Burning Season Research Articles

Season of prescribed fire determines grassland restoration outcomes after fire exclusion and overgrazing

AbstractFire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C4 grasslands and savannas. Although interest is growing in prescribed fire for grassland restoration, we have little long‐term experimental evidence of the influence of burn season on the recovery of herbaceous plant communities, encroachment by trees and shrubs, and invasion by exotic grasses. We conducted a prescribed fire experiment (seven burns between 2001 and 2019) in historically fire‐excluded and overgrazed grasslands of central Texas. Sites were assigned to one of four experimental treatments: summer burns (warm season, lightning season), fall burns (early cool season), winter burns (late cool season), or unburned (fire exclusion). To assess restoration outcomes of the experiment, in 2019, we identified old‐growth grasslands to serve as reference sites. Herbaceous‐layer plant communities in all experimental sites were compositionally and functionally distinct from old‐growth grasslands, with little recovery of perennial C4 grasses and long‐lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C3 grasses and forbs; and fall and winter sites were intermediate in composition to the unburned and summer sites. Despite compositional differences, all treatments had comparable plot‐level plant species richness (range 89–95 species/1000 m2). At the local‐scale, summer sites (23 species/m2) and old‐growth grasslands (20 species/m2) supported greater richness than unburned sites (15 species/m2), but did not differ significantly from fall or winter sites. Among fire treatments, summer and winter burns most consistently produced the vegetation structure of old‐growth grasslands (e.g., mean woody canopy cover of 9%). But whereas winter burns promoted the invasive grass Bothriochloa ischaemum by maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlled B. ischaemum invasion. Our results support a growing body of literature that shows that prescribed fire alone, without the introduction of plant propagules, cannot necessarily restore old‐growth grassland community composition. Nonetheless, this long‐term experiment demonstrates that prescribed burns implemented in the summer can benefit restoration by preventing woody encroachment while also controlling an invasive grass. We suggest that fire season deserves greater attention in grassland restoration planning and ecological research.

Effects of fire season on the reproductive success of the post-fire flowerer Doryanthes excelsa

Fire is an integral part of many ecosystems and recent record-breaking fires in natural systems around the world are indicative of changes occurring to the fire regime. Fire seasonality is one regime element that is shifting, and can impact the reproductive success of plant species, but rarely receives the spotlight when impacts of fire regimes are studied. We examined the effect of fire season on the reproductive effort and seed resourcing of the post-fire flowering Doryanthes excelsa from eastern Australia. We used field experiments at sites of different soil moisture levels (wet vs dry) burnt in different seasons (summer, autumn, spring) to measure the reproductive output of D. excelsa, including the percentage flowering and the number of seed pods. Seed vigour including seed germination and the percentages of lipids, carbohydrates, and crude proteins of seeds produced after different seasons of burn were analysed. There was a strong effect of fire season on D. excelsa reproduction, with percentage flowering, number of seed pods, germination and lipid levels in seeds all higher after summer compared to spring or autumn burns. Burns outside of summer also delayed flowering. The results of this study highlight the importance of fire and the season in which it occurs in the reproductive success of post-fire flowering species like D. excelsa, a species representative of a functional type often dominant in some fire-prone systems. Importantly, we found that the impacts of season of burn are not merely a product of differences in fire severity. Ongoing climatic changes driving shifts in the fire regime pose an imperative to consider fire season when managing to conserve this important functional group.

