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Wildfires are a global crisis, but current fire models fail to capture vegetation response to changing climate. With drought and elevated temperature increasing the importance of vegetation dynamics to fire behavior, and the advent of next generation models capable of capturing increasingly complex physical processes, we provide a renewed focus on representation of woody vegetation in fire models. Currently, the most advanced representations of fire behavior and biophysical fire effects are found in distinct classes of fine-scale models and do not capture variation in live fuel (i.e. living plant) properties. We demonstrate that plant water and carbon dynamics, which influence combustion and heat transfer into the plant and often dictate plant survival, provide the mechanistic linkage between fire behavior and effects. Our conceptual framework linking remotely sensed estimates of plant water and carbon to fine-scale models of fire behavior and effects could be a critical first step toward improving the fidelity of the coarse scale models that are now relied upon for global fire forecasting. This process-based approach will be essential to capturing the influence of physiological responses to drought and warming on live fuel conditions, strengthening the science needed to guide fire managers in an uncertain future., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

Post-fire predictions of forest recovery under future climate change and management actions are necessary for forest managers to make decisions about treatments. We applied the Climate-Forest Vegetation Simulator (Climate-FVS), a new version of a widely used forest management model, to compare alternative climate and management scenarios in a severely burned multispecies forest of Arizona, USA. The incorporation of seven combinations of General Circulation Models (GCM) and emissions scenarios altered long-term (100 years) predictions of future forest condition compared to a No Climate Change (NCC) scenario, which forecast a gradual increase to high levels of forest density and carbon stock. In contrast, emissions scenarios that included continued high greenhouse gas releases led to near-complete deforestation by 2111. GCM-emissions scenario combinations that were less severe reduced forest structure and carbon stock relative to NCC. Fuel reduction treatments that had been applied prior to the severe wildfire did have persistent effects, especially under NCC, but were overwhelmed by increasingly severe climate change. We tested six management strategies aimed at sustaining future forests: prescribed burning at 5, 10, or 20-year intervals, thinning 40% or 60% of stand basal area, and no treatment. Severe climate change led to deforestation under all management regimes, but important differences emerged under the moderate scenarios: treatments that included regular prescribed burning fostered low density, wildfire-resistant forests composed of the naturally dominant species, ponderosa pine. Non-fire treatments under moderate climate change were forecast to become dense and susceptible to severe wildfire, with a shift to dominance by sprouting species. Current U.S. forest management requires modeling of future scenarios but does not mandate consideration of climate change effects. However, this study showed substantial differences in model outputs depending on climate and management actions. Managers should incorporate climate change into the process of analyzing the environmental effects of alternative actions.

Contemporary wildfires in southwestern US ponderosa pine forests can leave uncharacteristically large patches of tree mortality, raising concerns about the lack of seed-producing trees, which can prevent or significantly delay ponderosa pine regeneration. We established 4-ha plots in high-severity burn patches in two Arizona wildfires, the 2000 Pumpkin and 2002 Rodeo-Chediski Fires, to determine if: (1) distance from forest edge influences the density and spatial patterns of regenerating ponderosa pine and sprouting tree species, (2) interactions with re-sprouting trees affect spatial patterns of ponderosa pine regeneration, and (3) distance from forest edge and species competition affect regenerating ponderosa pine height. Plots were located in high-severity burn patches (defined as 100% tree mortality) and either adjacent to residual live forest edges (edge plots), or >200 m from any residual live trees (interior plots). We found higher ponderosa pine regeneration densities in the edge plots (13–154 (median = 69) stems ha −1 ) than the interior plots (12–124 (median = 29) stems ha −1 ) on both wildfires, but no differences in spatial patterns between edge and interior plots. Ponderosa pine regeneration displayed patterns of small-scale spatial aggregation in all plots, except one edge and one interior plot on the Pumpkin Fire, which displayed random distributions. These patterns suggest both short- and long-distance dispersal play important roles in ponderosa pine regeneration in