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Historical and contemporary policies and practices, including the suppression of lightning-ignited fires and the removal of intentional fires ignited by Indigenous peoples, have resulted in over a century of fire exclusion across many of the USA’s landscapes. Within many designated wilderness areas, this intentional exclusion of fire has clearly altered ecological processes and thus constitutes a fundamental and ubiquitous act of trammeling . Through a framework that recognizes four orders of trammeling , we demonstrate the substantial, long-term, and negative effects of fire exclusion on the natural conditions of fire-adapted wilderness ecosystems. In order to un trammel more than a century of fire exclusion, the implementation of active programs of intentional burning may be necessary across some wilderness landscapes. We also suggest greater recognition and accommodation of Indigenous cultural burning, a practice which Tribes used to shape and maintain many fire-adapted landscapes for thousands of years before Euro-American colonization, including landscapes today designated as wilderness. Human-ignited fire may be critical to restoring the natural character of fire-adapted wilderness landscapes and can also support ecocultural restoration efforts sought by Indigenous peoples.

Yellow pine ( Pinus spp. L.) and mixed conifer (YPMC) forests of California, USA (Alta California), have been negatively affected since Euro-American settlement by a century or more of logging, fire exclusion, and other human activities. The YPMC forests in northwestern Mexico (northern Baja California) are found in the same climate zone as those of Alta California and support mostly the same dominant species, yet they are much less degraded, having suffered little logging and only 30 years of fire suppression. As such, the Baja California forests are believed to more closely approximate pre-Euro-American settlement conditions, and they have been proposed as reference ecosystems for restoration and management of Alta California forests. We studied fire severity trends in the Sierra de San Pedro Mártir National Park (SSPMNP), which supports the largest area of YPMC forest in Baja California, to determine whether fire severity is rising over the last three decades in the same manner that it is rising in the Sierra Nevada of Alta California. We used LANDSAT data to identify 32 fires that burned 26 529 ha in the Sierra de San Pedro Mártir National Park in the period 1984 to 2010. Of this, 1993 ha burned in YPMC forest types in 17 fires. We found no temporal trends in forest burned area or in the proportion of high severity fire, but we did find that the mean size of high severity patches within fires is rising. In the SSPMNP, the overall proportion of fire area burned at high severity averaged 3 % in both yellow pine and mixed conifer forests. We found no significant autoregressive effects of year in any of our analyses, but the year with the most burned area occurred after drier-than-average periods. In the SSPMNP data, there was no correlation between burned area and proportion of high severity fire; we interpreted this to mean that differences in fuels in SSPMNP were more important to fire behavior than weather conditions. The SSPMNP continues to burn at very low severities, even after 30 years of effective suppression of lightning-ignited fires. This is in stark contrast to similar forests in Alta California, which are experiencing fires of sizes and severities that fall far outside the historical range of variation. Current fire severities in the SSPMNP are very similar to the levels of severity described for Alta California YPMC forests before Euro-American settlement. Nonetheless, fire suppression policies in Mexican national parks in northern Baja California are causing increases in forest fuels and may be the cause of recent increases in high severity patch size. Current wildfire trends in YPMC forests in Alta California should serve as a warning to Mexican managers that continued fire exclusion in the Baja California YPMC forests is a recipe for ecological disaster in these unique and important ecosystems.

Whether ignited by lightning or by Native Americans, fire once shaped many North American ecosystems. Euro-American settlement and 20th-century fire suppression practices drastically altered historic fire regimes, leading to excessive fuel accumulation and uncharacteristically severe wildfires in some areas and diminished flammability resulting from shifts to more fire-sensitive forest species in others. Prescribed fire is a valuable tool for fuel management and ecosystem restoration, but the practice is fraught with controversy and uncertainty. Here, we summarize fire use in the forests and woodlands of North America and the current state of the practice, and explore challenges associated with the use of prescribed fire. Although new scientific knowledge has reduced barriers to prescribed burning, societal aversion to risk often trumps known, long-term ecological benefits. Broader implementation of prescribed burning and strategic management of wildfires in fire-dependent ecosystems will require improved integration of science, policy, and management, and greater societal acceptance through education and public involvement in land-management issues.

