Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

Wildland fires are a critical Earth-system process that impacts human populations in each settled continent[...]

Question: How frequent and variable were fire disturbances in longleaf pine ecosystems? Has the frequency and seasonality of fire events changed during the past few centuries? Location: Kisatchie National Forest, Western Gulf Coastal Plain, longleaf pine‐bluestem ecosystem, in relatively rough topography adjacent to the Red River, Louisiana, USA. Methods: Cross-sections of 19 remnant pines exhibiting 190 fire scars were collected from a 1.2-km 2 area. Tree-rings and fire scars were precisely dated and analysed for the purpose of characterizing past changes in fire and tree growth. Temporal variability in fire occurrence and seasonality was described for the pre- and post-European settlement periods. Seasonality of historic fires was determined by the scar position within the rings. The relationship between fire and drought was investigated using correlation and superposed epoch analysis. Results: The mean fire return interval for the period 1650-1905 was 2.2 years (range 0.5 to 12 yr). Significant new findings include: evidence for years of biannual burning, temporal variability in fire seasonality, an increase in fire frequency and percentage of trees scarred circa 1790, and synchronous growth suppression and subsequent release of trees coinciding with land-use changes near the turn of the 20th century. Drought conditions appeared unrelated to the occurrence of fire events or fire seasonality. Conclusions: Multi-century fire history records from longleaf pine ecosystems are difficult to obtain due to historic land-use practices and the species high resistance to scarring; however, our results indicate potential for reconstructing detailed fire histories in this ecosystem. Fire scars quantitatively documented one of the most frequent fire regimes known. Fire regime information, such as the temporal variability in fire intervals, prevalence of late-growing season fire events and biannual burning, provide a new perspective on the dynamics of longleaf pine fire regimes.

A comparison of Canadian and Russian boreal forest fire regimes

Aim As fire activity changes globally, we need to better understand the spatial and temporal characteristics of the individual events that, when aggregated, constitute fire regimes. Most global studies analyze point detections of burned area, without delineating or considering the properties of individual events. Furthermore, there is a critical need to understand fire patterns within the context of the geopolitical boundaries within which fires are managed. Location Global. Time Period 2003–2020. Major Taxa Studied Fire. Methods We divided 241 countries by Köppen‐Geiger climate classifications and quantified four event‐based fire regime metrics: size, duration, and mean and maximum growth rate; and four area‐based metrics: burned area, number of fires, season length, and season peak. We examined the correlations among fire regime components, and between each fire regime component and climate normals. We quantified temporal trends, and used mixed models to analyze how climate and landcover change were associated with event‐based components of fire regimes. Results Event‐based metrics were weakly correlated with area‐based metrics. Countries with warmer and less variable climates had high burned area, more fire events, longer season lengths and shorter event durations. Countries with high annual temperature range and low precipitation tended to have fewer events but larger fires that were faster‐spreading and occurred later in the year. The growth rate and size of individual fire events are increasing in 18% and 21% of regions we analysed, respectively. Interannual variability in size and growth rate was associated with aridity increases in boreal areas, and landcover changes in arid areas. Main Conclusions Drivers of burned area and fire seasonality are well understood but largely unrelated to the properties of individual events. A more detailed understanding of the spatial and temporal aspects of fire events at broad scales will assist fire management efforts in preparing for a warmer future.

