Past Fire Research Articles

Evaluating the effect of prescribed burning on the reduction of wildfire extent in Portugal

Prescribed burning (PB) is increasingly recognised as a viable, cost-effective technique for reducing wildfire risk. Yet, quantification of the effect of PB on the reduction of wildfire extent in southern Europe is non-existent. We used 35 years of fire mapping data in Portugal to analyse wildfire regime metrics in nine landscapes before (1985–2004) and during (2005–2020), the period when systematic use of PB to reduce fuel load began. We employed robust linear mixed models and generalized linear mixed models to examine the role of annual wildfire extent drivers at landscape scales and quantified the effect of PB on the reduction of subsequent wildfire extent, i.e., leverage. We found evidence of larger wildfires during the PB era, suggesting that other influences are counteracting its putative effect. The most important variables explaining wildfire extent were fire weather and ignition density, with positive effect, and past wildfire and PB extent, with negative effect, corresponding to leverage. We obtained a total past fire (PB and wildfire) leverage of 0.28, i.e., approximately 4 ha of past fire decrease subsequent wildfire by 1 ha, increasing to 0.84 if only landscapes with high wildfire incidence are considered. The analyses failed to unequivocally identify a direct leverage effect of PB but a 1:5 return-for-effort is reasonable as a conservative estimate. Increasing the size of PB treatments and treatment effort in the landscape in combination with fire-modelling assisted planning would enhance PB leverage, making it more effective at reducing wildfire extent, rather than functioning only as a supplement to the effect of previous wildfires.

Multi-century spatiotemporal patterns of fire history in black pine forests, Turkey

In this study, we aimed to use tree-ring based fire reconstruction to understand the spatiotemporal patterns of past fires in different climate types of western Anatolia. We collected fire scarred wood samples from living trees as wedges and remnant woods from ten sites along a transect that represents a continental to Mediterranean climate gradient. We determined fire years and assigned seasonality of fires based on the intraring position of the fire scars. We calculated fire statistics and analysed fire-climate relationships. Breakpoints in our Anatolian regional fire chronology were estimated to determine the regime shifts. A decrease in fire frequency was recorded at most of the sites after the end of the 19th and the beginning of the 20th century. We observed two critical fire regime shift periods. The period between 1853 and 1934 is characterized by highly frequent (a total of 82 fires) and simultaneous fires occurring in multiple sites and this period overlapped with the longest and most severe drought period of the past 550 years. The fire frequency decline after 1934 coincided with the period of the first forest protection law in 1937. Dry, as well as prior wet conditions were main drivers of fires in the black pine forests in western Anatolia. We observed a decrease in fire frequency in the late 19th and early 20th centuries due to fire suppression activities. Continued fire suppression activities may cause fuel accumulation and pose a risk for more intense fires and thus a paradox for forests in the future. Based on future climate projections, we will face prolonged fire seasons as a consequence of increasing drought frequency, which may shift the fire regime from surface to crown fires with the accumulation of combustible material in the understory in black pine forests.

Spatially calibrating polycyclic aromatic hydrocarbons (PAHs) as proxies of area burned by vegetation fires: Insights from comparisons of historical data and sedimentary PAH fluxes

Many regions worldwide have experienced increasing wildfire activity in recent years and climate changes are predicted to result in more frequent and severe fires. Reconstruction of past fire activity offers paleoenvironmental context for modern and future burning. Pyrogenic polycyclic aromatic hydrocarbons (PAHs) have been increasingly used as a molecular biomarker for fire occurrence in the paleorecord and offer opportunity for nuanced reconstructions of fire characteristics. A suite of PAHs is produced during combustion, and the emission amount and assemblage is influenced by many variables including fuel type, fire temperature, and oxygen availability. Despite recent advances in understanding the controls and taphonomy of these biomass burning markers, the spatial scale of this proxy is unknown. In this paper, measurements of PAH fluxes preserved in a lake sediment archive from the Sierra Nevada, California were compared with a historical geographic information system (GIS) dataset of area burned up to 150 km distance from the lake to determine the spatial scales for which these biomarkers are reliable proxies of biomass burning. The PAH fluxes in the Swamp Lake sediments record a change in the relative anthropogenic and pyrogenic sourcing of PAHs. Anthropogenic pollution sources could explain why some PAHs (fluoranthene (Fl), pyrene (Py), benz[a]anthracene (BaA), retene (Ret), benzo[a]pyrene (BaP), dibenzo[a,h]anthracene (DA) and ideno[1,2,3-cd]pyrene (IP)) did not correlate with area burned within 150 km. This indicates that individual PAHs may have different efficacies in recording area burned and be more susceptible to masking of fire signals by pollution sources. Despite these complications, we find that the PAHs naphthalene (Na), acenaphthene (Ace), fluorene (F), and anthracene (An) are reliable local proxies of area burned (within 40 km), whereas the PAHs phenanthrene (Phe), chrysene (Ch), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), and benzo[g,h,i]perylene (Bghi) are both reliable local (within 36 km) as well as more regional (as much as 75 km for phenanthrene (Phe), chrysene (Ch), and benzo[g,h,i]perylene (Bghi) or 150 km for benzo[b]fluoranthene (BbF) and benzo[k]fluoranthene (BkF)) area burned proxies. Comparisons of PAH fluxes with charcoal accumulation rates in the same sediments suggest that pyrogenic particulate transport modulates low to mid-molecular weight PAHs via adsorption. Overall, the results show that PAH records integrate a combination of spatial signals of area burned and measurement of individual PAHs may enable cross-scale paleofire reconstructions.

