Fine Fuels Research Articles

Southern Great Plains Wildfire Outbreaks

Destructive wildfire outbreaks are a preeminent natural hazard on the grass-dominated landscape of the southern Great Plains. These southern Great Plains wildfire outbreaks (SGPWOs) are characterized by tens of wildfires that evolve on spatial and temporal scales closely tied to the passage of midlatitude cyclones when dormant herbaceous vegetation is particularly dry and abundant. Ten SGPWOs inflicted tragic losses of life and property across eastern New Mexico, west Texas, and Oklahoma between December 2005 and April 2009. This study reviews the conditions that promoted these dangerous phenomena. Texas A&M; Forest Service records reveal that enhanced seasonal wildfire activity and increased potential for SGPWOs typically occurs during El Niño Southern Oscillation cold phases (La Niña), especially when preceded by positive growing-season rainfall anomalies. The antecedent state of predominately fine grassland vegetative fuels associated with SGPWOs is quantified per Energy Release Component (ERC, fuel model G). Average ERC values >50 (>70th percentile) supported the 2005-2009 SGPWOs on the Great Plains of Texas. Meteorological composites that quantify mean synoptic patterns during SGPWOs are generated via Rapid Update Cycle analyses, and averaged vertical temperature, moisture, and wind profiles are presented. Further analyses of subsynoptic low and midlevel tropospheric temperatures and winds illustrate a tendency for wildfires to occur near 2-m and 850-hPa thermal ridges when overspread by 500-hPa wind maxima. The juxtaposition of these atmospheric features appears to be a useful meso-α-scale predictor of heightened wildfire risks. Recognition of the presented seasonal indicators toward a fire-prone regime influenced strategic preparations for the historic 2011 Texas wildfires. Operational use of composite pattern recognition-based forecasts in tactical decision support is demonstrated for the 27 February 2011 “firestorm”, a particularly damaging SGPWO during an unprecedented fire season. Average ERC values >75 (>95th percentile) additionally supported prolonged burn periods with the passage of subsequent fire outbreak-bearing weather systems during the spring of 2011. Lastly, seasonal trends and the chronology of climatic and environmental signals prior to SGPWOs are highlighted, per a summary of conditions that preceded all of the 2005-2011 episodes.

Moisture availability and ecological restoration limit fine fuels and modelled wildfire intensity following non‐native ungulate removal in Hawaii

Abstract The removal and exclusion of non‐native ungulates for conservation of biodiversity is common globally, including on tropical Pacific Islands. However, the poorly quantified effects of ungulate exclusion on fuels and wildfire may represent an important conservation trade‐off. We measured fuels (live and dead fuel loads, type, height and continuity) and modelled potential wildfire behaviour (flame height and rate of spread) inside and outside of 13 ungulate exclosures, three of which received active ecological restoration (e.g. planting of native shrubs and trees), across a 2,740 mm mean annual rainfall (MAR) gradient on the Island of Hawaii. Differences in fuel characteristics and modelled wildfire behaviour inside versus outside of ungulate exclosures were assessed using linear mixed effects analyses. Non‐native ungulate removal, in general, increased fine fuel loading (average change ranged from −0.7 to 11.3 Mg/ha), shrub fuel loading (−0.1 to 5.6 Mg/ha) and modelled flame lengths (−0.2 to 1.9 m). Post‐removal fine fuel loading and modelled flame lengths increased linearly and positively with moisture availability, by as much as 56% in the wettest sites. Dead fine fuels, a key driver of fire intensity and combustion rates, also increased with ungulate removal. Sites undergoing active ecological restoration exhibited reductions in fuel loading, with greater reductions over time since ungulate removal; by the 10th year following ungulate removal, fine fuel loading was reduced at restoration sites by 41%. Synthesis and applications. Dry and mesic environments where wildfire occurrence is more frequent are of particular concern following non‐native ungulate removal, but with drought even typically wet environments can be at high risk of wildfire following ungulate removal. Given limited resources, managers in mesic and wet forests might focus first non‐native ungulates in non‐native grass‐dominated areas that contribute disproportionately to wildfire spread and are adjacent to fenced higher value areas, while managers in dry to mesic areas might focus on removing non‐native ungulates. Where possible, land managers in all situations should invest in long‐term fuel management strategies such as active ecological restoration to reduce fuel loads and the occurrence and intensity of wildfires.