high-severity burn patches. Sprouting trees dominated tree regeneration on the Rodeo-Chediski Fire, but they were spatially independent of ponderosa pine and did not influence ponderosa pine height. Regenerating ponderosa pine height was positively correlated with neighboring ponderosa pine densities and height, suggesting that intraspecific facilitation or similar habitat preferences occur in high-severity burn patches. Collectively, these results indicate that ponderosa pines are re-establishing with heterogeneous spatial patterns in large high-severity burn patches, but often with low densities. Also, ponderosa pine regeneration could be more strongly influenced by intraspecific facilitation than interspecific competition from dense sprouting species. Future forest spatial patterns and composition are still unclear, but at this stage of development, these heterogeneous patches, characterized by drought-tolerant sprouting species or low pine densities, could be more resilient to climate change and severe wildfires than the overly-dense ponderosa pine forests that were present before the wildfires. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that bark beetles and fires can be interacting disturbances, whereby bark beetle-caused tree mortality can alter the risk and severity of subsequent wildland fires. However, there remains considerable uncertainty around the type and magnitude of the interaction between fires following bark beetle attacks, especially in drier forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). We used a full factorial design across a range of factors thought to control bark beetle-fire interactions, including the temporal phase of the outbreak, level of mortality, and wind speed. We used a three-dimensional physics-based model, HIGRAD/FIRETEC, to simulate fire behavior in fuel beds representative of 60 field plots across five national forests in northern Arizona, USA. The plots were dominated by ponderosa pine, and encompassed a gradient of bark beetle-caused mortality due to a mixture of both Ips and Dendroctonus species. Non-host species included two sprouting species, Gambel oak (Quercus gambelii Nutt.) and alligator juniper (Juniperus deppeana Steud.), as well as other junipers and pinyon pine (Pinus edulis Engelm.). The simulations explicitly accounted for the modifications of fuel mass and moisture distribution caused by bark beetle-caused mortality. We first analyzed the influence of the outbreak phase, level of mortality, and wind speed on the severity of a subsequent fire, expressed as a function of live and dead canopy fuel consumption. We then computed a metric based on canopy fuel loss to characterize whether bark beetles and fire are linked disturbances and, if they are, if the linkage is antagonistic (net bark beetle and fire severity being less than if the two disturbances occurred independently) or synergistic (greater combined effects than independent disturbances). Both the severity of a subsequent fire and whether bark beetles and fire are linked disturbances depended on the outbreak phase of the bark beetle mortality and attack severity, as well as the fire weather (here, wind). Greater fire severity and synergistic interactions were generally associated with the "red phase" (when dead needles remain on trees). In contrast, during the "gray phase" (when dead needles had fallen to the ground), fire severity was either similar to, or less than, green-phase fires and interactions were generally antagonistic, but included both synergistic and neutral interactions. The simulations also revealed that the magnitude of the linkage between these two disturbances was smaller for fires occurring during high wind conditions, especially in the red phase. This complexity might be a reason for the contrasted or controversial perception of bark beetle-fire interactions reported in the literature, since both fire severity and the type and magnitude of the linkage can vary strongly among studies. These results suggest that, for fires burning in the gray phase following moderate levels of mortality, bark beetle-caused mortality may buffer rather than exacerbate fire severity. However, for fires burning under high wind speeds, regardless of the outbreak phase or level of mortality, the near complete loss of canopy fuels may push this ecosystem into an alternative state dominated by sprouting species. Estudios previos han sugerido que los escarabajos de la corteza y el fuego pueden ser disturbios interactivos, por lo que la mortalidad de árboles causada por estos escarabajos puede alterar el riesgo y la severidad de incendios subsecuentes. Sin embargo, una considerable incertidumbre persiste en torno al tipo y magnitud de la interacción entre los incendios que siguen al ataque de insectos, especialmente en tipos de bosques secos como los dominados por pino ponderosa (Pinus ponderosa Lawson & C. Lawson). Usamos un diseño factorial a través de un rango de factores que pensamos controlaban la interacción entre el escarabajo de la corteza y los incendios, incluyendo la fase temporal