Past fires and post-fire impacts reconstructed from a southwest Australian stalagmite

Speleothem records of past environmental change provide an important opportunity to explore fire frequency and intensity in the past, and the antecedent climatic conditions leading to fire events. Here, fire sensitive geochemical signals in a stalagmite from Yonderup Cave, a shallow cave in Western Australia, are compared to well-documented wildfire events that occurred in recent decades. The results are extended to identify wildfires during the growth interval of the speleothem (1760 CE–2005 CE). Principal component analysis of the stalagmite time-series revealed distinct peaks in a combination of phosphorus and metal (aluminium, zinc, copper and lead) concentrations in response to known fire events, which are interpreted to have come from ash. Varying responses in the geochemical signal in the pre- and post-European colonisation period are likely linked to changes in land management, fire frequency and fire intensities. Of note is what we infer to be a particularly intense fire event concluded to have occurred in 1897 ± 5 CE, which climate sensitive proxies (δ18O and bedrock-derived elements) indicate was preceded by a multi-decadal dry period that began in the late 1860s. The intensity of the deduced fire event is supported by a peak in P that is 6.3 times higher than elsewhere in the speleothem, accompanied by a peak in Zn. It was also inferred that changes to the surface-cave hydrology occurred as a result of this event, indicated by increased input of colloidal organic material onto the stalagmite that was interpreted to be caused by shallow karst bedrock fracturing from the heat-induced deformation that is observed after intense fires. These findings show the potential for speleothems to provide dated records of fire intensity and recurrence intervals. Further development could lead to a better understanding of the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity.

The Reading Palaeofire Database: an expanded global resource to document changes in fire regimes from sedimentary charcoal records

Abstract. Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using IntCal20 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1676 records (entities) from 1480 sites worldwide. The database (RPDv1b – Harrison et al., 2021) is available at https://doi.org/10.17864/1947.000345.

First Spatial Reconstruction of Past Fires in Temperate Europe Suggests Large Variability of Fire Sizes and an Important Role of Human-Related Ignitions

The spatial component of past forest fires in temperate Europe has been little studied, despite the value of such data in quantifying human and natural factors driving fire activity and associated forest dynamics. Changes in fire regimes reported across a range of ecosystems call for a better understanding of variability in historic fires and may help define reference points that can be relied upon when discussing climate change effects. We provide the first dendrochronological reconstruction of historical fire sizes in Central Europe and analyze the minimum extent of fires during the last four centuries in a 9.2 km2 (920 ha) conifer-dominated section of Białowieża Forest, one of the largest continuous lowland forests of the subcontinent. We recorded 82 fires between 1666 and 1946, using 275 sample trees, while 92% of fires (76 out of 82) spread beyond the studied area. Fires varied considerably in size, from events recorded at only one site (1–200 ha) to fires recorded in more than half of the studied area, thus exceeding 500 ha in size. The fire cycle was 11 years over the whole study period, with three distinct periods revealed by the regime shift analysis. In the years 1670–1750, the fire cycle averaged 12 years. It shortened to 7 years between 1755–1840 and increased to 22 years over the 1845–1955 period. In comparison with present day data, the reconstructed fire density of 3.2 fires per 100 km2 (10 000 ha) and year exceeded lightning ignition density by one to two orders of magnitude, suggesting a significant contribution of human-related ignitions. Our results highlight the important role of fire disturbance in Białowieża Forest and provide critical baseline information to design biological conservation strategies for European forests.