Assessment of Respiratory morbidity burden from exposure to vegetation fire-PM2.5 in Upper Northern Thailand for the year 2018

BACKGROUND AND AIM: The seasonal occurrence of vegetation fire in the Mainland Southeast Asia (MSEA) has deteriorated air quality, which can increase respiratory diseases. Nevertheless, no study has assessed the health burden from exposure to vegetation-fire smoke in the MSEA. We aimed to estimate the number of hospital visits from respiratory diseases attributable to vegetation-fire PM2.5 in the Upper northern Thailand (UNT). METHODS: The modelled vegetation fire-PM2.5 were obtained from System for Integrated modeLling of Atmosheric coMposition (SILAM) model, which is a global-to-meso-scale air pollution dispersion model. Vegetation fire-PM2.5 were simulated based on fire radiative power data of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. We averaged the extracted grid-cell of fire-PM2.5 within district boundaries. The number of respiratory disease-related hospital visits attributable to fire-PM2.5 was estimated for the whole year and burning period (January to April) of the year 2018 by using the modelled daily fire-PM2.5, population data, and concentration-response function derived from the previous epidemiological study. RESULTS:In 2018, we estimated 17,817 hospital visits for respiratory diseases attributable to vegetation fire-PM2.5 in the UNT. Our estimation was accounted approximately 1% of the total hospital visits for respiratory diseases throughout the year and 3 % during the burning season. CONCLUSIONS:This study suggests that PM2.5 emitted from vegetation fire events poses a health burden across the UNT. KEYWORDS: Health burden estimation, Vegetation fires, Hospital visits, Respiratory diseases

Tree Growth Response to Low-Intensity Prescribed Burning in Pinus nigra Stands: Effects of Burn Season and Fire Severity

The study of the short-term post-burn tree growth in a mixed stand of Pinus nigra and Pinus pinaster and in a pure stand of P. nigra in the Cuenca Mountains (Spain) will enable us to determine the disturbance of prescribed burning conducted in two seasons. Dendrochronological methods and mixed modelling were used to investigate whether tree growth responses are influenced by stand and tree characteristics, fire season and fire severity variables. The findings revealed that prescribed burning scarcely affected tree growth. The type of stand (mixed or pure) was not critical for tree growth. The individual tree characteristics were significant factors in all the scenarios studied. The inclusion of some fire severity variables for the first time in tree growth models showed that the maximum scorch height determined a main part of the variability of tree growth. The time during which the temperature was above 60 °C in the cambium region and temperature was above 300 °C in the bark surface were only significant factors after spring burnings. The litterfall one year after the prescribed burning was not a significant factor in any of the models. Overall, the findings confirm the characteristic resistance of P. nigra to surface fires and favor the potential application of prescribed burning programs for this species in the Mediterranean Basin.

Fresh and aged PM2.5 and their ion composition in rural and urban atmospheres of Northern Thailand in relation to source identification

This study aims to investigate ion composition of PM2.5 in various sites and seasons and to identify the main sources on spatial and temporal basis. PM2.5 compositions of two urban and two rural areas in Northern Thailand in 2019 were investigated to distinguish urban traffic and rural open burning sources. During the burning season, average PM2.5 concentrations in rural areas (104 ± 45 μg m−3) were slightly higher than those in urban areas (94 ± 39 μg m−3). Source identification of PM2.5 by cluster analysis during burning season in urban sites and one rural site revealed mixed sources of aged aerosols from biomass burning, traffic and transboundary pollution, characterized by (NH4)2SO4 and KNO3. Only PM2.5 in one rural area (Chiang Dao), where intense open burning activities observed, contained significant KCl level in addition to other compounds. KCl is being used as a tracer for fresh aerosols from biomass burning as opposes to KNO3 for aged aerosols. It was found that KNO3 proportion in total ions increased with PM2.5 concentrations both in urban and rural areas, indicating prominent open burning influences in regional scale. Source identification in other seasons was more distinguishable between urban and rural areas, and more varied depending on local emissions. Urban PM2.5 sources were secondary inorganic aerosols from traffic gas conversion in contrast with rural PM2.5 which were mainly from biomass burning.

Multidecadal effects of fire in a grassland biodiversity hotspot: Does pyrodiversity enhance plant diversity?

