role fire plays in fire-adapted ecosystems. Nevertheless, humans living in the wildland urban interface (WUI) understandably regard fires as a threat to their safety, their property, or the natural resources and ecosystem services upon which they depend. As land development has expanded into the WUI, so has the demand for better spatial information regarding fire danger and fire effects, both short- and long-term.

This paper quantifies current fire severity distributions for 19 different fire-regime types in Yosemite National Park, California, USA. Landsat Thematic Mapper remote sensing data are used to map burn severity for 99 fires (cumulatively over 97 000 ha) that burned in Yosemite over a 20-year period. These maps are used to quantify the frequency distributions of fire severity by fire-regime type. A classification is created for the resultant distributions and they are discussed within the context of four vegetation zones: the foothill shrub and woodland zone; the lower montane forest zone; the upper montane forest zone and the subalpine forest zone. The severity distributions can form a building block from which to discuss current fire regimes across the Sierra Nevada in California. This work establishes a framework for comparing the effects of current fires on our landscapes with our notions of how fires historically burned, and how current fire severity distributions differ from our desired future conditions. As this process is refined, a new set of information will be available to researchers and land managers to help understand how fire regimes have changed from the past and how we might attempt to manage them in the future.

Continental-scale studies of western North America have attributed recent increases in annual area burned and fire size to a warming climate, but these studies have focussed on large fires and have left the issues of fire severity and ignition frequency unaddressed. Lightning ignitions, any of which could burn a large area given appropriate conditions for fire spread, could be the first indication of more frequent fire. We examined the relationship between snowpack and the ignition and size of fires that occurred in Yosemite National Park, California (area 3027 km2), between 1984 and 2005. During this period, 1870 fires burned 77 718 ha. Decreased spring snowpack exponentially increased the number of lightning-ignited fires. Snowpack mediated lightning-ignited fires by decreasing the proportion of lightning strikes that caused lightning-ignited fires and through fewer lightning strikes in years with deep snowpack. We also quantified fire severity for the 103 fires >40 ha with satellite fire-severity indices using 23 years of Landsat Thematic Mapper data. The proportion of the landscape that burned at higher severities and the complexity of higher-severity burn patches increased with the log10 of annual area burned. Using one snowpack forecast, we project that the number of lightning-ignited fires will increase 19.1% by 2020 to 2049 and the annual area burned at high severity will increase 21.9%. Climate-induced decreases in snowpack and the concomitant increase in fire severity suggest that existing assumptions may be understated – fires may become more frequent and more severe.