Fire Residence Time Research Articles

Predictive Understanding of Links Between Vegetation and Soil Burn Severities Using Physics‐Informed Machine Learning

AbstractBurn severity is fundamental to post‐fire impact assessment and emergency response. Vegetation Burn Severity (VBS) can be derived from satellite observations. However, Soil Burn Severity (SBS) assessment—critical for mitigating hydrologic and geologic hazards—requires costly and laborious field recalibration of VBS maps. Here, we develop a physics‐informed Machine Learning model capable of accurately estimating SBS while revealing the intricate relationships between soil and vegetation burn severities. Our SBS classification model uses VBS, as well as climatological, meteorological, ecological, geological, and topographical wildfire covariates. This model demonstrated an overall accuracy of 89% for out‐of‐sample test data. The model exhibited scalability with additional data, and was able to extract universal functional relationships between vegetation and soil burn severities across the western US. VBS had the largest control on SBS, followed by weather (e.g., wind, fire danger, temperature), climate (e.g., annual precipitation), topography (e.g., elevation), and soil characteristics (e.g., soil organic carbon content). The relative control of processes on SBS changes across regions. Our model revealed nuanced relationships between VBS and SBS; for example, a similar VBS with lower wind speeds—that is, higher fire residence time—translates to a higher SBS. This transferrable model develops reliable and timely SBS maps using satellite and publicly accessible data, providing science‐based insights for managers and diverse stakeholders.

Sensitivity of Simulated Fire‐Generated Circulations to Fuel Characteristics During Large Wildfires

AbstractCoupled fire‐atmosphere models often struggle to simulate important fire processes like fire generated flows, deep flaming fronts, extreme updrafts, and stratospheric smoke injection during large wildfires. This study uses the coupled fire‐atmosphere model, WRF‐Fire, to examine the sensitivities of some of these phenomena to the modeled total fuel load and its consumption. Specifically, the 2020 Bear Fire and 2021 Caldor Fire in California's Sierra Nevada are simulated using three fuel loading scenarios (1X, 4X, and 8X LANDFIRE derived surface fuel), while controlling the fire rate of spread using observations. This approach helps isolate the fuel loading and consumption needed to produce fire‐generated winds and plume rise comparable to radar observations of these events. Increasing fuel loads and corresponding fire residence time in WRF‐Fire leads to deep plumes in excess of 10 km, strong vertical velocities of 40–45 m s−1, and combustion fronts several kilometers in width (in the along wind direction). These results indicate that LANDFIRE‐based surface fuel loads in WRF‐Fire likely under‐represent fuel loading, having significant implications for simulating landscape‐scale wildfire processes, associated impacts on spread, and fire‐atmosphere feedbacks.

Effects of Fire Frequency Regimes on Flammability and Leaf Economics of Non-Graminoid Vegetation

Fire is an ecological factor that strongly influences plant communities and functional traits. Communities respond differently to fire, either decreasing or increasing in flammability and resource acquisition strategies. This study aimed to investigate the influence of fire over traits associated with flammability and the plant economic spectrum in a stressful and infertile mountainous grassland located in the Espinhaço mountain range in Brazil. Non-graminoid plant species were sampled in 60 5 m × 5 m plots distributed in three fire frequency categories. We measured several traits related to flammability—leaf dry matter content (LDMC), twig dry matter content, leaf area, bark thickness, branching architecture, plant height, leaf toughness (LT), and specific leaf area (SLA). Traits responded differently to the increase in fire frequency. For instance, the LDMC and LT were lower while the SLA was higher at high fire frequencies, indicating a trend towards reduced heat release and fire residence time. This shift resulted in the dominance of plants with a relatively more acquisitive strategy. This study brings evidence that traits respond coordinately towards a reduction of flammability with the increase in fire frequency and are strong indicators of the filtering role that fire plays as a disturbance on rupestrian grassland vegetation.