ESTIMATING FINE DEAD FUEL MOISTURE CONTENT UNDER EQUATORIAL CLIMATE CONDITIONS

The measurement of the fine dead fuel moisture content (FDFMC) is extremely important for forest fire prevention and suppression activities, as it has a great influence on the ignition probability and fire behavior. The Fine Fuel Moisture Code (FFMC) from the Fire Weather Index (FWI), is one of the most used models to estimate the FDFMC. Nevertheless, studies that assess the efficiency of this model in Brazil or in low latitude regions are rare. The present study aimed to evaluate the efficiency of the FFMC in an equatorial climate area and to develop a new model capable of estimating the FDFMC with greater precision. For this purpose, 861 random samples of fine dead fuel had their moisture content determined through oven drying. The obtained values were compared with those estimated by the FFMC and correlated with meteorological parameters to build a regression model. The results obtained show that the FDFMC was overestimated by the FFMC. The independent variables with the greatest influence on the FDFMC were, in decreasing order of significance: air relative humidity, air temperature, amount of rainfall in the last 24 hours and number of days without rainfall. The developed model presented good statistical parameters (r2 = 0.86; p <0.0001; RMSE = 0.22) and can be used, in areas with similar characteristics of the study area, to estimate the daily fire risk and to determine ideal conditions for prescribed burns.

Online moisture measurement of dead fine fuel on the forest floor using near-infrared reflectometry.

A method for online measurement of the levels of moisture in dead fine fuels on the forest floor is proposed based on near-infrared reflectometry. A linear relationship is found between the moisture content of dead fine fuels and the intensity of light absorption at a wavelength of 1450nm, and this forms the basic principle of measurement for different forest species. An online moisture-monitoring device is designed and developed based on this principle. This uses long-distance wireless data-transmission to adapt to the lack of telecoms service in remote forests. Moisture-measurement experiments were conducted continuously for 24 h in rainless weather during the summer of 2019 in the Harbin Urban Forestry Demonstration Base of Northeast Forestry University using leaves from four different forest species, specifically, larch, dryland willow, silver birch, and walnut. These measurements were verified using simultaneous measurements employing a weighing method. The results from the field experiments agreed well with the weighing results. The uncertainty in the moisture-content measurements using the proposed system was ∼3.4% when the true moisture content was 20.0% or less. This proposed system has the advantages of online measurement and high accuracy. This method for monitoring moisture levels in dead fine fuels is an important advance in terms of improving the accuracy and simultaneity of measurements for prediction of forest-fire-risk ratings and fire behavior.

Large-scale wildfire reduces population growth in a peripheral population of sage-grouse