Tropical peatlands in the anthropocene: Lessons from the past

The status of tropical peatlands, one of Earth’s most efficient natural carbon stores, is of increasing international concern as they experience rising threat from deforestation and drainage. Peatlands form over thousands of years, where waterlogged conditions result in accumulation of organic matter. Vast areas of Southeast Asian peatlands have been impacted by land use change and fires, whilst lowland tropical peatlands of Central Africa and South America remain largely hydrologically intact. To predict accurately how these peatlands may respond to potential future disturbances, an understanding of their long-term history is necessary. This paper reviews the palaeoecological literature on tropical peatlands of Southeast Asia, Central Africa and South America. It addresses the following questions: (i) what were the past ecological dynamics of peatlands before human activity?; (ii) how did they respond to anthropogenic and natural disturbances through the palaeoanthropocene, the period from whence evidence for human presence first appeared?; and, (iii) given their past ecological resilience and current exposure to accelerating human impacts, how might the peatlands respond to drivers of change prevalent in the anthropocene? Through y synthesising palaeoecological records, this review demonstrates how tropical peatland ecosystems have responded dynamically, persisting through fire (both natural and anthropogenic), climatic and human-induced disturbances in the palaeoanthropocene. Ecosystem resilience does, however, appear to be compromised in the past c . 200 years in Southeast Asian peatlands, faced with transformative anthropogenic impacts. In combination, this review’s findings present a pantropical perspective on peatland ecosystem dynamics, providing useful insights for informing conservation and more responsible management. • Tropical peatlands are threatened by transformative human impact in the anthropocene. • Predicting future peatland resilience requires knowledge of past ecosystem dynamics. • Palaeoecology is used to explore past ecosystem responses to disturbance. • Tropical peatlands have recovered from past fire, climatic & human disturbances. • Past responses reveal possibilities for future sustainable human-peatland interaction.

Model-based supervised sensor placement optimization to detect propellant leak in a liquid rocket engine

The sensor selection for leak detection and diagnosis in a reusable liquid rocket engine was studied. Sufficient amounts of normal and leaked training data were generated by Monte Carlo simulations considering variations in system conditions based on past firing test results. A multivariate supervised analysis, in which measurements are linearly projected onto a vector that characterizes the difference between distributions of normal and leaked cases, successfully detected the simulated leaks that could not be detected by conventional univariate red-line judgment and unsupervised principal component analysis. Significant sensors for leak detection were selected using the greedy approach based on the detection performance score. It was found that only 11 of the 27 sensors were sufficient to maintain the detection performance.

Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

Safety at waste and recycling industry: Detection and mitigation of waste fire accidents

In this study, NASA’s VIIRS (Visible Infrared Imaging Radiometer Suite) fire hotspots and data of the Swedish Civil Contingencies Agency (MSB), collected between 2012 and 2018, was integrated to characterize waste fire incidents that were detected by VIIRS and reported to MSB (DaR), detected by VIIRS but not reported to MSB (DbNR) and that are reported to MSB but not detected by VIIRS (RbND). Results show that the average number of open waste fire incidents per million capita per year (AFIPMC) in Sweden, for the period 2012–2018, ranges from 2.4 to 4.7. Although a weak correlation exists (r = 0.44, P = 0.1563, one tailed) between years and number of fire incidents (MSB + VIIRS fires), a continuous increase in number of fire incidents was recorded between 2014 and 2018. It is concluded that the use of satellite data of fire anomalies, in-combination with the use of incident reports, will help in formalizing more reliable and comprehensive waste fire statistics. Another focus area of the article is to consolidate the recommendations and routines for safe storage of waste and biofuels and to present the lessons that can be learnt from past fire incidents. The article also discusses the technical, political, economic, social, and practical aspects of waste fires and provide a baseline for future research and experimentation.