Effects of large mammalian herbivory, previous fire, and year of burn on fire behavior in an African savanna

AbstractFire, herbivores, and climatic factors are all major drivers of savanna and grassland dynamics, and they interact in complex ways, which are still in the process of being explored. In particular, herbivores can reduce fire intensity by removal of biomass, and this could be reinforced by herbivores’ attraction to recently burned sites, although grassland resilience may limit the temporal depth of such effects. Fire temperature is the most common fire metric reported for grassland fire, but additional aspects of fire behavior can also be measured. Using a set of controlled, replicated experiments, we examined the effects of year of burn, herbivory by livestock and wildlife, previous burn, and weather history on fire behavior in an African savanna. Multiple measures of fire behavior (minimum fire temperature, flame front speed, fire residence time, maximum flame height, and flame length) in 36 controlled burns were positively intercorrelated. Burns conducted in 2018 were significantly cooler, especially at heights >0.5 m above the ground, than those in 2013, a wetter year with more grass fuel. Grass fuel loads and fire temperatures were reduced by the presence of livestock and wildlife. Our sampling methods did not for the most part reveal expected differences in fire temperatures or other behaviors between the reburned plots and those burned for the first time in 2018, with or without herbivores, suggesting strong postfire resilience in this semiarid savanna rangeland.

Perennial pasture grass invasion changes fire behaviour and recruitment potential of a native forb in a temperate Australian grassland

Invasive grasses can modify fire regimes of native ecosystems leading to changed ecosystem structure, composition, and functioning. Temperate grasslands in Australia are currently being invaded by a suite of exotic perennial pasture grasses, but their effects on ecosystems remain largely unknown. We aimed to determine the effect of invasion by the exotic perennial grass Phalaris aquatica on fire behaviour, as well as the regeneration potential of an endangered forb in temperate native grasslands in south-eastern Australia. Frequently burnt native grasslands invaded by exotic grasses were found to have two times more fuel than grasslands dominated by native grasses; in less-frequently burned native grasslands, exotic grasses contributed to fuel loads that were five times higher than native grasslands. Exotic-dominated grasslands burned differently than native grasslands; fire intensities were three times higher in exotic-dominated grasslands and had a wide variability in fire residence times. Soil heating was positively related to fire residence time but had no clear relationship with fire intensity. Seed germinability of Leucochrysum albicans var. tricolor (Hoary Sunray, Asteraceae) was reduced by exotic grass-fueled fire and increasing fire residence times. The observed changes in fire behaviour represent an invasion-driven shift in the ecosystem’s fire regime. By increasing fuel mass, fire residence time increased, and this influenced seed survival and subsequent germinability. Increased fire intensity following invasion highlights that invasive grasses can increase the fire-risk of grasslands. Maintaining native grasslands free of invasive pasture grasses therefore has environmental and fire-risk benefits.

Dynamics of topsoil carbon stocks after prescribed burning for pasture restoration in shrublands of the Central Pyrenees (NE-Spain)

Prescribed burning has been recently readopted as a management practice in the Central Pyrenees (NE-Spain) to stop shrub encroachment processes and recover pasturelands. The immediate effects of prescribed burning on soil C stocks and related biological properties and their evolution in the short-to mid-term after burning were assessed. The study was conducted during three autumnal prescribed burnings in the Central Pyrenees in the municipalities of Buisán, Asín de Broto and Yebra de Basa. At each site, the topsoil Ah horizon was sampled at soil depths of 0–1, 1–2 and 2–3 cm immediately before and immediately after burning. Additionally, seasonal samplings were conducted every 6 months up to one year in the case of the Asín and Yebra sites and up to 24 months at the Buisán site. The total soil organic C stock (SOCS) total N stock (NS), microbial biomass C (MBC), soil basal respiration (SR) and β-D-glucosidase activity were analyzed. The maximum temperatures recorded at the soil surface were 438 °C (Buisán), 768 °C (Asín) and 595 °C (Yebra). At the Buisán site, burning significantly decreased the SOCS (−52%), NS (−44%), MBC (−57%), SR (−72%) and glucosidase activity (−66%) at 0–1 cm depth, whereas fire had no direct effects on soil at the Asín and Yebra sites. The contrasting effects of burning on soil that were observed among sites were found to be related to differences in fire residence time. The prescribed fire at the Buisán site was on a plain slope under slow winds (<8 km h−1) at a burning rate of 0.64 ha h−1, which produced greater impacts on the soil properties than the burnings at the Asín and Yebra sites, where fire spread rapidly (2.72 and 1.43 ha h−1, respectively). At the Buisán site, the SOCS and NS recovered to the unburned values 24 months after burning. One year after burning, the SOCS at Asín were 60% higher than those of the unburned soils at 0–1 cm depth. At all sites a decreasing trend in soil biological activity in the short- and mid-term was observed. From the results it can be concluded that: 1) the direct effects of burning on soil are highly dependent on the environmental conditions, 2) in the mid-term, the reduction in soil biological activity and the incorporation of ashes and charred plant remains led to an increase in the SOCS of the burned soils.