BackgroundDrastic increases in wildfire size and frequency threaten western North American sagebrush (Artemisia L. spp.) ecosystems. At relatively large spatial scales, wildfire facilitates type conversion of sagebrush-dominated plant communities to monocultures of invasive annual grasses (e.g., Bromus tectorum L.). Annual grasses provide fine fuels that promote fire spread, contributing to a positive grass–fire feedback cycle that affects most sagebrush ecosystems, with expected habitat loss for resident wildlife populations. Greater sage-grouse (Centrocercus urophasianus Bonaparte, 1827) are sagebrush obligate species that are indicators of sagebrush ecosystem function because they rely on different components of sagebrush ecosystems to meet seasonal life history needs. Because wildfire cannot be predicted, chronic impacts of wildfire on sage-grouse populations have been largely limited to correlative studies. Thus, evidence from well-designed experiments is needed to understand the specific mechanisms by which wildfire is detrimental to sage-grouse population dynamics.ResultsFollowing a significant wildfire event in the southwest periphery of sage-grouse range, we implemented a before-after-control-impact study with long-term paired (BACIP) datasets of male sage-grouse surveyed from traditional breeding grounds (leks) within and outside the wildfire boundary. We estimated sage-grouse population rate of change in apparent abundance ( hat{uplambda} ) at burned and unburned areas before and after wildfire and derived BACIP ratios, which provide controlled evidence of wildfire impact. We found that hat{uplambda} at leks within the wildfire boundary decreased approximately 16% relative to leks at control sites. Furthermore, we estimated a 98.5% probability that the observed change in hat{uplambda} could be attributed to the wildfire.ConclusionsWe demonstrated adverse wildfire impacts on sage-grouse population growth using an experimental BACIP design, which disentangled the effect of wildfire disturbance from natural population fluctuations. Our results underscore the importance of active and comprehensive management actions immediately following wildfire (i.e., seeding coupled with planting sagebrush), that might offset short-term impacts of wildfire by timing rapid recovery of sagebrush to meet short-term species’ habitat requirements. Burned leks likely have substantial immediate impacts that may extend beyond wildfire boundaries, especially if critical source habitats are removed. Such impacts could fragment habitat and disrupt connectivity, thereby affecting larger populations and possibly contributing to more widespread declines in sage-grouse populations.

Opportunities and challenges for savanna burning emissions abatement in southern Africa

Savanna fires occurring in sub-Saharan Africa account for over 60% of global fire extent, of which more than half occurs in the Southern Hemisphere contributing ~29% of global fire emissions. Building on experience in reducing savanna fire emissions in fire-prone north Australian savannas through implementation of an internationally accredited ‘savanna burning’ emissions abatement methodology, we explore opportunities and challenges associated with the application of a similar approach to incentivise emissions reduction in fire-prone southern African savannas. We first show that for a focal region covering seven contiguous countries, at least 80% of annual savanna large fire (>250 ha) extent and emissions occur under relatively severe late dry season (LDS) fire-weather conditions, predominantly in sparsely inhabited areas. We then assess the feasibility of adapting the Australian emissions abatement methodology through exploratory field studies at the Tsodilo Hills World Heritage site in north-west Botswana, and the Niassa Special Reserve in northern Mozambique. Our assessment demonstrates that application of a savanna burning emissions abatement method focused on the undertaking of strategically located early dry season (EDS) burning to reduce LDS wildfire extent and resultant emissions meets key technical criteria, including: LDS fine fuels tend to be markedly greater than EDS fuels given seasonal leaf litter inputs; LDS fires tend to be significantly more severe and combust more fuels; methane and nitrous oxide emission factors are essentially equivalent in EDS and LDS periods under cured fuel conditions. In discussion we consider associated key implementation challenges and caveats that need to be addressed for progressing development of savanna burning methods that incentivise sustainable fire management, reduce emissions, and support community livelihoods in wildfire-dominated southern African savannas.

Measuring a Fire. The Story of the January 2019 Fire Told from Measurements at the Warra Supersite, Tasmania

Non-stand-replacing wildfires are the most common natural disturbance in the tall eucalypt forests of Tasmania, yet little is known about the conditions under which these fires burn and the effects they have on the forest. A dry lightning storm in January 2019 initiated the Riveaux Road fire. This fire burnt nearly 64,000 ha of land, including tall eucalypt forests at the Warra Supersite. At the Supersite, the passage of the fire was recorded by a suite of instruments measuring weather conditions and fluxes (carbon, water and energy), while a network of permanent plots measured vegetation change. Weather conditions in the lead-up and during the passage of the fire through the Supersite were mild—a moderate forest fire danger index. The passage of the fire through the Supersite caused a short peak in air temperature coinciding with a sharp rise in CO2 emissions. Fine fuels and ground vegetation were consumed but the low intensity fire only scorched the understorey trees, which subsequently died and left the Eucalyptus obliqua canopy largely intact. In the aftermath of the fire, there was prolific seedling regeneration, a sustained reduction in leaf area index, and the forest switched from being a carbon sink before the fire to becoming a carbon source during the first post-fire growing season.