The drivers of background tree mortality rates—the typical low rates of tree mortality found in forests in the absence of acute stresses like drought—are central to our understanding of forest dynamics, the effects of ongoing environmental changes on forests, and the causes and consequences of geographical gradients in the nature and strength of biotic interactions. To shed light on factors contributing to background tree mortality, we analyzed detailed pathological data from 200,668 tree-years of observation and 3,729 individual tree deaths, recorded over a 13-yr period in a network of old-growth forest plots in California's Sierra Nevada mountain range. We found that: (1) Biotic mortality factors (mostly insects and pathogens) dominated (58%), particularly in larger trees (86%). Bark beetles were the most prevalent (40%), even though there were no outbreaks during the study period; in contrast, the contribution of defoliators was negligible. (2) Relative occurrences of broad classes of mortality factors (biotic, 58%; suppression, 51%; and mechanical, 25%) are similar among tree taxa, but may vary with tree size and growth rate. (3) We found little evidence of distinct groups of mortality factors that predictably occur together on trees. Our results have at least three sets of implications. First, rather than being driven by abiotic factors such as lightning or windstorms, the \"ambient\" or \"random\" background mortality that many forest models presume to be independent of tree growth rate is instead dominated by biotic agents of tree mortality, with potentially critical implications for forecasting future mortality. Mechanistic models of background mortality, even for healthy, rapidly growing trees, must therefore include the insects and pathogens that kill trees. Second, the biotic agents of tree mortality, instead of occurring in a few predictable combinations, may generally act opportunistically and with a relatively large degree of independence from one another. Finally, beyond the current emphasis on folivory and leaf defenses, studies of broad-scale gradients in the nature and strength of biotic interactions should also include biotic attacks on, and defenses of, tree stems and roots.

The increasing extent of wildfires has prompted investigation into alternative fire management approaches to complement the traditional strategies of fire suppression and fuels manipulation. Wildfire prevention through ignition reduction is an approach with potential for success, but ignitions result from a variety of causes. If some ignition sources result in higher levels of area burned, then ignition prevention programmes could be optimised to target these distributions in space and time. We investigated the most common ignition causes in two southern California sub-regions, where humans are responsible for more than 95% of all fires, and asked whether these causes exhibited distinct spatial or intra-annual temporal patterns, or resulted in different extents of fire in 10–29-year periods, depending on sub-region. Different ignition causes had distinct spatial patterns and those that burned the most area tended to occur in autumn months. Both the number of fires and area burned varied according to cause of ignition, but the cause of the most numerous fires was not always the cause of the greatest area burned. In both sub-regions, power line ignitions were one of the top two causes of area burned: the other major causes were arson in one sub-region and power equipment in the other. Equipment use also caused the largest number of fires in both sub-regions. These results have important implications for understanding why, where and how ignitions are caused, and in turn, how to develop strategies to prioritise and focus fire prevention efforts. Fire extent has increased tremendously in southern California, and because most fires are caused by humans, ignition reduction offers a potentially powerful management strategy, especially if optimised to reflect the distinct spatial and temporal distributions in different ignition causes.

Fire is one of the most important natural disturbance processes in the western United States and ecosystems differ markedly with respect to their ecological and evolutionary relationships with fire. Reference fire regimes in forested ecosystems can be categorized along a gradient ranging from \"fuel-limited\" to \"climate-limited\" where the former types are often characterized by frequent, lower-severity wildfires and the latter by infrequent, more severe wildfires. Using spatial data on fire severity from 1984-2011 and metrics related to fire frequency, we tested how divergence from historic (pre-Euroamerican settlement) fire frequencies due to a century of fire suppression influences rates of high-severity fire in five forest types in California. With some variation among bioregions, our results suggest that fires in forest types characterized by fuel-limited fire regimes (e.g., yellow pine and mixed conifer forest) tend to burn with greater proportions of high-severity fire as either time since last fire or the mean modern fire return interval (FRI) increases. Two intermediate fire regime types (mixed evergreen and bigcone Douglas-fir) showed a similar relationship between fire frequency and fire severity. However, red fir and redwood forests, which are characterized by more climate-limited fire regimes, did not show significant positive relationships between FRI and fire severity. This analysis provides strong evidence that for fuel-limited fire regimes, lack of fire leads to increasing rates of high-severity burning. Our study also substantiates the general validity of \"fuel-limited\" vs. \"climate-limited\" explanations of differing patterns of fire effects and response in forest types of the western US.