Charcoal morphotypes and potential fuel types from a Mexican lake during MIS 5a and MIS 3

Charcoal particles vary in size and form, but they can preserve morphological characteristics used to infer the type of vegetation being burnt. To deduce potential fuel and fire type, we reconstructed local fire activity analyzing macrocharcoal morphologies and pollen data of the sediment core CHA08-V from Lake Chalco from the sub-stage MIS 5a to MIS 3, with CHAR data and Z-Score analysis. We identified the following fuel types: i) wood, characterized by blocky and quadrilateral morphologies with solid, foliated or striated surface textures; ii) grass, represented by rectangular-porous and rectangular-latticed morphologies; and iii) herbs, characterized by long-thin filaments and polygonal porous particles. Arboreal pollen components were associated to wood morphotypes whereas non-arboreal pollen related to herbs and grass morphotypes, indicating that variation in vegetation composition was important regulating factor determining fire types in the Basin of Mexico. This study provides the first long-term record of charcoal morphotypes for Mexico region to emphasize the importance of charcoal morphotypes analyses to complement paleoenvironmental interpretation of past fire regimes.

Perspectives on biotic responses to repeated wildfires from decades of long-term empirical studies

ABSTRACT Fire can have marked impacts on biodiversity and on ecosystem condition. However, it is the sequence of multiple fires over a prolonged period of time which can have the most marked effects on biodiversity and on ecosystem condition. A good understanding of these effects comes from long-term studies. In this article we outline some of the key perspectives on the effects of fire on ecosystems and biodiversity from two large-scale, long-term monitoring studies in south-eastern Australia. These are studies in the montane ash forests of the Central Highlands of Victoria and at Booderee National Park in the Jervis Bay Territory. These studies have shown that the effects of fires are strongly influenced by: (1) The condition of an ecosystem before a fire (e.g. the age of a forest at the time it is burnt). (2) Conditions after the fire such as the extent of herbivory in regenerating vegetation and whether the ecosystem is subject to post-fire (salvage) logging. (3) Fire history (e.g. the number of past fires and the time since the previous fire). And, (4) Interactions between fire and other ecosystem drivers such as logging. We discuss some of the key implications for conservation and resource management that arise from these studies including the need to: (a) Reduce the number of stressors in some ecosystems to facilitate post-fire recovery. (b) Recognize that pre-fire human disturbances can elevate fire severity in some forest ecosystems, with corresponding negative effects on elements of the biota, and, (c) Acknowledge the inherent patchiness of wildfires and the value of unburnt areas and places burnt at low severity as critical refugia for some species; it is critical that these locations are managed accordingly (e.g. by limited additional disturbances within them). Finally, many of the insights discussed in this article have emerged only through long-term studies. More long-term monitoring and research is needed to truly understand and better manage fire in Australian ecosystems.

Estimating visitor preferences for recreation sites in wildfire prone areas

Development into the wildland–urban interface, combined with heat and drought, contribute to increasing wildfires in the U.S. West and a range of damages including recreation site closures and longer-term effects on recreation areas. A choice experiment survey is used to estimate visitor preferences for vegetation and the effects of past fire at recreation sites. Intercept interviews are used to randomly select visitors at national forest sites near Los Angeles. The choice model results reveal that recreation sites with waterbodies and sites with tree cover, instead of shrubs or barren areas, are highly desirable, while evidence of past fires decreases the value of a site. We find the effects of past fire depend on vegetation type, fire intensity and time since the fire ended. Older forest fires and shrub fires are undesirable, but forest fires that reach the crowns of trees are least desirable. The findings add to evidence that fire damage to recreation areas extends beyond closures and depends on vegetation, which can inform the allocation of firefighting and prevention resources.

Fire scar characteristics in two tropical montane conifer species from central Mexico

Fire scar analysis is a fundamental tool for reconstructing fire regimes in conifer forests. However, little is known about fire scar properties in tropical montane conifers, where some assumptions limit dendroecological research. These include that fir species do not exhibit external fire scars and that pines without external fire scars have not experienced past fires. This study describes fire scar patterns in two conifer species growing in Mexican temperate forests: sacred fir (Abies religiosa) and smooth-bark Mexican pine (Pinus pseudostrobus). We extracted cross-sections from 110 trees and measured tree age, basal diameter, bark thickness, fire scar size and seasonality, the number of scars per tree, years and basal diameter from pith to the first scar, and the proportion of external and buried fire scars. Most trees had three fire scars, which appeared during the dry season after the first 15 years measured from the pith. Old and large-diameter trees did not have more fire scars, but the time between fire scars influenced fire scar closure in sacred firs. Bark thickness and the proportion of visible and buried fire scars were similar in both species. Our results suggest that the absence of visible fire scars in smooth-bark Mexican pines does not imply the absence of fire, and sacred firs can exhibit external fire scars like pines.