When do grasses resprout after fire?

This article is a Commentary on Simpson et al. (2021), 230: 832–844.

Effects of plant hydraulic traits on the flammability of live fine canopy fuels

Abstract Plant species vary in how they regulate moisture and this has implications for their flammability during wildfires. We explored how fuel moisture is shaped by variation within five hydraulic traits: saturated moisture content, cell wall rigidity, cell solute potential, symplastic water fraction and tissue capacitance. Using pressure–volume curves, we measured these hydraulic traits in twigs and distal shoots (i.e. twigs + leaves) in 62 plant species across four wooded communities in south‐eastern Australia. Moisture content of fine fuels was then estimated for circumstances typical of fire weather. These projections were made assuming that under the hot, dry, windy conditions typical of large wildfires, leaves and fine twigs would function at internal water pressures close to wilting point (i.e. turgor loss point, TLP). The effect of different moisture contents at TLP on ignition time was then modelled using a fully mechanistic, finite element model of biomass ignition based on standard principles of physical chemistry. We also measured predawn water potential, an indication of plant access to soil water that is influenced by root architecture. These data were used to model how root traits influence fuel moisture and ignition time. Most variation among species in fuel moisture under fire weather conditions arose from differences in saturated moisture content (3.4‐ to 3.6‐fold variation). Twig capacitance was also an important driver of fuel moisture under these weather conditions (1.9‐ to 2.2‐fold variation in moisture content). A suite of other leaf and root traits influencing how much shoots dry out as they approach wilting point each contributed 1.0‐ to 1.6‐fold variation in projected fuel moisture during fire weather. Fuel moisture variation in turn drove variation in flammability by modifying predicted ignition time. Two main life‐history types in fire‐prone habitats are obligate seeders and resprouters. There were no significant differences between these species groups in estimated fuel moisture during fire weather, nor in any measured hydraulic traits. Live fuel moisture is an important determinant of wildfire activity. Our data show that variation in tissue saturated moisture content among co‐occurring species represents an important ecological store of variation in flammability in the study communities. A free Plain Language Summary can be found within the Supporting Information of this article.

Biomass burning combustion efficiency observed from space using measurements of CO and NO<sub>2</sub> by the TROPOspheric Monitoring Instrument (TROPOMI)

Abstract. The global fire emission inventories depend on ground and airborne measurements of species-specific emission factors (EFs), which translate dry matter losses due to fires to actual trace gas and aerosol emissions. The EFs of nitrogen oxides (NOx) and carbon monoxide (CO) can function as a proxy for combustion efficiency to distinguish flaming from smoldering combustion. The uncertainties in these EFs remain large as they are limited by the spatial and temporal representativeness of the measurements. The global coverage of satellite observations has the advantage of filling this gap, making these measurements highly complementary to ground-based or airborne data. We present a new analysis of biomass burning pollutants using space-borne data to investigate the spatiotemporal efficiency of fire combustion. Column measurements of nitrogen dioxide and carbon monoxide (XNO2 and XCO) from the TROPOspheric Monitoring Instrument (TROPOMI) are used to quantify the relative atmospheric enhancements of these species over different fire-prone regions around the world. We find spatial and temporal patterns in the ΔXNO2 ∕ ΔXCO ratio that point to distinct differences in biomass burning behavior. Such differences are induced by the burning phase of the fire (e.g., high-temperature flaming vs. low-temperature smoldering combustion) and burning practice (e.g., the combustion of logs, coarse woody debris and soil organic matter vs. the combustion of fine fuels such as savanna grasses). The sampling techniques and the signal-to-noise ratio of the retrieved ΔXNO2 ∕ ΔXCO signals were quantified with WRF-Chem experiments and showed similar distinct differences in combustion types. The TROPOMI measurements show that the fraction of surface smoldering combustion is much larger for the boreal forest fires in the upper Northern Hemisphere and peatland fires in Indonesia. These types of fires cause a much larger increase (3 to 6 times) in ΔXCO relative to ΔXNO2 than elsewhere in the world. The high spatial and temporal resolution of TROPOMI also enables the detection of spatial gradients in combustion efficiency at smaller regional scales. For instance, in the Amazon, we found higher combustion efficiency (up to 3-fold) for savanna fires than for the nearby tropical deforestation fires. Out of two investigated fire emission products, the TROPOMI measurements support the broad spatial pattern of combustion efficiency rooted in GFED4s. Meanwhile, TROPOMI data also add new insights into regional variability in combustion characteristics that are not well represented in the different emission inventories, which can help the fire modeling community to improve their representation of the spatiotemporal variability in EFs.