Native grasslands have been vastly transformed with the expansion of human activities. Applied fire regimes offer conservation‐based management an opportunity to enhance remaining grassland biodiversity and secure its persistence into the future. Fire regimes have complex interactions with abiotic and biotic ecosystem components that influence environmental heterogeneity and biodiversity. We examined the pyrodiversity–biodiversity hypothesis, which suggests that more species are supported where pyrodiversity, that is, the level of environmental heterogeneity associated with different fire regimes, is greater. A mesocosm‐type field experiment, maintained for 38 yr, was used to determine the response of plant diversity to 1‐, 2‐, 5‐ and 12‐yr fire‐return interval treatments, with early‐dormant, middormant and early–growing season burns. Our sampling regime was designed to assess the influence of fire treatments and combinations thereof, over spatial scale, on plant diversity. Pyrodiversity was maximized where fire regime diversity, simulated by varying the size of patches with different fire treatments, was greatest. Species richness was predicted to be reduced at short and long extremes of fire‐return interval, as suggested by the intermediate‐disturbance hypothesis. The influence of fire treatments on alpha and beta diversity, and plant functional groups, were tested using multivariate and Bayesian models. Multilevel models of plant height and growth form, with fire‐return interval, reflected the strong indirect influence of fire‐return interval on sward structure and the plant environment. The pyrodiversity–biodiversity and intermediate‐disturbance hypotheses were only partially supported and depended on the plant group and spatial scale of assessment. Although both frequent and infrequent burns made important contributions to overall species richness, richness peaked where 20–40% of the area was protected from frequent fires. The larger contribution of frequent burning to diversity was due to an interaction with scale and forb turnover over the trial area. Extremes in fire‐return intervals reduced forb richness, supporting the predictions of the intermediate‐disturbance hypothesis. Spring burns had a weak negative influence on forb alpha diversity, but only at small scales. For a meaningful contribution of management to plant diversity, traditional fixed biennial burns need to be supplemented with smaller patches burned with longer fire‐return intervals, and extremes in fire‐return intervals avoided.

Source apportionment of black carbon over Delhi: A case study of extreme biomass burning events and Diwali festival

Present study investigates the temporal and diurnal variations of Black Carbon (BC) and quantifies the contribution of fossil fuel (BCff) combustion and biomass burning (BCbb) to the BC concentrations in Delhi using the Aethalometer model. The average BC mass concentration and Absorption Angstrom Exponent were 12.6 ± 8.2 μg/m3 (range: 1.73–37.0 μg/m3) and 1.30 ± 0.16 (range: 1.06–1.82), respectively. The average BCff and BCbb mass concentrations were 7.90 ± 4.98 μg/m3 (range: 1.13–19.98 μg/m3) and 4.73 ± 5.23 μg/m3 (range: 0.21–26.85 μg/m3), respectively. The contribution of BCbb was 31.15% and BCff was 68.85% to the overall BC concentration in the study period. The highest BCbb contribution, amounting to 52.04%, was found in the month of November (stubble burning season). The effect of Diwali firecrackers on BC concentrations was also investigated which was transient, lasted for few hours only. The probable major source regions of BC to the receptor site were Rajasthan, Gujarat, Punjab, Indo-Gangetic Plain and parts of Pakistan during monsoon season while Punjab, Haryana, and parts of Uttar Pradesh during post-monsoon season. The results revealed that the biomass burning activities in Punjab and Haryana contribute significantly to the Delhi's post-monsoon air pollution suggesting urgency to implement the carbonaceous aerosols emission abatement strategies over northern India.