Research in the last several years has indicated that fire size and frequency are on the rise in western U.S. forests. Although fire size and frequency are important, they do not necessarily scale with ecosystem effects of fire, as different ecosystems have different ecological and evolutionary relationships with fire. Our study assessed trends and patterns in fire size and frequency from 1910 to 2008 (all fires > 40 ha), and the percentage of high-severity in fires from 1987 to 2008 (all fires > 400 ha) on the four national forests of northwestern California. During 1910-2008, mean and maximum fire size and total annual area burned increased, but we found no temporal trend in the percentage of high-severity fire during 1987-2008. The time series of severity data was strongly influenced by four years with region-wide lightning events that burned huge areas at primarily low-moderate severity. Regional fire rotation reached a high of 974 years in 1984 and fell to 95 years by 2008. The percentage of high-severity fire in conifer-dominated forests was generally higher in areas dominated by smaller-diameter trees than in areas with larger-diameter trees. For Douglas-fir forests, the percentage of high-severity fire did not differ significantly between areas that re-burned and areas that only burned once (10% vs. 9%) when re-burned within 30 years. Percentage of high-severity fire decreased to 5% when intervals between first and second fires were >30 years. In contrast, in both mixed-conifer and fir/high-elevation conifer forests, the percentage of high-severity fire was less when re-burned within 30 years compared to first-time burned (12% vs. 16% for mixed conifer; 11% vs. 19% for fir/high-elevation conifer). Additionally, the percentage of high-severity fire did not differ whether the re-burn interval was less than or greater than 30 years. Years with larger fires and greatest area burned were produced by region-wide lightning events, and characterized by less winter and spring precipitation than years dominated by smaller human-ignited fires. Overall percentage of high-severity fire was generally less in years characterized by these region-wide lightning events. Our results suggest that, under certain conditions, wildfires could be more extensively used to achieve ecological and management objectives in northwestern California.

ContextIn fire-excluded forests across western North America, recent intense wildfire seasons starkly contrast with fire regimes of the past. The last 100 years mark a transition between pre-colonial and modern era fire regimes, providing crucial context for understanding future wildfire behavior.ObjectivesUsing the greatest time depth of digitized fire events in Canada, we identify distinct phases of wildfire regimes from 1919 to 2019 by evaluating changes in mapped fire perimeters (> 20-ha) across the East Kootenay region (including the southern Rocky Mountain Trench), British Columbia.MethodsWe detect transitions in annual number of fires, burned area, and fire size; explore the role of lightning- and human-caused fires in driving these transitions; and quantify departures from historical fire frequency at the regional level.ResultsRelative to historical fire frequency, fire exclusion has created a significant fire deficit in active fire regimes, with a minimum of 1–10 fires missed across 46.4-percent of the landscape. Fire was active from 1919 to 1939 with frequent and large fire events, but the regime was already altered by a century of colonization. Fire activity decreased in 1940, coinciding with effective fire suppression influenced by a mild climatic period. In 2003, the combined effects of fire exclusion and accelerated climate change fueled a shift in fire regimes of various forest types, with increases in area burned and mean fire size driven by lightning.ConclusionsThe extent of fire regime disruption warrants significant management and policy attention to alter the current trajectory and facilitate better co-existence with wildfire throughout this century.

With the development of computer technology, forest fire spread simulation using computers has gradually developed. According to the existing research on forest fire spread, the models established in various countries have typical regional characteristics. A fire spread model established in a specific region is only suitable for the local area, and there is still a great deal of uncertainty as to whether or not the established model is suitable for fire spread simulation for the same fuel in other regions. Although many fire spread models have been established, the fuel characteristics applicable to each model, such as the fuel loading, fuel moisture content, combustibility, etc., are not similar. It is necessary to evaluate the applicability of different fuel characteristics to different fire spread models. We combined ground investigation, historical data collection, model improvements, and statistical analysis to establish a multi-model forest fire spread simulation method (FIRER) that shows the burning time, perimeter, burning area, overlap area, and spread rate of fire sites. This method is a large-scale, high-resolution fire growth model based on fire spread in eight directions on a regular 30 m grid. This method could use any one of four different physical models (McArthur, Rothermel, FBP, and Wang Zhengfei (China)) for fire behavior. This method has an option to represent fire breaks from roads, rivers, and fire suppression. We can evaluate which model is more suitable in a specific area. This method was tested on a single historical lightning fire in the Daxing’an Mountains. Different scenarios were tested and compared: using each of the four fire behavior models, with fire breaks on or off, and with a single or suspected double fire ignition location of the historical fire. The results show that the Rothermel model is the best model in the simulation of the Hanma lightning fire; the overlap area is 5694.4 hm2. Meanwhile, the real fire area in FIRER is 5800.9 hm2; both the Kappa and Sørensen values exceed 0.8, providing high accuracy in fire spread simulations. FIRER performs well in the automatic identification of fire break zones and multiple ignited points. Compared with FARSITE, FIRER performs well in predicting accuracy. Compared with BehavePlus, FIRER also has advantages in simulating large-scale fire spread. However, the complex data preparation stage of FIRER means that FIRER still has great room for improvement. This research provides a practical basis for the comparison of the practicability and applicability of various fire spread models and provides more effective practical tools and a scientific basis for decision-making and the management of fighting forest fires.