Postfire treatments alter forest canopy structure up to three decades after fire

We evaluated the effects of postfire management on forest structure in mixed-conifer forests of northeastern Washington, USA. Postfire treatments were harvest-only, harvest combined with planting, planting-only, and postfire prescribed fire. We used aerial light detection and ranging (LiDAR) to measure vertical and horizontal components of postfire forest structure over a period of 2 to 32 years after fires. We compared treated areas to control areas with similar bioclimatic environments and past fire severity. We used niche overlap statistics to quantify distributions of individual forest structure components and PERMANOVA to assess forest structural response to the presence or absence of treatments, past fire severity, time since treatment, and bioclimatic setting. Harvest alone after fire decreased dominant tree height and reduced vertical canopy complexity and the cover of tall trees. Harvest combined with planting increased dominant tree height, vertical complexity, and cover in lower height strata. Planting and prescribed fires showed little difference in forest structure relative to untreated controls. Overall, the burn severity of the initial fire was the strongest influence on postfire structure, and many aspects of vertical and horizontal forest structure showed little difference with increasing time since fire.

Palaeoecological records as a guide for fire management in Killarney National Park, Ireland

Climate change is allowing fire to expand into previously unburnt ecosystems and regions. While management policies such as fire suppression have significantly altered their frequency and intensity. To prevent future biodiversity/ecosystem services loss, and the large financial burden of wildfires, management plans will be required to adapt to future climate and land use changes. Long-term ecological data offer a unique perspective to assess fire variability under different climate and land-use conditions. In this study, we focus on Killarney National Park, Ireland. An area which today is under threat from an increase in fire activity. Comparing palaeoecological and archaeological records, we reconstruct the past fire dynamic and its impact on the landscape, and evaluate the role of climate vs humans in influencing the natural fire regime over the millennial time-scale. Our results indicate that fire has been present in the landscape since the beginning of the Holocene, with fire in the early Holocene being largely controlled by climate and microsite conditions, and in the late Holocene being increasingly influenced by human activity. The knowledge of past fire regimes can help inform future management in order to protect the semi-natural native woodland. The park's present landscape mosaic, could be preserved by limiting forest encroachment through moderate grazing and burning, while also protecting any fragmented forest from excessive grazing and large/intense fires, via traditional fire management strategies such as fuel load management. However, a fire management strategy should only be implemented following careful consideration of all ecosystem factors and controls.

Pottery of Early Iron Age from the Glinjeni II-La Șanț (North-Western Pontic Sea Region): Composition, Technology and Raw Material Sources

Transition to the Early Iron Age was marked by the appearance of innovations such as iron technology and changes in the lifestyle of local societies on the territory of the North-Western Pontic Sea region. One of the most interesting sites of this period is the Glinjeni II-La Șanț fortified settlement, located in the Middle Dniester basin (Republic of Moldova). Materials of different cultural traditions belonged to the Cozia-Saharna culture (10th–9th cc. BC) and the Basarabi-Șoldănești culture (8th–beginning of 7th cc. BC) were found on this site. The article presents the results of a multidisciplinary approach to the study of ceramic sherds from these archaeological complexes and cultural layers as well as raw clay sources from this area. The archaeometry analysis, such as the XRF-WD, the thin section analysis, SEM-EDX of ceramics, m-CT of pottery were carried out. The study of ancient pottery through a set of mineralogical and geochemical analytic methods allowed us to obtain new results about ceramic technology in different chronological periods, ceramic paste recipes and firing conditions. Correlation of archaeological and archaeometry data of ceramics from the Glinjeni II-La Șanț site gives us the possibility to differ earlier and later chronological markers in the paste recipes of pottery of 10th–beginning of 7th cc. BC in the region of the Middle Dniester basin.

~23% rear side poly-Si/SiO2 passivated silicon solar cell with optimized ion-implanted boron emitter and screen-printed contacts