AimOver the past several decades, wildfires have become larger, more frequent, and/or more severe in many areas. Simultaneously, anthropogenic ignitions are steadily growing. We have little understanding of how increasing anthropogenic ignitions are changing modern fire regimes.LocationConterminous United States.Time period1984–2016.Major taxa studiedVegetation.MethodsWe aggregated fire radiative power (FRP)‐based fire intensity, event size, burned area, frequency, season length, and ignition type data from > 1.8 million government records and remote sensing data at a 50‐km resolution. We evaluated the relationship between fire physical characteristics and ignition type to determine if and how modern U.S.A. fire regimes are changing sensu stricto given increased anthropogenic ignitions, and how those patterns vary over space and time.ResultsAt a national scale, wildfires occur over longer fire seasons (17% increase) and have become larger (78%) and more frequent (12%), but not necessarily more intense. Further, human ignitions have increased 9% proportionally. The proportion of human ignitions has a negative relationship with fire size and FRP and a positive relationship with fire frequency and season length. Areas dominated by lightning ignitions experience fires that are 2.4 times more intense and 9.2 times larger. Areas dominated by human ignitions experience fires that are twice as frequent and have a fire season that is 2.4 times longer. The effect of human ignitions on fire characteristics varies regionally. Ecoregions in the eastern U.S.A. and in some parts of the coastal western U.S.A. have no areas dominated by lightning ignitions. For the remaining ecoregions, more intense and larger fires are associated with lightning ignitions, and longer season lengths are associated with human ignitions.Main conclusionsIncreasing anthropogenic ignitions – in tandem with climate and land cover change – are contributing to a ‘new normal’ of fire activity across continental scales.

202 years of changes in Mediterranean fire regime in Pinus nigra forest, Corsica

Large fires account for the majority of burned area and are an important focus of fire management. However, ‘large’ is typically defined by a fire size threshold, minimizing the importance of proportionally large fires in less fire-prone ecoregions. Here, we defined ‘large fires’ as the largest 10% of wildfires by ecoregion (n = 175,222 wildfires from 1992 to 2015) across the United States (U.S.). Across ecoregions, we compared fire size, seasonality, and environmental conditions (e.g., wind speed, fuel moisture, biomass, vegetation type) of large human- and lighting-started fires that required a suppression response. Mean large fire size varied by three orders of magnitude: from 1 to 10 ha in the Northeast vs. >1000 ha in the West. Humans ignited four times as many large fires as lightning, and were the dominant source of large fires in the eastern and western U.S. (starting 92% and 65% of fires, respectively). Humans started 80,896 large fires in seasons when lightning-ignited fires were rare. Large human-started fires occurred in locations and months of significantly higher fuel moisture and wind speed than large lightning-started fires. National-scale fire policy should consider risks to ecosystems and economies by these proportionally large fires and include human drivers in large fire risk assessment.

BackgroundSteep elevational gradients bring multiple forest types and fire regimes together in close proximity. The San Francisco Peaks/Dook’o’oosłííd in northern Arizona rise to 3851 m elevation with slopes that span many of the major forest types of the southwestern US mountains. To reconstruct past fire regimes across this broad elevational gradient, we sampled fire-scarred trees across the south face of the Peaks, complementing previous research on forest structure, composition, and origin of aspen stands.ResultsAt the highest elevations, Rocky Mountain bristlecone pine forests had a mean fire interval (MFI) of 19.7 years prior to a modern fire exclusion period beginning after 1879. Other high-elevation (> 2800 m) mixed conifer forests had MFI = 5.7 years and low-elevation (< 2,800 m) pine forests had MFI = 4.0 years. After 1879, there were no large fires through the end of the twentieth century. Before 1879, fires occurred in the early to middle growing season, and fire event years were linked to climate across all elevations, with a stronger association to drought (i.e., the Palmer Drought Severity Index) than to El Niño-Southern Oscillation phase. Pulses of forest regeneration were associated with the fire regime, with the largest pulse occurring shortly after fire exclusion. In addition to fire exclusion, other factors such as post-fire sprouting and regeneration after tree harvesting likely contributed to the current dense forest structure on the Peaks.ConclusionsFollowing over a century of fire exclusion, fire activity has increased on the Peaks over the past two decades, with large recent fires of uncharacteristic severity raising concerns about tree mortality, erosion, flooding, and infrastructure damage in surrounding human communities. Past fire regimes provide useful insight into fire-climate-forest interactions and the conditions under which existing forest communities were well adapted, but adaption to future conditions is likely to be challenging due to the rapid pace of projected environmental changes.