Characterisation of initial fire weather conditions for large spring wildfires in Alberta, Canada

We evaluated surface and 500-hPa synoptic weather patterns, and fire weather indices from the Canadian Forest Fire Danger Rating System for 80 large wildfires during 1990–2019 in Alberta that started in May and grew to over 1000 ha. Spread days were identified during the first 4 days of wildfire activity. We observed two distinct synoptic weather patterns on these days. Pre-frontal and frontal passage activity was the predominant feature associated with 48% of the calendar spread days. Strong south–south-east winds from a surface high centred east of Alberta (west of Hudson Bay) and supported by an upper ridge, and a surface low located south-west of the ridge occurred on 26% of the calendar spread days. Surface analysis indicates the spring wildfire season in Alberta is driven by very high to extreme Initial Spread Index, a rating of the expected wildfire rate of spread based on Fine Fuel Moisture Code and wind. Very high to extreme values of Buildup Index, a rating of the amount of fuel available for consumption, are not a prerequisite for large wildfires in May. For Alberta, this means large wildfires in May can occur after only a few days of dry, windy weather.

Influence of climatic factor of changes in forest fire danger and fire season length in Turkey.

In contrast to the expectations of an increase in annual fire activity and the severity of fire season due to climate change and large fires, which have been occurring in recent years, a downtrend has been identified in fire activity in many studies conducted for the whole of Europe in recent years. Similarly, in Turkey, according to the General Directorate of Forestry statistics, while there is an increase in the number of annual fires, the burnt area has a downtrend pattern. In this study, fire activity and climate data statistics for Turkey were examined along with the fire season length and severity. The results obtained conform with the studies conducted in places from Spain at the westernmost part of Mediterranean Europe to Israel at the easternmost part of the Mediterranean. Considering the changes in temperatures, temperature rise of 2 to 3 °C was detected at all stations in the study area. No decrease was observed in the average temperatures at any of the stations within the study period between 1940 and 2018. On the other hand, the precipitation trend varied according to the stations. Although there have been increases in precipitation in Fethiye, Isparta, and Marmaris since 1960, the decrease in precipitation by 132 mm in Afyon since 1970 and the decrease in precipitation by 137 mm in Bodrum since 1940 are attention-grabbing. These stations are followed by Izmir station with 66 mm and Cesme station with 37 mm of decrease, despite being smaller decreases. In the study, the long-term (1940-2018) data of the meteorological stations discussed within the study, the Canadian Fire Weather Index (FWI) and the Fine Fuel Moisture Code (FFMC) values were calculated. According to the FWI results used in determining the severity and length of fire season on the coastline of Turkey from the northern Aegean to Antalya, the likelihood of large fires decreased by about 52% in 2018 compared to 1970. This decrease in FWI value indicates that the fire severity is reduced. The specified decrease in fire severity also explains the reason of the decrease in the burnt area that occurred over the years in Turkey. No significant change was observed in the FFMC values indicating the possibility of human-induced fires between 1970 and 2018.

Green and sustainable route for oxidative depolymerization of lignin: New platform for fine chemicals and fuels.

