Fire, Volume 2, Issue 4 (December 2019) – 10 articles

Blackgum (Nyssa sylvatica) is a “consummate subordinate” hardwood tree species consigned to the mid-canopy of many eastern North American forests. Despite its wide distribution and ecological amplitude, blackgum is an underutilized tree species in fire history reconstructions within its range. In this study, I analyzed cross-section samples collected from 19 fire-scarred blackgum trees at a dry, nutrient-poor ridgetop study area in northeastern Pennsylvania. All but two of these samples were successfully crossdated, each containing between one and six fire scars. Fires recorded by blackgum occurred frequently, with site-level mean fire intervals between approximately three and five years. There was an increase in blackgum growth within two years following fire events, but this increase was not statistically significant and it was dependent on local fire regime characteristics. In addition, the blackgum fire-scar data increased the temporal and spatial resolution of an existing local fire history. These results provide evidence for the potential use of blackgum in fire history reconstructions, but applications may be limited by tree age, complacent growth that prevents crossdating, and the degree of rot resistance after scarring. Full article

Most wildfires in North America are quickly extinguished during initial attack (IA), the first phase of suppression. While rates of success are high, it is not clear how much IA suppression reduces annual fire risk across landscapes. This study introduces a method of estimating IA effectiveness by pairing burn probability (BP) analysis with containment probability calculations based on initial fire intensity, spread rate, and crew response time. The method was demonstrated on a study area in Kootenay National Park, Canada by comparing burn probabilities with and without modeled IA suppression. Results produced landscape-level analyses of three variables: burn probability, suppression effectiveness, and conditional escape probability. Overall, IA reduced mean study area BP by 78% as compared to a no-suppression scenario, but the primary finding was marked spatial heterogeneity. IA was most effective in recently burned areas (86% reduction), whereas mature, contiguous fuels moderated its influence (50%). Suppression was least effective in the designated wildfire exclusion zone, suggesting supplementary management approaches may be appropriate. While the framework includes assumptions about IA containment, results offer new insight into emergent risk patterns and how management strategies alter them. Managers can adopt these methods to anticipate, quantify, and compare fine-scale policy outcomes. Full article

Coordinated approaches to wildfire risk mitigation strategies that cross-ownership and management boundaries are found in many policies and programs worldwide. The “all lands” approach of the United States (US) National Cohesive Strategy, for example, attempts to address the mismatches between biophysical risk and the social potential to address risks by improving multijurisdictional coordination and collaboration. Local capacity to coordinate wildfire risk mitigation, therefore, may be an important influence on whether risk reduction planning makes success stories out of at-risk communities, or turns what would appear a manageable problem into a disaster waiting to happen. We analyzed the relationship between predicted housing exposure to wildfire and local self-assessment of community competence to mitigate wildfire risks in 60 communities in the western US. Results generally demonstrate that (1) the number of sources of wildfire risk influences local housing exposure to wildfire, and (2) perceived community-competence is associated with predicted exposure to wildfire. We suggest that investments in ongoing updates to community risk planning and efforts to build multi-jurisdictional risk management networks may help to leverage existing capacity, especially in moderate capacity communities. The analysis improves the social-ecological understanding of wildfire risks and highlights potential causal linkages between community capacity and wildfire exposure. Full article

Forests store a large amount of terrestrial carbon, but this storage capacity is vulnerable to wildfire. Combustion, and subsequent tree mortality and soil erosion, can lead to increased carbon release and decreased carbon uptake. Previous work has shown that non-constant fire return intervals over the past 4000 years strongly shaped subalpine forest carbon trajectories. The extent to which fire-regime variability has impacted carbon trajectories in other subalpine forest types is unknown. Here, we explored the interactions between fire and carbon dynamics of 14 subalpine watersheds in Colorado, USA. We tested the impact of varying fire frequency over a ~2000 year period on ecosystem productivity and carbon storage using an improved biogeochemical model. High fire frequency simulations had overall lower carbon stocks across all sites compared to scenarios with lower fire frequencies, highlighting the importance of fire-frequency in determining ecosystem carbon storage. Additionally, variability in fire-free periods strongly influenced carbon trajectories across all the sites. Biogeochemical trajectories (e.g., increasing or decreasing total ecosystem carbon and carbon-to-nitrogen (C:N) ratios) did not vary among forest types but there were trends that they may vary by elevation. Lower-elevations sites had lower overall soil C:N ratios, potentially because of higher fire frequencies reducing carbon inputs more than nitrogen losses over time. Additional measurements of ecosystem response to fire-regime variability will be essential for improving estimates of carbon dynamics from Earth system models. Full article