Direct effects of climate change (i.e. temperature rise, changes in seasonal precipitation, wind patterns and atmospheric stability) affect fire regimes of boreal forests by altering fire behaviour, fire seasons and fuel moisture. Climate change also alters species composition and fuel characteristics, which subsequently alter fire regimes. However, indirect effects of climate change are often simplified or neglected in the direct climate–fire relationship models and dynamic global vegetation models. This may result in high uncertainties associated with existing projections of fire regimes for climate change scenarios. Moreover, few studies have examined fire regime predictions beyond the 21st century, and consequently, how the fire regimes of boreal forests would respond to climate change at the long term (&gt;100 years) are not clear. We develop a coupled modelling framework integrating direct and indirect effects of climate change to predict fire occurrence probability and burned area for boreal forests in northeastern China. We applied repeated measures ANOVA to quantify direct and indirect effects of climate change on fire regimes in the short (0–50 years), medium (60–100 years) and long term (150–200 years). Results showed that for the 21st century, direct effects of climate change are likely to exert a stronger influence on fire regimes than indirect effects. However, increases in fire occurrence probability and burned area will accelerate the transition of boreal forests to temperate forests in the period 2100–2200, and thereby reduce fire occurrence probability and burned area. This suggests that vegetation change will mediate direct effects of climate change on fire regimes of boreal forests at the long term. Synthesis and applications. Vegetation change will mediate direct effects of climate change on fire regimes of boreal forests at the long term. This finding suggested that policymakers may consider adaptive management by planting deciduous species to reduce fire occurrence probability and resistant management by reducing competition to promote boreal species under changing climate conditions.

Large fires or small fires, will they differ in affecting shifts in species composition and distributions under climate change?

Fire regimes were reconstructed from fire-scarred trees on five large forested study sites (135–810 ha) on the North and South Rims at Grand Canyon National Park. Adequacy of sampling was tested with cumulative sample curves, effectiveness of fire recording on individual trees, tree age data, and the occurrence of 20th Century fires which permitted comparison of fire-scar data with fire-record data, a form of modern calibration for the interpretation of fire-scar results. Fire scars identified all 13 recorded fires &amp;gt;8 ha on the study sites since 1924, when record keeping started. Records of fire season and size corresponded well with fire-scar data. We concluded that the sampling and analysis methods were appropriate and accurate for this area, in contrast to the suggestion that these methods are highly uncertain in ponderosa pine forests. Prior to 1880, fires were most frequent on low-elevation ‘islands’ of ponderosa pine forest formed by plateaus or points (Weibull Median Probability Intervals [WMPI] 3.0–3.9 years for all fires, 6.3–8.6 years for ‘large’ fires scarring 25% or more of the sampled trees). Fires were less frequent on a higher-elevation ‘mainland’ site located further to the interior of the North Rim (WMPI 5.1 years all fires, 8.7 years large fires), but fires tended to occur in relatively drier years and individual fires were more likely to burn larger portions of the study site. In contrast to the North Rim pattern of declining fire frequency with elevation, a low-elevation ‘mainland’ site on the South Rim had the longest fire-free intervals prior to European settlement (WMPI 6.5 years all fires, 8.9 years large fires). As in much of western North America, surface fire regimes were interrupted around European settlement, 1879 on the North Rim and 1887 on the South Rim. However, either two or three large surface fires have burned across each of the geographically remote point and plateau study sites of the western North Rim since settlement. To some extent, these sites may be rare representatives of nearly-natural conditions due to the relatively undisrupted fire regimes in a never-harvested forest setting.