Depolymerization of lignin biomass to its value-added chemicals and fuels is pivotal for achieving the goals for sustainable society, and therefore has acquired key interest among the researchers worldwide. A number of distinct approaches have evolved in literature for the deconstruction of lignin framework to its mixture of complex constituents in recent decades. Among the existing practices, special attention has been devoted for robust site selective chemical transformation in the complex structural frameworks of lignin. Despite the initial challenges over a period of time, oxidation and oxidative cleavage process of aromatic building blocks of lignin biomass toward the fine chemical synthesis and fuel generation has improved substantially. The development has improved in terms of cost effectiveness, milder reaction conditions, and purity of compound individuals. These aforementioned oxidative protocols mainly involve the breaking of C-C and C-O bonds of complex lignin frameworks. More precisely in the line with environmentally friendly greener approach, the catalytic oxidation/oxidative cleavage reactions have received wide spread interest for their mild and selective nature toward the lignin depolymerization. This mini-review aims to provide an overview of recent developments in the field of oxidative depolymerization of lignin under greener and environmentally benign conditions. Also, these oxidation protocols have been discussed in terms of scalability and recyclability as catalysts for different fields of applications.

Developing a New Hourly Forest Fire Risk Index Based on Catboost in South Korea

Forest fires can cause enormous damage, such as deforestation and environmental pollution, even with a single occurrence. It takes a lot of effort and long time to restore areas damaged by wildfires. Therefore, it is crucial to know the forest fire risk of a region to appropriately prepare and respond to such disastrous events. The purpose of this study is to develop an hourly forest fire risk index (HFRI) with 1 km spatial resolution using accessibility, fuel, time, and weather factors based on Catboost machine learning over South Korea. HFRI was calculated through an ensemble model that combined an integrated model using all factors and a meteorological model using weather factors only. To confirm the generalized performance of the proposed model, all forest fires that occurred from 2014 to 2019 were validated using the receiver operating characteristic (ROC) curves and the area under the ROC curve (AUC) values through one-year-out cross-validation. The AUC value of HFRI ensemble model was 0.8434, higher than the meteorological model. HFRI was compared with the modified version of Fine Fuel Moisture Code (FFMC) used in the Canadian Forest Fire Danger Rating Systems and Daily Weather Index (DWI), South Korea’s current forest fire risk index. When compared to DWI and the revised FFMC, HFRI enabled a more spatially detailed and seasonally stable forest fire risk simulation. In addition, the feature contribution to the forest fire risk prediction was analyzed through the Shapley Additive exPlanations (SHAP) value of Catboost. The contributing variables were in the order of relative humidity, elevation, road density, and population density. It was confirmed that the accessibility factors played very important roles in forest fire risk modeling where most forest fires were caused by anthropogenic factors. The interaction between the variables was also examined.

5-Hydroxymethylfurfural Hydrodeoxygenation to 2,5-Dimethylfuran in Continuous-Flow System over Ni on Nitrogen-Doped Carbon

Waste lignocellulosic biomass is sustainable and an alternative feedstock to fossil resources. Among the lignocellulosic derived compounds, 2,5-dimethylfuran (DMF) is a promising building block for chemicals, e.g., p-xylene, and a valuable biofuel. DMF can be obtained from 5-hydroxymethylfurfural (HMF) via catalytic deoxygenation using non-noble metals such as Ni in the presence of H2. Herein, we present the synthesis of DMF from HMF using 35 wt.% Ni on nitrogen-doped carbon pellets (35Ni/NDC) as a catalyst in a continuous flow system. The conversion of HMF to DMF was studied at different hydrogen pressures, reaction temperatures, and space times. At the best reaction conditions, i.e., 423 K, 8.0 MPa, and space time 6.4 kgNi h kgHMF−1, the 35Ni/NDC catalyst exhibited high catalytic activity with HMF conversion of 99 mol% and 80 mol% of DMF. These findings can potentially contribute to the transition toward the production of sustainable fine chemicals and liquid transportation fuels.