Previously burned areas can influence the occurrence, extent, and severity of subsequent wildfires, which may influence expenditures on large fires. We develop a conceptual model of how interactions of fires with previously burned areas may influence fire management, fire behavior, expenditures, and test hypotheses using regression models of wildfire size and suppression expenditures. Using a sample of 722 large fires from the western United States, we observe whether a fire interacted with a previous fire, the percent area of fires burned by previous fires, and the percent perimeter overlap with previous fires. Fires that interact with previous fires are likely to be larger and have lower total expenditures on average. Conditional on a fire encountering a previous fire, a greater extent of interaction with previous fires is associated with reduced fire size but higher expenditures, although the expenditure effect is small and imprecisely estimated. Subsequent analysis suggests that fires that interact with previous fires may be systematically different from other fires along several dimensions. We do not find evidence that interactions with previous fires reduce suppression expenditures for subsequent fires. Results suggest that previous fires may allow suppression opportunities that otherwise might not exist, possibly reducing fire size but increasing total expenditures. Full article

Wildland fire is a major producer of aerosols from combustion of vegetation and soils, but little is known about the abundance and composition of smoke’s biological content. Bioaerosols, or aerosols derived from biological sources, may be a significant component of the aerosol load vectored in wildland fire smoke. If bioaerosols are injected into the upper troposphere via high-intensity wildland fires and transported across continents, there may be consequences for the ecosystems they reach. Such transport would also alter the concept of a wildfire’s perimeter and the disturbance domain of its impact. Recent research has revealed that viable microorganisms are directly aerosolized during biomass combustion, but sampling systems and methodology for quantifying this phenomenon are poorly developed. Using a series of prescribed fires in frequently burned forest ecosystems, we report the results of employing a small rotary-wing unmanned aircraft system (UAS) to concurrently sample aerosolized bacteria and fungi, particulate matter, and micrometeorology in smoke plumes versus background conditions. Airborne impaction-based bioaerosol sampling indicated that microbial composition differed between background air and smoke, with seven unique organisms in smoke vs. three in background air. The air temperature was negatively correlated with the number of fungal colony-forming units detected. Our results demonstrate the utility of a UAS-based sampling platform for active sampling of viable aerosolized microbes in smoke arising from wildland fires. This methodology can be extended to sample viable microbes in a wide variety of emissions sampling pursuits, especially those in hazardous and inaccessible environments. Full article

For this study, we characterized the dependence of fire counts (FCs) on soil moisture (SM) at global and sub-global scales using 15 years of remote sensing data. We argue that this mathematical relationship serves as an effective way to predict fire because it is a proxy for the semi-quantitative fire–productivity relationship that describes the tradeoff between fuel availability and climate as constraints on fire activity. We partitioned the globe into land-use and land-cover (LULC) categories of forest, grass, cropland, and pasture to investigate how the fire–soil moisture (fire–SM) behavior varies as a function of LULC. We also partitioned the globe into four broadly defined biomes (Boreal, Grassland-Savanna, Temperate, and Tropical) to study the dependence of fire–SM behavior on LULC across those biomes. The forest and grass LULC fire–SM curves are qualitatively similar to the fire–productivity relationship with a peak in fire activity at intermediate SM, a steep decline in fire activity at low SM (productivity constraint), and gradual decline as SM increases (climate constraint), but our analysis highlights how forests and grasses differ across biomes as well. Pasture and cropland LULC are a distinctly human use of the landscape, and fires detected on those LULC types include intentional fires. Cropland fire–SM curves are similar to those for grass LULC, but pasture fires are evident at higher SM values than other LULC. This suggests a departure from the expected climate constraint when burning is happening at non-optimal flammability conditions. Using over a decade of remote sensing data, our results show that quantifying fires relative to a single physical climate variable (soil moisture) is possible on both cultivated and uncultivated landscapes. Linking fire to observable soil moisture conditions for different land-cover types has important applications in fire management and fire modeling. Full article

