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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.)

Fuel moisture content (FMC) is a crucial driver of forest fires in many regions world-wide. Yet, the dynamics of FMC in forest canopies as well as their physiological and environmental determinants remain poorly understood, especially under extreme drought. We embedded a FMC module in the trait-based, plant-hydraulic SurEau-Ecos model to provide innovative process-based predictions of leaf live fuel moisture content (LFMC) and canopy fuel moisture content (CFMC) based on leaf water potential ( ψ Leaf ). SurEau-Ecos-FMC relies on pressure-volume (p-v) curves to simulate LFMC and vulnerability curves to cavitation to simulate foliage mortality. SurEau-Ecos-FMC accurately reproduced ψ Leaf and LFMC dynamics as well as the occurrence of foliage mortality in a Mediterranean Quercus ilex forest. Several traits related to water use (leaf area index, available soil water, and transpiration regulation), vulnerability to cavitation, and p-v curves (full turgor osmotic potential) had the greatest influence on LFMC and CFMC dynamics. As the climate gets drier, our results showed that drought-induced foliage mortality is expected to increase, thereby significantly decreasing CFMC. Our results represent an important advance in our capacity to understand and predict the sensitivity of forests to wildfires., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation. This article has been contributed to by US Government employees and their work is in the public domain in the USA.)

Modeling wildfire activity is crucial for informing science-based risk management and understanding the spatiotemporal dynamics of fire-prone ecosystems worldwide. Models help disentangle the relative influences of different factors, understand wildfire predictability, and provide insights into specific events. Here, we develop Firelihood, a two-component, Bayesian, hierarchically structured, probabilistic model of daily fire activity, which is modeled as the outcome of a marked point process: individual fires are the points (occurrence component), and fire sizes are the marks (size component). The space-time Poisson model for occurrence is adjusted to gridded fire counts using the integrated nested Laplace approximation (INLA) combined with the stochastic partial differential equation (SPDE) approach. The size model is based on piecewise-estimated Pareto and generalized Pareto distributions, adjusted with INLA. The Fire Weather Index (FWI) and forest area are the main explanatory variables. Temporal and spatial residuals are included to improve the consistency of the relationship between weather and fire occurrence. The posterior distribution of the Bayesian model provided 1,000 replications of fire activity that were compared with observations at various temporal and spatial scales in Mediterranean France. The number of fires larger than 1 ha across the region was coarsely reproduced at the daily scale, and was more accurately predicted on a weekly basis or longer. The regional weekly total number of larger fires (10-100 ha) was predicted as well, but the accuracy degraded with size, as the model uncertainty increased with event rareness. Local predictions of fire numbers or burned areas also required a longer aggregation period to maintain model accuracy. The estimation of fires larger than 1 ha was also consistent with observations during the extreme fire season of the 2003 unprecedented heat wave, but the model systematically underrepresented large fires and burned areas, which suggests that the FWI does not consistently rate the actual danger of large fire occurrence during heat waves. Firelihood enabled a novel analysis of the stochasticity underlying fire hazard, and offers a variety of applications, including fire hazard predictions for management and projections in the context of climate change., (© 2021 by the Ecological Society of America.)

Identifying the links between fire danger metrics and fire activity is critical in various operational and research fields. A common methodology consists in analysing the relationship between cumulative burnt areas and fire danger metrics. Building on this approach, it has been proposed that fuel moisture content (FMC) drives fire activity in some ecosystems through between one and three breakpoints corresponding to the onset or saturation of fire activity. We demonstrate, through two different approaches, that this methodology is incorrect, because it is biased by the frequency distribution of FMC values. From comparison with a neutral fire distribution and correction for the frequency bias, we show that cumulative burnt area breakpoints are spurious: an upper breakpoint might exist (but would be higher than expected), while no evidence of reduced fire danger was detected for the lowest values of FMC (on the contrary, a secondary increase was detected). Our findings clearly suggest that previous breakpoints resulting from this methodology should be considered with caution, as erroneous conclusions regarding fire danger breakpoints could have major consequences for both fire safety and science outcomes. Finally, we discuss widespread confusion between fire danger breakpoints and fire danger levels, which explains most previous erroneous conclusions. Identifying breakpoints in fire danger metrics is critical for various applications. We show that the widespread cumulative burnt area method can lead to spurious breakpoints and recommend that previous datasets be reanalysed. [ABSTRACT FROM AUTHOR]

Wind and slope are commonly accepted to be major environmental factors affecting the manner in which wildfires propagate. Fire-line width has been observed as having a significant effect on fire behaviour in some experimental fires. Most wildfire behaviour models and fire behaviour prediction systems take wind and slope effects into account when determining the rate of spread, but do not take into account the influence of fire width on the coupled effects of slope and wind. In the present study, the effect of topographic slope on rate of spread under weak (1 m s[sup -1]), moderate (5 m s[sup -1]) and strong (12 m s[sup -1]) wind speeds is investigated for two different initial fire widths (20 and 50 m) in a typical Mediterranean garrigue, using the coupled atmosphere-fire HIGRAD-FIRETEC model. The results show non-trivial combined effects and suggest a strong effect of fire width under low-wind conditions, especially for steep slopes. Simulated spread rates were compared with predictions of existing models of operational systems and a reasonable agreement was found. Additional exploratory simulations of fire behaviour in small canyons are provided. These simulations show how combined effects of wind, slope and fire-front size can induce different fire behaviours that operational models could fail to predict and provide insight that could be valuable for analysis of blow-up fires. These preliminary results also suggest that 3D physically based models could be used in the future to investigate how operational models can include non-local effects of fire propagation. The combined effects of wind, slope and fire size were investigated with the physically based model FIRETEC. This study identifies configurations where current operational models could fail to predict hazardous fire behaviour. [ABSTRACT FROM AUTHOR]

Abstract: This study was conducted to increase understanding of possible roles and importance of local three-dimensionality in the forward spread of wildfire models. A suite of simulations was performed using a coupled atmosphere–fire model, HIGRAD/FIRETEC, consisting of different scenarios that varied in domain width and boundary condition implementation. A subset of the simulations was strictly two-dimensional in the streamwise and vertical directions, while another subset of simulations involved igniting a finite-length fireline. The remaining simulations were all three-dimensional and employed periodic boundary conditions in the cross-stream direction and a fireline spanning the entire cross-stream extent of the domain. The three-dimensional periodic simulations were compared with the two-dimensional simulations, and then briefly with the finite-length fireline simulations. The two-dimensional scenarios were constrained in their ability to represent inherently three-dimensional physical phenomena such as horizontal flow penetrating through the fireline between plumes of rising hot gas, and cross-stream heterogeneity in the windfield. Elimination of these three-dimensional flow patterns in two-dimensional simulations resulted in over prediction of spread rates in low velocity situations and under predicted spread rates in high wind speed scenarios. In the three-dimensional simulations, local cross-stream heterogeneities in temperature and velocities lead to penetration of hot gases through the fireline and onto unburned fuel. Three-dimensional fires presented a positive correlation between increasing ambient wind speed and rate of spread. Further investigation of finite length fires is required in order to understand the ramifications of fireline curvature. [Copyright &y& Elsevier]