Claire M. Belcher

Charring temperatures are driven by the fuel types burned in a peatland wildfire

Peatlands represent a globally important carbon store; however, the human exploitation of this ecosystem is increasing both the frequency and severity of fires on drained peatlands. Yet, the interactions between the hydrological conditions (ecotopes), the fuel types being burned, the burn severity, and the charring temperatures (pyrolysis intensity) remain poorly understood. Here we present a post-burn assessment of a fire on a lowland raised bog in Co. Offaly, Ireland (All Saints Bog). Three burn severities were identified in the field (light, moderate, and deeply burned), and surface charcoals were taken from 17 sites across all burn severities. Charcoals were classified into two fuel type categories (either ground or aboveground fuel) and the reflectance of each charcoal particle was measured under oil using reflectance microscopy. Charcoal reflectance shows a positive relationship with charring temperature and as such can be used as a temperature proxy to reconstruct minimum charring temperatures after a fire event. Resulting median reflectance values for ground fuels are 1.09 ± 0.32%Romedian, corresponding to estimated minimum charring temperatures of 447°C ± 49°C. In contrast, the median charring temperatures of aboveground fuels were found to be considerably higher, 646°C ± 73°C (3.58 ± 0.77%Romedian). A mixed-effects modeling approach was used to demonstrate that the interaction effects of burn severity, as well as ecotope classes, on the charcoal reflectance is small compared to the main effect of fuel type. Our findings reveal that the different fuel types on raised bogs are capable of charring at different temperatures within the same fire, and that the pyrolysis intensity of the fire on All Saints Bog was primarily driven by the fuel types burning, with only a weak association to the burn severity or ecotope classes.

Fire-adapted traits of Pinus arose in the fiery Cretaceous

• The mapping of functional traits onto chronograms is an emerging approach for the identification of how agents of natural selection have shaped the evolution of organisms. Recent research has reported fire-dependent traits appearing among flowering plants from 60 million yr ago (Ma). Although there are many records of fossil charcoal in the Cretaceous (65-145 Ma), evidence of fire-dependent traits evolving in that period is lacking. • We link the evolutionary trajectories for five fire-adapted traits in Pinaceae with paleoatmospheric conditions over the last 250 million yr to determine the time at which fire originated as a selective force in trait evolution among seed plants. • Fire-protective thick bark originated in Pinus c. 126 Ma in association with low-intensity surface fires. More intense crown fires emerged c. 89 Ma coincident with thicker bark and branch shedding, or serotiny with branch retention as an alternative strategy. These innovations appeared at the same time as the Earths paleoatmosphere experienced elevated oxygen levels that led to high burn probabilities during the mid-Cretaceous. • The fiery environments of the Cretaceous strongly influenced trait evolution in Pinus. Our evidence for a strong correlation between the evolution of fire-response strategies and changes in fire regime 90-125 Ma greatly backdates the key role that fire has played in the evolution of seed plants.

Limits for Combustion in Low O2 Redefine Paleoatmospheric Predictions for the Mesozoic

Several studies have attempted to determine the lower limit of atmospheric oxygen under which combustion can occur; however, none have been conducted within a fully controlled and realistic atmospheric environment. We performed experimental burns (using pine wood, moss, matches, paper, and a candle) at 20°C in O2 concentrations ranging from 9 to 21% and at ambient and high CO2 (2000 parts per million) in a controlled environment room, which was equipped with a thermal imaging system and full atmospheric, temperature, and humidity control. Our data reveal that the lower O2 limit for combustion should be increased from 12 to 15%. These results, coupled with a record of Mesozoic paleowildfires, are incompatible with the prediction of prolonged intervals of low atmospheric O2 levels (10 to 12%) in the Mesozoic.

Baseline intrinsic flammability of Earth’s ecosystems estimated from paleoatmospheric oxygen over the past 350 million years

Atmospheric oxygen (O2) is estimated to have varied greatly throughout Earth’s history and has been capable of influencing wildfire activity wherever fuel and ignition sources were present. Fires consume huge quantities of biomass in all ecosystems and play an important role in biogeochemical cycles. This means that understanding the influence of O2 on past fire activity has far-reaching consequences for the evolution of life and Earth’s biodiversity over geological timescales. We have used a strong electrical ignition source to ignite smoldering fires, and we measured their self-sustaining propagation in atmospheres of different oxygen concentrations. These data have been used to build a model that we use to estimate the baseline intrinsic flammability of Earth’s ecosystems according to variations in O2 over the past 350 million years (Ma). Our aim is to highlight times in Earth’s history when fire has been capable of influencing the Earth system. We reveal that fire activity would be greatly suppressed below 18.5% O2, entirely switched off below 16% O2, and rapidly enhanced between 19–22% O2. We show that fire activity and, therefore, its influence on the Earth system would have been high during the Carboniferous (350–300 Ma) and Cretaceous (145–65 Ma) periods; intermediate in the Permian (299–251 Ma), Late Triassic (285–201 Ma), and Jurassic (201–145 Ma) periods; and surprisingly low to lacking in the Early–Middle Triassic period between 250–240 Ma. These baseline variations in Earth’s flammability must be factored into our understanding of past vegetation, biodiversity, evolution, and biogeochemical cycles.