Seasonality of Biennial Burning Has No Adverse Effects on Mature Longleaf Pine Survival or Productivity

Emulating natural disturbance has become an increasingly important restoration strategy. In the fire-maintained woodlands of the southeastern United States, contemporary restoration efforts have focused on approximating the historical fire regime by burning at short intervals. Due to concerns over escape and damage to mature trees, most prescribed burning has occurred in the dormant season, which is inconsistent with the historical prevalence of lightning-initiated fire in the region. This discordance between contemporary prescribed burning and what is thought to be the historical fire regime has led some to question whether dormant season burning should remain the most common management practice; however, little is known about the long-term effects of repeated growing season burning on the health and productivity of desirable tree species. To address this question, we report on a long-term experiment comparing the effects of seasonal biennial burning (winter, spring, and summer) and no burning on the final survival status, height, diameter, and volume growth of 892 mature longleaf pine (Pinus palustris) over 23 years in three mature even-aged stands in southern Alabama, United States. Overall, longleaf pine survival across all treatments averaged 81 ± 2% [s.e]. Among seasonal burn treatments, survival was highest in the spring burns (82 ± 4%) but did not vary significantly from any other treatment (summer – 79 ± 4%, winter – 81 ± 4%, unburned – 84 ± 4%). However, survival was statistically influenced by initial diameter at breast height, as survival of trees in the largest size class (30 cm) was 40% higher than trees in the smallest size class (5 cm). Productivity of longleaf pine was not significantly different among treatment averages in terms of volume (38.9–44.1 ± 6.0 m3 ha–1), diameter (6.0–6.7 ± 0.3 cm), and height (2.5–3.4 ± 0.4 m) growth. Collectively, our results demonstrate that burning outside the dormant season will have little impact on mature longleaf pine survival and growth. This finding has important implications for the maintenance of restored southeastern woodlands, as interest in burning outside the dormant season continues to grow.

Using a New Top‐Down Constrained Emissions Inventory to Attribute the Previously Unknown Source of Extreme Aerosol Loadings Observed Annually in the Monsoon Asia Free Troposphere

AbstractThe contribution of biomass burning to the total aerosol loading over Monsoon Asia is both significant and continuing to increase. To better match the spatio‐temporal distribution of aerosols and trace gasses observed in the free troposphere, this work applied a 3‐D constrained emission inventory based on top‐down remotely sensed NO2 measurement to investigate the most extreme of the annually occurring biomass burning seasons in Monsoon Asia. In 2016, this constituted an extreme event observed over a 6‐day period covering millions of square kilometers, including over regions that are typically in the rainy phase of the Asian Monsoon. The results are shown to be consistent with respect to Tropical Rainfall Measuring Mission precipitation, AERONET measurements, MODIS AOD, MOPITT CO, and reanalysis meteorology, over both the biomass burning source as well as the millions of square kilometers downwind both to the East and to the Southwest. Reproducing the observed long‐range transport pattern requires the time of biomass burning to be increased, regions not previously identified as burning to be actual source regions, and the emissions of Black Carbon (BC) to be 6.6 to 11.9 times larger than current inventories. The underlying mechanism for this long‐range transport involves a new 3‐D pathway that can occur during the transition from the North phase to the South phase of the Asian Monsoon. The results are also consistent with the new idea that large loadings of BC in the lower free troposphere may significantly affect the meteorological field and the overall vertical distribution of aerosols in the tropical troposphere.

Environmental and Management Effects on Demographic Processes in the U.S. Threatened Platanthera leucophaea (Nutt.) Lindl. (Orchidaceae).

Populations of the U.S. threatened orchid, Platanthera leucophaea, are restricted to fragmented grassland and wetland habitats. We address the long-term (1998–2020) interactive effects of habitat (upland prairie vs. wetland), fire management (burned vs. unburned) and climatic variation, as well as pollination crossing effects, on population demography in 42 populations. Our analysis revealed the consistent interactive effects of habitat, dormant season burning, and climatic variation on flowering, reproduction, and survival. Burning increased flowering and population size under normal or greater than normal precipitation but may have a negative effect during drought years apparently if soil moisture stress reduces flowering and increases mortality. Trends in the number of flowering plants in populations also correspond to precipitation cycles. As with flowering and fecundity, survival is significantly affected by the interactive effects of habitat, fire, and climate. This study supports previous studies finding that P. leucophaea relies on a facultative outcrossing breeding system. Demographic modeling indicated that fire, normal precipitation, and outcrossing yielded greater population growth, and that greater fire frequency increased population persistence. It also revealed an ecologically driven demographic switch, with wetlands more dependent upon survivorship than fecundity, and uplands more dependent on fecundity than survivorship. Our results facilitate an understanding of environmental and management effects on the population demography of P. leucophaea in the prairie region of its distribution. Parallel studies are needed in the other habitats such as wetlands, especially in the eastern part of the range of the species, to provide a more complete picture.