Long-term fire ecology can help to better understand the major role played by fire in driving vegetation composition and structure over decadal to millennial timescales, along with climate change and human agency, especially in fire-prone areas such as the Mediterranean basin. Investigating past ecosystem dynamics in response to changing fire activity, climate, and land use, and how these landscape drivers interact in the long-term is needed for efficient nature management, protection, and restoration. The Toledo Mountains of central Spain are a mid-elevation mountain complex with scarce current anthropic intervention located on the westernmost edge of the Mediterranean basin. These features provide a perfect setting to study patterns of late Holocene fire activity and landscape transformation. Here, we have combined macroscopic charcoal analysis with palynological data in three peat sequences (El Perro, Brezoso, and Viñuelas mires) to reconstruct fire regimes during recent millennia and their linkages to changes in vegetation, land use, and climatic conditions. During a first phase (5000–3000 cal. BP) characterized by mixed oak woodlands and low anthropogenic impact, climate exerted an evident influence over fire regimes. Later, the data show two phases of increasing human influence dated at 3000–500 cal. BP and 500 cal. BP–present, which translated into significant changes in fire regimes increasingly driven by human activity. These results contribute to prove how fire regimes have changed along with human societies, being more related to land use and less dependent on climatic cycles. Full article

Socio-ecological systems are complex, dynamic structures driven by cross-scale interactions between climate, disturbance and subsistence strategies. We synthetize paleoecological data to explore the emergence and evolution of anthropogenic landscapes in southwestern Europe and northern Africa. Specifically, we estimate trends in vegetation and fire, and assess how changes in climate and resource exploitation altered ecosystem dynamics over the last 10,000 years. Pollen data reveal that a complex vegetation mosaic resulted from the conversion of forests into areas suitable for crops, especially after 7000 cal yr BP. Cross-scale analysis shows a progressive decoupling of climate and ecosystem trajectories, which displayed an overall south-to-north time-transgressive pattern consistent with models of population expansion. As human impact increased, so did the use of fire, and after 4000 cal yr BP, levels of biomass burning became homogeneous across the region. This region-wide rise in burning suggests that land-management overrode the effects of climate, fuel and topography. Thus, while increasing the returns and predictability of resources, rapidly-growing communities created a new form of frequent and extensive disturbance that led to profound and persistent changes in the landscape, including shrub encroachment, increased erosion and soil impoverishment. Full article

Previous attempts to identify the environmental factors associated with firefighter entrapments in the United States have suggested that there are several common denominators. Despite the widespread acceptance of the assumed commonalities, few studies have quantified how often entrapments actually meet these criteria. An analysis of the environmental conditions at the times and locations of 166 firefighter entrapments involving 1202 people and 117 fatalities that occurred between 1981 and 2017 in the conterminous United States revealed some surprising results. Contrary to general assumptions, we found that at broad spatial scales firefighter entrapments happen under a wide range of environmental conditions, including during low fire danger and on flat terrain. A cluster-based analysis of the data suggested that entrapments group into four unique archetypes that typify the common environmental conditions: (1) low fire danger, (2) high fire danger and steep slopes, (3) high fire danger and low canopy cover, and (4) high fire danger and high canopy cover. There are at least three important implications from the results of this study; one, fire environment conditions do not need to be extreme or unusual for an entrapment to occur, two, the region and site specific context is important, and, three, non-environmental factors such as human behavior remain a critical but difficult to assess factor in wildland firefighter entrapment potential. Full article