We report on the understanding and optimization of ion-implanted boron emitters in combination with screen-printed contacts to produce very low recombination current density and high-efficiency cells with rear poly-Si/SiO2 passivated contact. Due to high bulk lifetime in n-base Si and very low recombination in the rear-side n+ passivated contact, recombination in the emitter limits the efficiency potential of manufacturable single-side passivated contact cells. Emitter recombination is governed by recombination under metal contacts (J0e,metal) and in the passivated regions in between (J0e,pass). Emitter profile engineering and dielectric passivation can lower the J0e,pass while paste chemistry and firing conditions can reduce J0e,metal by altering the extent of screen-printing induced emitter surface etching and the percentage of unetched dielectric islands under the metal contacts. We optimized implanted boron profiles, surface concentration and passivation, and junction depth in the sheet resistance range of 48-200 Ω/□ and achieved very low J0e,pass (<15 fA/cm2) for textured boron emitters with Rsheet≥140 Ω/□. Additionally, screen-printing paste and firing optimization reduced J0e,metal from 1172 to 707 fA/cm2 on 170 Ω/□ boron emitter by virtually eliminating emitter surface etching and creating distributed local contacts with unetched dielectric islands (~40% area coverage) underneath the grid lines. This resulted in very low total metallized J0e of 30 fA/cm2 with 3% metal coverage. Our device modeling showed the optimum boron sheet resistance is in the range of 120-170 Ω/□. The simulated results were validated by fabricating fully screen-printed 100 cm2 22.9% efficient bifacial cell with n+ poly-Si/SiO2 passivated rear contact.

Negligible Quantities of Particulate Low-Temperature Pyrogenic Carbon Reach the Atlantic Ocean via the Amazon River.

Particulate pyrogenic carbon (PyC) transported by rivers and aerosols, and deposited in marine sediments, is an important part of the carbon cycle. The chemical composition of PyC is temperature dependent and levoglucosan is a source‐specific burning marker used to trace low‐temperature PyC. Levoglucosan associated to particulate material has been shown to be preserved during riverine transport and marine deposition in high‐ and mid‐latitudes, but it is yet unknown if this is also the case for (sub)tropical areas, where 90% of global PyC is produced. Here, we investigate transport and deposition of levoglucosan in suspended and riverbed sediments from the Amazon River system and adjacent marine deposition areas. We show that the Amazon River exports negligible amounts of levoglucosan and that concentrations in sediments from the main Amazon tributaries are not related to long‐term mean catchment‐wide fire activity. Levoglucosan concentrations in marine sediments offshore the Amazon Estuary are positively correlated to total organic content regardless of terrestrial or marine origin, supporting the notion that association of suspended or dissolved PyC to biogenic particles is critical in the preservation of PyC. We estimate that 0.5–10 × 106 g yr−1 of levoglucosan is exported by the Amazon River. This represents only 0.5–10 ppm of the total exported PyC and thereby an insignificant fraction, indicating that riverine derived levoglucosan and low‐temperature PyC in the tropics are almost completely degraded before deposition. Hence, we suggest caution in using levoglucosan as tracer for past fire activity in tropical settings near rivers.

Ecological resilience and vegetation transition in the face of two successive large wildfires

Abstract Wildfire can both promote and erode resilience to future disturbances in fire‐adapted ecosystems. Through a combination of past fire exclusion and climate change, fire patterns and successional trajectories are shifting with potentially negative consequences for forest resilience. In particular, high‐severity short‐interval reburns can lead to permanent transitions from forested to persistent non‐forested ecosystems. To test conditions under which wildfires promote resilience or initiate vegetation transitions we leveraged high‐resolution LiDAR data, field data and a natural experiment where two uncharacteristically severe wildfires burned the same area in California's Sierra Nevada mountains. Specifically, we evaluate what factors influence resistance to high‐severity reburn and whether early forest recovery is evident following vegetation transition. Our findings indicate that topography and vegetative structure influenced resistance to high‐severity effects of a second wildfire and that environmental heterogeneity played an important role. Forests that survived the initial burn were most resistant to subsequent high‐severity fire when they were characterized by relatively dense but heterogeneous upper strata and a sparse understorey, located in variable and mesic terrain and burned under milder fire weather conditions. Early seral vegetation was most likely to resist repeat high‐severity fire and potentially continue post‐fire forest recovery when it was located in variable and mesic terrain and was characterized by relatively sparse understorey vegetation and a heterogeneous subcanopy. Some early seral areas that reburned at lower severity showed signs of conifer forest recovery. Vegetation structure and composition of areas that repeatedly burned at high severity are consistent with a transition to persistent shrubland or hardwood forests. Synthesis. Short‐interval reburns close to historical fire intervals but of unusually high burn severity can create challenges for maintaining resilient forests, as sequential fires can expand upon and stabilize non‐forest vegetation. However, forest communities that survive such disturbances appear partially restored with increased structural heterogeneity and greater resistance to future high‐severity fire. If climate and fire regime trends continue, we are likely to see broadscale shifts towards vegetation types and species able to recover quickly from high‐severity fire at the expense of forests and species resistant to frequent low‐severity fire.