Fireball passes and nothing burns¿The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America

High soot contents have been reported in Cretaceous-Tertiary (K-T) sedimentary rocks, leading to the suggestion that the amount of thermal power delivered from the Chicxulub impact was sufficient to have ignited wildfires. Soot cannot be used to indicate fire location, however, as soot from one large fire could spread globally. Sources other than biomass burning could also yield soot. Charcoal in nonmarine sedimentary rocks (here quantified in situ in polished blocks) provides a unique tool to record the distribution of wildfires and therefore assess the extent of any thermal radiation associated with the impact at Chicxulub. The K-T and lowermost Tertiary sedimentary rocks of six nonmarine sequences (Colorado to Saskatchewan) contain no charcoal or below-background levels of charcoal and a significant quantity of noncharred organic materials, revealing that there was no distinctive wildfire across the North American continent related to the K-T event. This finding indicates that the K-T impact cannot have delivered a peak irradiance of >95 kW.m - 2 of thermal power to the atmosphere and <19 kW.m - 2 to the ground. Therefore, the thermal power delivered from the impact to North America did not have the destructive potential previously predicted. High amounts of thermal radiation were not responsible for the environmental perturbations or extinctions associated with the K-T event.

Combustion of fossil organic matter at the Cretaceous-Paleogene (K-P) boundary

Recognition of elevated concentrations of aciniform soot in Cretaceous-Paleogene (K-P) boundary sediments worldwide led to the hypothesis that global-scale forest wildfires could have been generated by the K-P boundary bolide impact and might have contributed directly to the extinction event. The wildfires are estimated to have injected 10 13 t of CO 2 into the atmosphere, resulting in an interval of greenhouse warming. Yet minimal amounts of charred plant remains and abundant noncharred material occur in various K-P boundary locations across North America. This refutes the inference that wildfires occurred on a global scale, and requires an alternative explanation for the aciniform soot. Here we describe significant concentrations of carbon cenospheres in K-P boundary sediments from New Zealand, Denmark, and Canada. Carbon cenospheres are thought to derive solely from incomplete combustion of pulverized coal or fuel-oil droplets, which suggests that the impact may have combusted organic-rich target crust. The Chicxulub impact crater is located adjacent to the Cantarell oil reservoir, one of the most productive oil fields on Earth. This indicates that abundance of organic carbon in the Chicxulub target crust was likely to have been above global mean values. But even if we discount Chicxulub9s organic-rich locality, the global mean crustal abundance for fossil organic matter is more than adequate to account for observed concentrations of both carbon cenospheres and aciniform soot, therefore making the global wildfire hypothesis unnecessary.

Constraints on the thermal energy released from the Chicxulub impactor: new evidence from multi-method charcoal analysis

It has been suggested by various workers that an extraterrestrial impact at the K–T boundary delivered sufficient thermal power to ignite globally extensive wildfires. Numerous models have sought to predict the amount of thermal power released by the impact, but none have considered the distribution of wildfire indicators in K–T rocks. Probably the most distinctive product from combustion of biomass is charcoal. The abundance of charcoal across the K–T boundary at eight non-marine sites in North America, stretching from Colorado in the south to Saskatchewan in the north, is recorded using three separate methods that allow quantitative analyses of microscopic to macroscopic charcoal particles. This study not only provides the first extensive study of charcoals across the K–T boundary but also uses the presence or absence of charred material to predict the extent and severity of the thermal pulse released by the K–T impact across the area predicted to have suffered the most extreme environmental effects. The K–T rocks contain on average between four and eight times (according to the method used) less charcoal than the Cretaceous rock record and non-charred plant remains are abundant in the K–T rocks. The below-background charcoal abundance and the high proportion of noncharred material in the K–T and lowermost Tertiary rocks across the Western Interior of North America suggest that there were no significant wildfires in this area associated with the K–T event. Although soot and polyaromatic hydrocarbons (PAHs) have been reported in the K–T rocks we suggest that the soot morphology and PAH types are more consistent with a source from the vaporization of hydrocarbons rather than biomass. For spontaneous ignition of vegetation temperatures >545 °C are necessary, whereas smouldering will begin at 325 °C. The below-background levels of charcoal in the K–T rocks allow the ground temperatures following the K–T impact to be constrained to between no more than 545 °C at any point and not above 325 °C for any significant period. This implies a maximum irradiance of <19 kW m−2 at the ground surface and that no more than 6 kW m−2 of thermal power was delivered to the ground for more than a few hours. Therefore our results show that the fossil record indicates that the impact at Chixculub did not generate sufficient thermal power to ignite extensive wildfires.