Evolution of Plume Core Structures and Turbulence during a Wildland Fire Experiment

Micrometeorological observations were made during a prescribed fire experiment conducted in a region of complex terrain with grass fuels and weak ambient winds of 3 m s−1. The experiment allowed for the analysis of plume and turbulence structures including individual plume core evolution during fire front passage. Observations were made using a suite of in situ and remote sensing instruments strategically placed at the base of a gully with a 24° slope angle. The fire did not spread upwards along the gully because the ambient wind was not in alignment with the slope, demonstrating that unexpected fire spread can occur under weak wind conditions. Our observational results show that plume overturning caused downward heat transport of −64 kW m−2 to occur and that this mixing of warmer plume air downward to the surface may result in increased preheating of fine fuels. Plume evolution was associated with the formation of two plume cores, caused by vigorous entrainment and mixing into the plume. Furthermore, the turbulence kinetic energy observed within the plume was dominated by horizontal velocity variances, likely caused by increased fire-induced circulations into the plume core. These observations highlight the nature of plume core separation and evolution and provide context for understanding the plume dynamics of larger and more intense wildfires.

Harvest Intensity Effects on Carbon Stocks and Biodiversity Are Dependent on Regional Climate in Douglas-Fir Forests of British Columbia

Temperate forests provide crucial ecosystems services as living sinks for atmospheric carbon (C) and repositories of biodiversity. Applying harvesting at intensities that minimize losses offers one means for mitigating global change. However, little is known of overstory retention levels that best conserve ecosystem services in different regional climates, and likewise as climate changes. To quantify the effect of harvest intensity on C stocks and biodiversity, we compared five harvesting intensities (clearcutting,seedtree retention, 30% patch retention, 60% patch retention, and uncut controls) across a climatic aridity gradient that ranged from humid to semi-arid in the Douglas-fir (Pseudostuga menziesii) forests of British Columbia. We found that increased harvesting intensity reduced total ecosystem, aboveground, and live tree C stocks one year post-harvest, and the magnitude of these losses were negatively correlated with climatic aridity. In humid forests, total ecosystem C ranged from 50% loss following clearcut harvest, to 30% loss following large patch retention harvest. In arid forests this range was 60% to 8% loss, respectively. Where lower retention harvests are sought, the small patch retention treatment protected both C stocks and biodiversity in the arid forests, whereas the seedtree method performed as well or better in the humid forests. Belowground C stocks declined by an average of 29% after harvesting, with almost all of the loss from the forest floor and none from the mineral soil. Of the secondary pools, standing and coarse deadwood declined in all harvesting treatments regardless of cutting intensity or aridity, while C stocks in fine fuels and stumps increased. The understory plant C pool declined across all harvesting intensities in the humid forests, but increased in arid forests. Shannon’s diversity and richness of tree and bryoid species declined with harvesting intensity, where tree species losses were greatest in the humid forests and bryoid losses greatest in arid forests. Shrub and herb species were unaffected. This study showed that the highest retention level was best at reducing losses in C stocks and biodiversity, and clearcutting the poorest, and while partial retention of canopy trees can reduce losses in these ecosystem services, outcomes will vary with climatic aridity.

Seed traits determine species' responses to fire under varying soil heating scenarios