Pyrogenic carbon from tropical savanna burning: production and stable isotope composition

Abstract. Widespread burning of mixed tree–grass ecosystems represents the major natural locus of pyrogenic carbon (PyC) production. PyC is a significant, pervasive and yet poorly understood "slow-cycling" form of carbon present in the atmosphere, hydrosphere, soils and sediments. We conducted 16 experimental burns on a rainfall transect through northern Australian savannas with C4 grasses ranging from 35 to 99% of total biomass. Residues from each fire were partitioned into PyC and further into recalcitrant (HyPyC) components, with each of these fluxes also partitioned into proximal components (&gt;125 μm), likely to remain close to the site of burning, and distal components (&lt;125 μm), likely to be transported from the site of burning. The median (range) PyC production across all burns was 16.0 (11.5) % of total carbon exposed (TCE), with HyPyC accounting for 2.5 (4.9) % of TCE. Both PyC and HyPyC were dominantly partitioned into the proximal flux. Production of HyPyC was strongly related to fire residence time, with shorter duration fires resulting in higher HyPyC yields. The carbon isotope (δ13C) compositions of PyC and HyPyC were generally lower by 1–3‰ relative to the original biomass, with marked depletion up to 7‰ for grasslands dominated by C4 biomass. δ13C values of CO2 produced by combustion were computed by mass balance and ranged from ~0.4 to 1.3‰. The depletion of 13C in PyC and HyPyC relative to the original biomass has significant implications for the interpretation of δ13C values of savanna soil organic carbon and of ancient PyC preserved in the geologic record, as well as for global 13C isotopic disequilibria calculations.

Impact of solid fuel particle size upon the propagation of a surface fire through a homogeneous vegetation layer

The aim of this paper is to investigate the role played by the size (thickness) of solid fuel particles upon both the heat transfer and the propagation of a surface fire through a homogeneous vegetation layer. Because all the interactions (mass, momentum, and heat transfer) between the solid fuel layer and the gas phase occur at the interface between these two media, the surface area to volume (SA/V) ratio (inversely proportional to the thickness of the solid fuel particles), which appears in the expression of the specific surface separating these two phases, must affect, more or less significantly, the fire dynamics. This problem has been studied numerically, using a multiphase formulation. Various variables, such as the temperature of the solid fuel, the temperature of the gas, the fire residence time and the heat flux by radiation and convection have been analyzed, in order to understand the role played by the SA/V upon the behaviour of the fire.

Numerical study of the effect of fuel moisture content (FMC) upon the propagation of a surface fire on a flat terrain

This paper was devoted to clarify and evaluate how fuel moisture content (FMC) characterising a homogeneous vegetation layer (grass or shrubs), can affect the behaviour of surface fire. The approach used in this study was based on numerical simulations performed using a detailed fire physical model. The numerical results were analysed in terms of fire residence time, fire front depth, mass loss rate and rate of spread (ROS). Two windy conditions (calm and weak) were studied to evaluate the decay of the rate of spread (ROS) resulting from an increase of the fuel moisture content. The effect of wind velocity upon marginal burning conditions was also analysed. The numerical results were compared with empirical data of the literature.

Low tolerance of New Caledonian secondary forest species to savanna fires

AbstractQuestionsHow do early secondary successional forest species that grow in savannas differ in their tolerance to surface fires? What are the consequences of these fire tolerances for savanna–forest dynamics and landscape management?LocationAnthropogenic savannas in the New Caledonian biodiversity hotspot (SW Pacific).MethodsWe estimated the range of fire intensity in New Caledonian savannas using field survey of fuels and the BehavePlus fire behaviour model. Within the predicted range of fire line intensity, we assessed theoretical fire injury to the cambium and crown for 11 species: the dominant tree of New Caledonian savannas (Melaleuca quinquenervia) and early secondary successional forest species. Using empirical models, for each species we estimated cambium damage from depth of necrosis (as a function of fire line intensity and fire residence time) and bark thickness, and crown damage from scorch height (as a function of fire line intensity) and tree height. We compared bark thickness and tree height increment patterns among species as well as species potential fire tolerance.ResultsThe 11 species had very contrasting capacity to avoid fire injury to the bole cambium due to differences in bark investment patterns, but were all very exposed to scorching and crown injury. Overall, most of sampled individuals are likely top‐killed by low intensity fires (&lt;1000 kW·m−1), which are frequent according to our simulations.ConclusionsThe early secondary successional forest species growing in New Caledonian savannas are poorly adapted to fire, in comparison with literature on worldwide trees in savannas. As a result, their juveniles are unlikely to reach adult size in fire‐prone areas. Restoration using the most fire‐tolerant species and fire prevention may be complementary strategies to manage such tropical landscapes in order to conserve biodiversity and ecosystem services.