Understanding agricultural fire dynamics in the southern Yucatán Peninsular Region using the MODIS (C6) active fire product

ABSTRACT Smallholder farmers commonly use fire for land clearing and agricultural maintenance in the Yucatán Peninsula of Mexico. The southern Yucatán Peninsular Region (SYPR) has experienced increasing fire frequency since the 1960s due to expanding smallholder agriculture and recurrent droughts. Beginning in January 2019, a government-subsidized programme named Sembrando Vida encouraged further smallholder agricultural development throughout Mexico, resulting in increased burning for cropland preparation. This study uses the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 1 km Active Fire product to characterize agricultural burning trends in the SYPR between 2010 and 2019, focusing on the impacts of the Sembrando Vida programme. Comparison of MODIS Active Fire locations to visible agricultural burns in coincident Landsat imagery showed a 29.5% (21/71) agreement. MODIS Active Fire time series data exhibited a biennial trend in fire frequency that was caused by interannual rainfall variability. An inverse relationship was found between total rainfall in the burning season and annual fire frequency. A combination of low rainfall and the implementation of Sembrando Vida in 2019 caused a 98% increase in state-authorized burn permitting compared to the 2015–2018 average, and MODIS Active Fire data exhibited a 33% increase in agricultural burns in 2019 compared to previous years that had >1000 fires.

Assessment of the effects of straw burning bans in China: Emissions, air quality, and health impacts

Open biomass burning (OBB) plays an important role in air pollution and climate change by releasing short-term but intensive amounts of particulate matter and gaseous air pollutants. During past years, policies with respect to prohibition on open straw burning have been issued in China in order to mitigate the air pollution problems and the effectiveness of these straw burning bans in different regions remains to be evaluated. In this study, open crop straw burning (OCSB) emissions during 2010–2018 were analyzed based on a commonly used emission inventory with high spatial and temporal resolution. High emissions concentrated over Northeast China (31.8% of national total PM2.5 emissions in 2018), East China (24.0%), and North China (16.6%). Simulations based on an integrated meteorology-air quality modeling system and an exposure-response function show that OCSB emissions could increase monthly PM2.5 concentration by as much as 10 μg/m3 during burning seasons in Northeast China and were associated with 4741 premature deaths in 2018. Spatial heterogeneities were observed with respect to the trends of OCSB emissions during 2010–2018. In East China, North China, and Central China, OCSB emissions showed a general declining trend since 2013 while an opposing increasing trend was observed in Northeast China with peak emissions in 2017. Comparing 2013 (before intensive implementation of straw burning bans) and 2018 (after), national total PM2.5 emissions from OCSB activities decreased by 46.9%, ranging from −14.1% to +70% depending on the specific regions. Northeast China is the only region that showed higher OCSB emissions in 2018 compared to 2013, probably associated with the relatively delayed implementation of the straw burning bans. Avoided number of premature deaths due to reduced OCSB emissions was estimated to be 4256 on a national scale, with most health benefits gained in East and Central China. Results from this study demonstrate the importance of OCSB contribution to PM2.5 concentrations and spatial heterogeneities exist in terms of the effectiveness of the straw burning bans in reducing OCSB emissions and gained health benefits.

Are fire temperatures and residence times good predictors of survival and regrowth for resprouters in Florida, USA, scrub?