Geochemical evidence for combustion of hydrocarbons during the K-T impact event.

It has been proposed that extensive wildfires occurred after the Cretaceous–Tertiary (K-T) impact event. An abundance of soot and pyrosynthetic polycyclic aromatic hydrocarbons (pPAHs) in marine K-T boundary impact rocks (BIRs) have been considered support for this hypothesis. However, nonmarine K-T BIRs, from across North America, contain only rare occurrences of charcoal yet abundant noncharred plant remains. pPAHs and soot can be formed from a variety of sources, including partial combustion of vegetation and hydrocarbons whereby modern pPAH signatures are traceable to their source. We present results from multiple nonmarine K-T boundary sites from North America and reveal that the K-T BIRs have a pPAH signature consistent with the combustion of hydrocarbons and not living plant biomass, providing further evidence against K-T wildfires and compelling evidence that a significant volume of hydrocarbons was combusted during the K-T impact event.

Determinants of flammability in savanna grass species.

Summary Tropical grasses fuel the majority of fires on Earth. In fire‐prone landscapes, enhanced flammability may be adaptive for grasses via the maintenance of an open canopy and an increase in spatiotemporal opportunities for recruitment and regeneration. In addition, by burning intensely but briefly, high flammability may protect resprouting buds from lethal temperatures. Despite these potential benefits of high flammability to fire‐prone grasses, variation in flammability among grass species, and how trait differences underpin this variation, remains unknown. By burning leaves and plant parts, we experimentally determined how five plant traits (biomass quantity, biomass density, biomass moisture content, leaf surface‐area‐to‐volume ratio and leaf effective heat of combustion) combined to determine the three components of flammability (ignitability, sustainability and combustibility) at the leaf and plant scales in 25 grass species of fire‐prone South African grasslands at a time of peak fire occurrence. The influence of evolutionary history on flammability was assessed based on a phylogeny built here for the study species. Grass species differed significantly in all components of flammability. Accounting for evolutionary history helped to explain patterns in leaf‐scale combustibility and sustainability. The five measured plant traits predicted components of flammability, particularly leaf ignitability and plant combustibility in which 70% and 58% of variation, respectively, could be explained by a combination of the traits. Total above‐ground biomass was a key driver of combustibility and sustainability with high biomass species burning more intensely and for longer, and producing the highest predicted fire spread rates. Moisture content was the main influence on ignitability, where species with higher moisture contents took longer to ignite and once alight burnt at a slower rate. Biomass density, leaf surface‐area‐to‐volume ratio and leaf effective heat of combustion were weaker predictors of flammability components. Synthesis. We demonstrate that grass flammability is predicted from easily measurable plant functional traits and is influenced by evolutionary history with some components showing phylogenetic signal. Grasses are not homogenous fuels to fire. Rather, species differ in functional traits that in turn demonstrably influence flammability. This diversity is consistent with the idea that flammability may be an adaptive trait for grasses of fire‐prone ecosystems.

Chapter 6 – Infrared Image Analysis as a Tool for Studying the Horizontal Smoldering Propagation of Laboratory Peat Fires

Smoldering fires in peatlands can consume large areas of peat and release important amounts of carbon to the atmosphere as they self-propagate. This chapter focuses on the use of infrared images to characterize the horizontal propagation of smoldering fires in laboratory experiments. In these laboratory experiments an infrared camera takes images of the peat surface at regular intervals during the experiment. We present methods to process and analyze these infrared images that identify the shape and position of the smoldering front, quantify the maximum energy flux, the spread rate and direction of the front and its variability to time. To demonstrate our methods we analyze images from experiments that record the smoldering of dry peats (25% moisture content, mass of water per mass of dry peat) and wet peats (100% moisture content). Infrared images are used to quantify the effect of moisture content upon the smoldering fronts. Our methods demonstrate that smoldering combustion in dry peats has a wider front (6.8 ± 1 cm for the dry peat, 2.4 ± 0.7 cm for the wet peat), a faster spread rate (4.3 ± 1 cm/h for dry peat, 2.6 ± 0.7 cm/h for wet peat), and a lower peak of radiative energy flux (7.1 ± 0.7 kW/m2 for dry peat, 10.51 ± 2.1 kW/m2 for wet peat). Our infrared image analysis is a useful tool to characterize peat fires at an experimental scale. These methods can be applied to peats with different characteristics to identify and compare smoldering propagation dynamics.