Prescribed fire is widely accepted as a conservation tool because fire is essential to the maintenance of native biodiversity in many terrestrial communities. Approaches to this land-management technique vary greatly among continents, and sharing knowledge internationally can inform application of prescribed fire worldwide. In North America, decisions about how and when to apply prescribed fire are typically based on the historical-fire-regime concept (HFRC), which holds that replicating the pattern of fires ignited by lightning or preindustrial humans best promotes native species in fire-prone regions. The HFRC rests on 3 assumptions: it is possible to infer historicalfire regimes accurately; fire-suppressed communities are ecologically degraded and reinstating historical fire regimes is the best course of action despite the global shift toward novel abiotic and biotic conditions. We examined the underpinnings of these assumptions by conducting a literature review on the use of historical fire regimes to inform the application of prescribed fire. We found that the practice of inferring historical fire regimes for entire regions or ecosystems often entails substantial uncertainty and can yield equivocal results; ecological outcomes of fire suppression are complex and may not equate to degradation, depending on the ecosystem and context; and habitat fragmentation, invasive species, and other modern factors can interact with fire to produce novel and in some cases negative ecological outcomes. It is therefore unlikely that all 3 assumptions will befully upheld for any landscape in which prescribed fire is being applied. Although the HFRC is a valuable starting point, it should not be viewed as the sole basisfor developing prescribed fire programs. Rather, fire prescriptions should also account for other specific, measurable ecologica parameters on a case-by-case basis. To best achieve conservation goals, researchers should seek to understand contemporary fire-biota interactions across trophic levels, functional groups, spatial and temporal scales, and management contexts. Los incendios prescritos están aceptados ampliamente como una herramienta de la conservación pues elfuego es esencial para el mantenimiento de la biodiversidad nativa en muchas comunidades terrestres. Las estrategias de esta técnica de manejo de suelos varían enormemente entre continentes y compartir el conocimiento a nivel internacional puede informar a la aplicación de los incendios prescritos en todo el mundo. En América del Norte las decisiones sobre cómo y cuándo aplicar los incendios prescritos están basadas típicamente en el concepto de régimen histórico de incendios (CRHI), el cual sostiene que replicar el patrón de incendios iniciados por relámpagos o humanos pre-industriales promueve de mejor manera a las especies nativas en las regiones propensas a incendios. El CRHI se apoya en tres suposiciones: es posible inferir con certeza los regímenes históricos de incendios; las comunidades contenidas por incendios son degradas ecológicamente; y la reincorporación de los regímenes históricos de incendios es la mejor acción a seguir a pesar del cambio global hacia las condiciones bióticas y abióticas innovadoras. Examinamos los apuntalamientos de estas suposiciones por medio de una revisión a la literatura sobre el uso de regímenes históricos de incendios para informar a la aplicación de incendios prescritos. Encontramos que la práctica de la inferencia de los regímenes históricos de incendios para regiones o ecosistemas enteros implica generalmente una incertidumbre sustancial y puede producir resultados equivocados; los resultados ecológicos de la contención por incendios son complejos y pueden no ser iguales a la degradación, dependiendo del ecosistema y el contexto; y la fragmentación del habitat, las especies invasoras, y otros factores modernos pueden interactuar con los incendios para producir resultados ecológicos innovadores y, en algunos casos, negativos. Por lo tanto, no es probable que las tres suposiciones se mantengan totalmente para cualquier paisaje en el que los incendios prescritos se estén aplicando. Aunque el CRHI es un punto de inicio valioso, no debe ser visto como el fundamento único para desarrollar programas de incendios prescritos. En su lugar, la prescripción de incendios debería considerar otros parámetros ecológicos mediblesy específicos con una base de casapor-caso. Para conseguir de manera más efectiva los objetivos de conservación, los investigadores deberían buscar entender las interacciones contemporáneas entre los incendios y la biota en todos los niveles tróficos, grupos funcionales, escalas espaciales y temporales, y en contextos de manejo.