del estallido del insecto, el nivel de mortalidad y la velocidad del viento. Para simular el comportamiento del fuego en camas de combustible en 60 parcelas representativas a lo largo de cinco bosques nacionales en el norte de Arizona, EEUU, usamos el modelo físico tridimensional HIGRAD/FIERETEC. Las parcelas estaban dominadas por pino ponderosa, y abarcaban un gradiente de mortalidad causado por escarabajos de diversas especies, tanto de los géneros Ips como de Dendroctonus. Las especies no hospedantes de estos escarabajos incluían a dos rebrotantes como el roble de Gambela (Quercus gambelii Nutt.) y el táscate (Juniperus deppeana Steud.), como así también otras especies de juníperos y de pino edulis (Pinus edulis Engelm.). Las simulaciones representaron explícitamente las modificaciones de la biomasa combustible y la distribución de humedad causada por la mortalidad inducida por estos escarabajos. Primeramente analizamos la influencia de la fase del estallido poblacional, el nivel de mortalidad, y la velocidad del viento, en la severidad de un incendio subsecuente, expresado en función del consumo de combustible vivo y muerto del dosel. Luego computamos una medida basada en la pérdida del combustible del dosel para caracterizar si los escarabajos de la corteza y el fuego son disturbios relacionados entre sí y, si lo son, si esa relación es antagónica (daño por el escarabajo y severidad del fuego son menores que cuando ambos disturbios ocurren independientemente), o sinérgica (mayores efectos combinados que si ocurriesen independientemente). Ambos, la severidad de un incendio subsecuente y si los escarabajos y el fuego son disturbios relacionados, dependen de la fase del estallido, de la mortalidad, y de la severidad del ataque, y también del entorno meteorológico del fuego, en este caso la velocidad del viento. Una mayor severidad e interacciones sinérgicas fueron generalmente asociadas con la fase "roja" (cuando las acículas muertas permanecen en los árboles). En contraste con esto, durante la fase "gris" (cuando las acículas han caído al suelo), la severidad del fuego fue similar a, o menor que, las fases de fuego "verde" y las interacciones fueron generalmente antagónicas, pero incluyeron también interacciones sinérgicas y neutras. Las simulaciones también revelaron que la magnitud entre ambos disturbios fue menor durante la ocurrencia de vientos fuertes, especialmente en la fase roja. Esta complejidad podría ser una razón sobre la percepción controversial o contrastante de las interacciones entre estos escarabajos y el fuego reportados en la literatura, dado que ambos, la severidad y el tipo y magnitud de la relación, pueden variar fuertemente entre estudios. Estos resultados sugieren que, para incendios que queman en la fase gris seguidos de moderados niveles de mortalidad, la causa de mortalidad debida a los escarabajos puede atemperar, más que exacerbar, la severidad del fuego. Sin embargo, para incendios que queman bajo fuertes velocidades de viento, independientemente de la fase del estallido o el nivel de mortalidad, el casi completo consumo del dosel puede derivar este ecosistema a un estado alternativo dominado por especies rebrotantes. [ABSTRACT FROM AUTHOR]

Increasing size and severity of wildfires have led to an interest in the effectiveness of forest fuels treatments on reducing fire severity and post-wildfire fuels. Our objective was to contrast stand structure and surface fuel loadings on treated and untreated sites within the 2002 Rodeo-Chediski Fire area. Data from 140 plots on seven paired treated-untreated sites indicated that pre-wildfire treatments reduced fire severity compared with untreated sites. In 2011, coarse woody debris loading (woody material >7.62 cm in diameter) was 257% higher and fine woody debris (woody material <7.62 cm) was 152% higher on untreated sites than on treated sites. Yet, in spite of higher levels of coarse woody debris on untreated sites, loadings did not exceed recommended ranges based on published literature and many treated sites fell below recommendations. By 2011, basal area and stand density on treated sites and stand density on untreated sites met management guidelines for ponderosa pine forests, but untreated sites had basal areas well below recommendations. Snags declined over this period and only three plots had snags that met minimum size and density requirements for wildlife habitat by 2011. The effects of pre-wildfire treatments are long-lasting and contribute to changes in both overstorey and understorey fuel complexes. Pre-wildfire thinning and fuel reduction treatments increased tree survival compared with untreated sites following the 2002 Rodeo-Chediski Fire. A decade later, treated sites have stand basal areas that fall within target ranges for increasing resiliency to future fires. Surface fuels were within acceptable levels and are unlikely to increase substantially. [ABSTRACT FROM AUTHOR]