BackgroundFire is a dominant ecological disturbance in many ecosystems. Post-fire resprouting is a widespread response to fire, but resprouting vigor varies with many components of the fire regime, including fire intensity. We measured responses in 46 species of resprouting plants of Florida, USA, scrub and related habitats, marking and measuring individual plants before fires. We then measured fire temperatures and residence times (time above 60 °C) during 13 fires, at the base of each plant. We measured post-fire plant sizes for up to six years. We hypothesized that high temperatures and long residence times would reduce survival and growth, and that these vital rates would vary by burn season, habitat type, and species group.ResultsFires had variable intensities with maximum temperatures ranging from 47 to 890 °C (mean temperature = 549 °C) and residence times ranging from 0 to 83 minutes (mean time = 10 minutes). Consumed plants experienced higher fire intensity than scorched plants, and residence times were higher during the wet season (July through October), than the dry season (November through April), and fire season (May to June) and with drier conditions. Across all species affected by fire, 86% of plants survived and resprouted post fire. First year survival was unrelated to fire variables, with high survival across all maximum temperatures and residence times. Burn season, habitat, and species group did not significantly affect survival. On average across all species, post-fire growth recovered to pre-fire heights within four years. Growth was not significantly affected by species group or burn season.ConclusionResprouting perennial plants that dominate Florida scrub and surrounding habitats appear resilient to a wide range of fire intensities, as measured by maximum temperatures and residence times. Post-fire growth was rapid, with recovery of pre-fire heights in four years. Species groups varied in post-fire recovery rates. In these habitats, fire is critical to maintain the habitat structure for many animals and plants, including many rare species. The slower recovery of biomass for resprouting shrubs results in the longer availability of gaps for rare herbaceous species. In addition, variability in post-fire survival and growth among species groups likely contributes to the persistence of species diversity and complexity across habitats. However, variation in fire intensity appears to have little effect on resprouting species or subsequent habitat structure. A range of fire regimes can be used in prescribed fire management without having major impacts on resprouting plants in Florida scrub and related vegetation.

Improving the understanding between climate variability and observed extremes of global NO2 over the past 15 years

This work addresses the relationship between major dynamical forcings and variability in NO2 column measurements. The dominating impact in Northern Southeast Asia is due to El Niño-Southern Oscillation (ENSO); in Indonesia, Northern Australia and South America is due to Indian Ocean Dipole (IOD); and in Southern China Land and Sea, Populated Northern China, Siberia, Northern and Arctic Eurasia, Central and Southern Africa, and Western US and Canada is due to North Atlantic Oscillation (NAO). That NO2 pollution in Indonesia is modulated by IOD contradicts previous work claiming that the emissions in Indonesia are a function of El Niño impacting upon Aerosol Optical Depth and Fire Radiative Power. Simultaneous impacts of concurrent and lagged forcings are derived using multi-linear regression, demonstrating ENSO impacts future NO2 variability, enhancing NO2 emissions 7–88 weeks in the future, while IOD and NAO in some cases increase the emissions from or the duration of the future burning season as measured by NO2. This impact will also extend to co-emitted aerosols and heat, which may impact the climate. In all cases, lagged forcings exhibit more impact than concurrent forcings, hinting at non-linearity coupling with soil moisture, water table, and other dynamical effects. The regression model formed demonstrates that dynamical forcings are responsible for over 45% of the NO2 emissions variability in most non-urban areas and over 30% in urban China and sub-arctic regions. These results demonstrate the significance of climate forcing on short-lived air pollutants.