Abstract Many plant species in fire‐prone environments maintain persistence through fire via soil seedbanks. However, seeds stored within the soil are at risk of mortality from elevated soil temperatures during fire. Seeds may be protected from fire‐temperature impacts by burial, however, those buried too deeply may germinate but fail to emerge. Thus, successful post‐fire seed regeneration is contingent upon a trade‐off between burial depth and survival through fire. We examined the relationships between seedling emergence behaviour, seed survival and soil temperatures during fire in 13 native and four non‐native woodland species in southwestern Australia. We assessed total seedling emergence per depth, maximum seedling emergence depth and seedling emergence speed from seeds planted at eight depths (0, 1, 2, 3, 4, 5, 7, 10 cm). Soil temperatures were quantified using distributed temperature sensing in optic fibre (DTS), measured continuously between 1 and 10 cm in depth (temperatures were subsequently categorized into 1 cm increments for analysis) during five experimental fires in beds with fine fuels manipulated between 8 and 20 t/ha. Using seed survival and emergence success relative to soil temperatures, we determined vulnerability of seedling emergence relative to soil temperatures generated by combustion of fuel quantities typically observed in woodlands. Maximum depth of emergence varied between species from 2 to >10 cm, with a positive linear correlation to seed mass. Maximum soil temperatures from the two highest fuel masses exceeded seed lethal thresholds (T50—representing temperatures lethal to 50% of seeds) of at least five species. Lethal temperatures were exceeded at all potential emergence depths for all three grass species, and all four non‐native species studied. Of the remaining 10 species, temperatures did not exceed the lethal thresholds under any of the fuel mass levels tested. We found no relationship between lethal temperature thresholds and maximum emergence depth. Our data demonstrate that seeds exhibit variation in their response to soil heating and capacity to emerge from depth, with three distinct functional responses amongst our study species, which enable persistence through, and recruitment following, fire. Such variation in species attributes and fuel mass may lead to heterogeneity (within fires) or divergent trajectories (among fires) in community response under changed fire regime. A free Plain Language Summary can be found within the Supporting Information of this article.

Frequent fire slows microbial decomposition of newly deposited fine fuels in a pyrophilic ecosystem.

Frequent fires maintain nearly 50% of terrestrial ecosystems, and drive ecosystem changes that govern future fires. Since fires are dependent on available plant or fine fuels, ecosystem processes that alter fine fuel loads like microbial decomposition are particularly important and could modify future fires. We hypothesized that variation in short-term fire history would influence fuel dynamics in such ecosystems. We predicted that frequent fires within a short-time period would slow microbial decomposition of new fine fuels. We expected that fire effects would differ based on dominant substrates and that fire history would also alter soil nutrient availability, indirectly slowing decomposition. We measured decomposition of newly deposited fine fuels in a Longleaf pine savanna, comparing plots that burned 0, 1, 2, or 3 times between 2014 and 2016, and which were located in either close proximity to or away from overstory pines (Longleaf pine, Pinus palustris). Microbial decomposition was slower in plots near longleaf pines and, as the numbers of fires increased, decomposition slowed. We then used structural equation modeling to assess pathways for these effects (number of fires, 2016 fuel/fire characteristics, and soil chemistry). Increased fire frequency was directly associated with decreased microbial decomposition. While increased fires decreased nutrient availability, changes in nutrients were not associated with decomposition. Our findings indicate that increasing numbers of fires over short-time intervals can slow microbial decomposition of newly deposited fine fuels. This could favor fine fuel accumulation and drive positive feedbacks on future fires.

Preceding Fall Drought Conditions and Overwinter Precipitation Effects on Spring Wildland Fire Activity in Canada

Spring fire activity has increased in parts of Canada, particularly in the west, prompting fire managers to seek indicators of potential activity before the fire season starts. The overwintering adjustment of the Canadian Fire Weather Index System’s Drought Code (DC) is a method to adjust and carry-over the previous season’s drought conditions into the spring and potentially point to what lies ahead. The occurrence of spring fires is most strongly influenced by moisture in fine fuels. We used a zero-inflated Poisson regression model to examine the impact of the previous end of season Drought Code (DCf) and overwinter precipitation (Pow) while accounting for the day-to-day variation in fine fuel moisture that drives ignition potential. Impacts of DCf and Pow on area burned and fire suppression effectiveness were also explored using linear and logistic regression frameworks. Eight fire management regions across the boreal forests were analyzed using data from 1979 to 2018. For the majority of regions, drier fall conditions resulted in more human-caused spring fires, but not in greater area burned or reduced suppression effectiveness. The influence of Pow was much more variable pointing to the conclusion that Pow alone is not a good indicator of spring drought conditions.