Disturbance interactions can impact resilience mechanisms of forests

Interactions between multiple disturbances are of special concern in ecology due to their potential for non-linear behavior and long-lasting legacies on landscape structure and function. If multiple disturbances overcome the ecological resilience of a system, alternate stable states are possible. Increases in the frequency and severity of disturbance events as a result of climate change heighten this concern. This study directly addresses the question of ecosystem resilience in the face of multiple disturbances. We investigated a gradient of disturbance interaction severities between two events in a subalpine forest, a 1997 windstorm (variable severity) and a 2002 wildfire (high-severity). A third disturbance, salvage logging of blowdown (1999–2001) prior to the fire, served as a de facto experimental treatment. Ninety-nine study plots were established across the disturbance gradient, including fire-only areas for a baseline fire response. Modeling indicated that the combination of two severe disturbances created novel conditions which exceeded the resilience mechanisms of the system. Modeled mean fire residence time and temperature (First Order Fire Effects Model, FOFEM), as well as mean distance to potential seed sources, increased as a result of the interaction. Regeneration 8 years post-fire was essentially absent in medium- to high-severity blowdown + fire plots, whereas low-severity blowdown + fire and fire-only areas showed strong regeneration. Blowdown + salvage + fire had significantly higher regeneration than areas of comparable blowdown, suggesting that fuel loading drove the interaction. CART analysis supported this hypothesis. Multiple disturbances have the potential to create surprising situations and reduce the resilience of an ecosystem. Differential recovery as a result of a “novel disturbance” created by compounding events will likely have long lasting legacies across the landscape.

A soil heat and water transfer model to predict belowground grass rhizome bud death in a grass fire

AbstractQuestion:Grasses often resprout from surviving belowground buds following a fire in which aboveground matter is consumed. We used a soil heat and water transport model to present a general method for determining the potential mortality of rhizome buds due to fire for three tallgrass species (Andropogon gerardii, Sorghastrum nutans, andPanicum virgatum).Methods:Soil heating was described by physical processes that include heat conduction through the soil and heating and evaporation of soil water. We considered the following factors: soil moisture, texture, mineral thermal conductivity, maximum surface temperature, and fire residence time. Simulated soil temperature profiles were combined with measured belowground bud distributions to determine the proportion of buds expected to be heated to lethal temperatures under various conditions.Location:Wisconsin, USA.Results:Lethal temperatures for buds do not occur below ∼2 cm, and at least 30% of rhizome buds remain below lethal temperatures, even under extreme conditions.Conclusions:The model explains the possible mechanisms for grass belowground rhizome bud survival in fires. Changes in fire and soil conditions do not notably impact soil temperatures and rhizome bud survival.

Determination of Fire-Residence Time, and Its Role in the Survival of Eucalypts After a Bushfire

Bushfires occur frequently in drought-prone eucalypt forests of Australia, and their intensity can vary within very wide limits. Permanent damage is caused when a tree's vascular cambium layer is heated above 60°C, but this may be prevented in eucalypts by a thick layer of bark. It can take several minutes of flaming combustion during a bushfire for sufficient heat to penetrate the bark and kill the cambium. By heating some small sections of wood or bark in an oven to temperatures between 300°C and 600°C, then grinding and polishing a transverse section, reflectance measurements have been made in vertical incident light through a microscope fitted with a photomultiplier. These readings show a linear correlation between the logarithm of reflectance and temperature. Using this correlation, some maximum temperature profiles were obtained across transverse bark sections cut from trees subjected to a low-intensity bushfire. These have been compared with some graphs which relate the maximum temperatures reached within bark to the distance from the outer surface after progressively longer periods. By using reflectance measurements made on sections of charred bark or, more simply, by measuring the thickness of the black outer layer of charred bark an estimate of the fire residence time of a bushfire can be obtained.