Impact of smoke and non-smoke aerosols on radiation and low-level clouds over the southeast Atlantic from co-located satellite observations

Abstract. Data derived from instruments on board the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat satellites as well as meteorological parameters from reanalysis are used to explore situations when moist aerosol layers overlie stratocumulus clouds over the southeast Atlantic during the biomass burning season (June to October). To separate and quantify the impacts of aerosol loading, aerosol type, and humidity on the radiative fluxes (including cloud top cooling), the data are split into different levels of aerosol and moisture loadings. The aerosol classification available from the CALIPSO products is used to compare and contrast situations with pristine air, with smoke, and with other (non-smoke) types of aerosols. A substantial number of cases with non-smoke aerosols above clouds are found to occur under similar meteorological conditions to the smoke cases. In contrast, the meteorology is substantially different for the pristine situations, making a direct comparison with the aerosol cases ambiguous. The moisture content is enhanced within the aerosol layers, but the relative humidity does not always increase monotonously with increasing optical depth. Shortwave (SW) heating rates within the moist aerosol plumes increase with increasing aerosol loading and are higher in the smoke cases compared to the non-smoke cases. However, there is no clear correlation between moisture changes and SW absorption. Cloud top cooling rates do not show a clear correlation with moisture within the overlying aerosol layers due to the strong variability of the cooling rates caused by other meteorological factors (most notably cloud top temperature). No clear influence of aerosol type or loading on cloud top cooling rates is detected. Further, there is no correlation between aerosol loading and the thermodynamic structure of the atmosphere nor the cloud top height.

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.

Global search for temporal shifts in fire activity: potential human influence on southwest Russia and north Australia fire seasons

Decadal trends in fire activity can reveal important human and climate-driven influences across a multitude of landscapes from croplands to savannas. We use 16 years of daily satellite observations from 2003 to 2018 to search globally for stationary temporal shifts in fire activity during the primary burning season. We focus on southwest Russia and north Australia as case study regions; both regions experienced nearly 40 d shifts over a 16 year period but in opposite directions. In southwest Russia, a major wheat-growing region, we trace the delay in post-harvest fires to several potential drivers: modernization in the agricultural system and recent droughts, followed by government restrictions on wheat exports. In north Australia, prescribed burns in the early dry season are a key practice in Aboriginal fire management of savannas, and the increasing trend of such fires has limited the size and extent of fast-spreading late dry season fires, thereby shifting overall fire activity earlier. In both regions, human action, through controlling fire ignition and extent, is an important driver of the temporal shifts in fire activity with climate as both a harbinger and an amplifier of human-induced changes.

Aerosol Direct Radiative Effects at the ARM SGP and TWP Sites: Clear Skies

AbstractThe clear‐sky aerosol direct radiative effect (DRE) was estimated at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) and Tropical Western Pacific (TWP) sites. The NASA Langley Fu‐Liou radiation model was used with observed inputs including aerosol vertical extinction profile from the Raman lidar; spectral aerosol optical depth (AOD), single‐scattering albedo and asymmetry factor from Aerosol Robotic Network; temperature and water vapor profiles from radiosondes; and surface shortwave (SW) spectral albedo from radiometers. A radiative closure experiment was conducted for clear‐sky conditions. The mean differences of modeled and observed surface downwelling SW total fluxes were 1 W m−2 at SGP and 2 W m−2 at TWP, which are within observational uncertainty. At SGP, the estimated annual mean clear‐sky aerosol DRE is −3.00 W m−2 at the top of atmosphere (TOA) and −6.85 W m−2 at the surface. The strongest aerosol DRE of −4.81 (−10.77) W m−2 at the TOA (surface) are in the summer when AODs are largest. The weakest aerosol DRE of −1.28 (−2.77) W m−2 at the TOA (surface) are in November–January when AODs and single‐scattering albedos are lowest. At TWP, the annual mean clear‐sky DRE is −2.82 W m−2 at the TOA and −10.34 W m−2 at the surface. The strongest aerosol DRE of −5.95 (−22.20) W m−2 at the TOA (surface) are in November (October) due to the biomass burning season’s peak. The weakest aerosol DRE of −0.96 (−4.16) W m−2 at the TOA (surface) are in March (April) when AODs are smallest.