The rationale for seeding following high-severity wildfires is to enhance plant cover and reduce bare ground, thus decreasing the potential for soil erosion and non-native plant invasion. However, experimental tests of the effectiveness of seeding in meeting these objectives in forests are lacking. We conducted three experimental studies of the effectiveness of seeding with non-native and native species following three Arizona wildfires. Seeding treatments were largely ineffective in increasing vegetative cover or decreasing exposed bare ground. At one treatment at one fire, wheat seeding at the Warm Fire, senesced seeded annuals increased litter cover and resulted in lower bare ground values than unseeded controls. Only on one fire, the Warm Fire, did seeded non-native annuals establish well, resulting in 20-29% vegetative cover. On the other two fires, seeded cereal grains accounted foro3% cover. At all fires, native seeded species contributed between <1 and ∼12% vegetative cover. Vegetative cover on all treatments, including unseeded treatments, was at or near 40% the first year following fire, at all three study sites. Non-native species richness and abundance did not differ among treatments at any fire. This study adds to growing evidence that post-fire seeding is ineffective in enhancing post-fire plant cover and reducing invasive non-native plants. [ABSTRACT FROM AUTHOR]

The rationale for seeding following high-severity wildfires is to enhance plant cover and reduce bare ground, thus decreasing the potential for soil erosion and non-native plant invasion. However, experimental tests of the effectiveness of seeding in meeting these objectives in forests are lacking. We conducted three experimental studies of the effectiveness of seeding with non-native and native species following three Arizona wildfires. Seeding treatments were largely ineffective in increasing vegetative cover or decreasing exposed bare ground. At one treatment at one fire, wheat seeding at the Warm Fire, senesced seeded annuals increased litter cover and resulted in lower bare ground values than unseeded controls. Only on one fire, the Warm Fire, did seeded non-native annuals establish well, resulting in 20-29% vegetative cover. On the other two fires, seeded cereal grains accounted for < 3% cover. At all fires, native seeded species contributed between < 1 and ∼12% vegetative cover. Vegetative cover on all treatments, including unseeded treatments, was at or near 40% the first year following fire, at all three study sites. Non-native species richness and abundance did not differ among treatments at any fire. This study adds to growing evidence that post-fire seeding is ineffective in enhancing post-fire plant cover and reducing invasive non-native plants. [ABSTRACT FROM AUTHOR]

Over the past three decades, wildfires in southwestern US ponderosa pine (Pinus ponderosa Lawson & C. Lawson) forests have increased in size and severity. These wildfires can remove large, contiguous patches of mature forests, alter dominant plant communities and increase woody debris, potentially altering fungal community composition. Additionally, post-fire conditions may shift dominant fungal functional groups from plant-symbiotic ectomycorrhizal (EM) fungi to more decomposer saprotrophic fungi. We investigated the long-term (13 years post-wildfire) effect of fire severity on 1) fungal sporocarp density, functional groups and community composition and 2) EM colonization and community composition from naturally regenerating ponderosa pine seedlings on the Pumpkin Fire that burned in 2000 in Arizona, USA. Plots were located in four burn severity classes: unburned, moderate-severity, and two high-severity (defined as 100% tree mortality) classes, either adjacent to residual live forest edges (edge plots), or >200 m from any residual live trees (interior plots). We found that high-severity burn plots had a unique sporocarp community composition, and a shift in dominant sporocarp functional groups, with 5–13 times lower EM sporocarp densities, and 4–7 times lower EM sporocarp species richness compared to unburned and moderate-severity plots. In contrast, saprotrophic sporocarp densities and richness were similar among burn severity classes, even with the large amount of coarse wood in the high-severity burn patches. Regenerating ponderosa pine seedlings had similar EM colonization and richness among severity classes, yet high-severity interior plots had a different community composition and a lower relative abundance of EM species compared to moderate-severity burn plots. Taken together, our results suggest that large patches of high-severity fire have long-term consequences for both EM sporocarp and root tip communities. Because EM fungal species vary in function, the limited species pool available in interior high-severity burn patches may influence pine recovery. • High-severity burns had a unique sporocarp community composition. • EM sporocarp densities were 5–13 times lower on high-severity vs. moderate burns. • Saprotrophic sporocarp density and richness were similar among treatments. • Pine seedlings >200 m from live trees had a different EM community composition. [ABSTRACT FROM AUTHOR]