Interannual and Seasonal Variability of Greenhouse Gases and Aerosol Emissions from Biomass Burning in Northeastern China Constrained by Satellite Observations

Biomass burning is a major source of greenhouse gases (GHGs) and particulate matter (PM) emissions in China. Despite increasing efforts of fire monitoring, it remains challenging to quantify the variability in interannual and seasonal emissions of GHGs and PM from biomass burning. In this study, we investigated the biomass burning emissions in Northeastern China based on fire radiative power (FRP) obtained from the Visible Infrared Imaging Radiometer Suites (VIIRS) active fires datasets during the period 2012 to 2019. Our results showed that the average annual emissions from biomass burning in Northeastern China during 2012–2019 were: 81.6 Tg for CO2, 260.2 Gg for CH4, 5.5 Gg for N2O, 543.2 Gg for PM2.5 and 573.6 Gg for PM10, respectively. Higher levels of GHGs and PM emissions were concentrated in the Songnen Plain and Sanjiang Plain, the main grain producing areas in this region, and were associated with dense fire points. There were two emission peaks observed each year: after harvesting (October to November) and before planting (March to April). During this study period, the total fire emissions initially increased and then decreased in a fluctuating pattern, with emissions peaking in 2015, the year when more emission regulations were introduced. Crop straw was the major source of GHGs and PM among all kinds of biomass burning. Following more stringent controls on burning and the utilization of crop straw, the main burning season changed from autumn to spring. The proportion from spring burning increased from 20.5% in 2013 to 77.1% in 2019, with an annual growth rate of 20%. The results of this study demonstrate the effectiveness of regulatory control in reducing GHGs and PM emissions, as well as satellite fire observations as a powerful means to assess such outcomes.

Seasonal patterns of atmospheric mercury in tropical South America as inferred by a continuous total gaseous mercury record at Chacaltaya station (5240 m) in Bolivia

Abstract. High-quality atmospheric mercury (Hg) data are rare for South America, especially for its tropical region. As a consequence, mercury dynamics are still highly uncertain in this region. This is a significant deficiency, as South America appears to play a major role in the global budget of this toxic pollutant. To address this issue, we performed nearly 2 years (July 2014–February 2016) of continuous high-resolution total gaseous mercury (TGM) measurements at the Chacaltaya (CHC) mountain site in the Bolivian Andes, which is subject to a diverse mix of air masses coming predominantly from the Altiplano and the Amazon rainforest. For the first 11 months of measurements, we obtained a mean TGM concentration of 0.89±0.01 ng m−3, which is in good agreement with the sparse amount of data available from the continent. For the remaining 9 months, we obtained a significantly higher TGM concentration of 1.34±0.01 ng m−3, a difference which we tentatively attribute to the strong El Niño event of 2015–2016. Based on HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) back trajectories and clustering techniques, we show that lower mean TGM concentrations were linked to either westerly Altiplanic air masses or those originating from the lowlands to the southeast of CHC. Elevated TGM concentrations were related to northerly air masses of Amazonian or southerly air masses of Altiplanic origin, with the former possibly linked to artisanal and small-scale gold mining (ASGM), whereas the latter might be explained by volcanic activity. We observed a marked seasonal pattern, with low TGM concentrations in the dry season (austral winter), rising concentrations during the biomass burning (BB) season, and the highest concentrations at the beginning of the wet season (austral summer). With the help of simultaneously sampled equivalent black carbon (eBC) and carbon monoxide (CO) data, we use the clearly BB-influenced signal during the BB season (August to October) to derive a mean TGM / CO emission ratio of (2.3±0.6)×10-7 ppbvTGM ppbvCO-1, which could be used to constrain South American BB emissions. Through the link with CO2 measured in situ and remotely sensed solar-induced fluorescence (SIF) as proxies for vegetation activity, we detect signs of a vegetation sink effect in Amazonian air masses and derive a “best guess” TGM / CO2 uptake ratio of 0.058 ±0.017 (ng m−3)TGM ppmCO2-1. Finally, significantly higher Hg concentrations in western Altiplanic air masses during the wet season compared with the dry season point towards the modulation of atmospheric Hg by the eastern Pacific Ocean.