Understanding wildfires in mainland Spain. A comprehensive analysis of fire regime features in a climate-human context

Summary Serotiny, the retention of mature seeds in closed fruits within the canopy for over a year, is a common trait in fire‐prone environments. When competition with adult plants prevents seedling establishment between fire events and in the absence of post‐release soil seed dormancy, strong serotiny, i.e. the retention of all seeds until the next fire, appears as the best strategy. Despite the low levels of inter‐fire seed recruitment for several species in both Australian and South African fire‐prone environments, considerable variation in the duration of fruit retention is nevertheless observed among species. Our aim is to predict optimal age‐specific reproductive schedules in a perennial, serotinous species, when cone maintenance is costly. We focus on species where adults are killed by fire, without a soil seed‐bank. We explicitly consider a trade‐off between growth (which determines plant survival), seed production and seed maintenance. In our model recruitment relies upon fire events. We use dynamic programming to determine, for given fire regimes, the optimal pattern of resource allocation. We further study the effect of changes in fire regime on the viability of populations adapted to some historical fire regime. We find that, whenever maximal plant survival probability is low, the optimal strategy consists in reducing resource allocation to seed maintenance while increasing resource allocation to annual seed production. This illustrates a trade‐off between current and future reproduction. A low rather than a strong level of serotiny should evolve whenever the variance of fire intervals is large and the mean fire interval is low. Low levels of serotiny could constitute a bet‐hedging strategy with decreasing predictability of the arrival of fire. Once adapted to some historical fire regime, serotinous populations are highly sensitive to a change in mean fire frequency and to an increase in the variance of fire intervals. Populations adapted to a historically high level of variance in fire return are more robust to changes in fire regime. Synthesis: Life‐history trade‐offs and low predictability of fire intervals may favour low rather than strong levels of serotiny even when recruitment essentially occurs just after fire events.

Analysis of complex spatio-temporal fire data is an important tool to assist the management and study of fire regimes. For fire ecologists, a useful visual aid to identify contrasting fire regimes is to map temporal sequences of data such as fire return intervals, seasons, and types (planned versus unplanned fire) across the landscape. However, most of the programs that map this information are costly and complex, requiring specialist training. We present a simple yet novel method for creating sequences of temporal data for mapping fire regimes using basic geographic information system (GIS) techniques and logical test functions in Microsoft® Excel 2003 (Microsoft, Bellevue, Washington, USA). Using fire history data (1972 to 2005) for southwestern Australia, we assigned integer classifications to fire return intervals (short, moderate, and long) and fire types and seasons (wildfires and prescribed burns in different seasons) and joined the integer classifications together to form a sequence of numbers representing the order of either fire return intervals or fire seasons in reverse time sequence. This sequence can be mapped in a GIS environment so that spatial dimensions formed by overlapping polygons are readily observed, and the temporal sequence of fire data within each polygon can be interpreted across the landscape. We applied the technique to examine experimental design options for investigating the effects of contrasting fire regimes on biota at the landscape scale. This investigation identified several important factors: 1) patterns were evident in fire types and seasons, 2) patterns were evident for fire return interval sequences, and 3) combining fire types and seasons with fire return intervals significantly constrained options for the study design. A visual analysis of this type highlights fire regime patterns in the landscape and permits a feasibility study for the development of study design options and the spatial arrangement of potential study sites.

The hummock grasslands of arid Australia are fire-prone ecosystems in which the perennial woody plants mostly resprout after fire. The resprouting ability among these species is poorly understood in relation to environmental variation; consequently, little is known about the impacts that contemporary fire regimes are having on vegetation within these systems. We examined the resprouting ability of adults and juveniles of four widespread Acacia species (A. aneura, A. kempeana, A. maitlandii, A. melleodora) by experimentally testing the effects of fire severity, interval and season. We found that fire severity and season strongly affected survival, but the magnitude of the effects was variable among the species. Unexpectedly, a short fire interval of 2 years did not have a strong negative effect on resprouting of any species. Fire severity had variable effects among the four species, with those species with more deeply buried buds being more resilient to high-severity soil heating than those with shallow buds. Season of fire also strongly affected survival of some species, and we propose that seasonal variation in soil heating and soil moisture mediated these effects. The species by environment interactions we observed within one functional group (resprouters with a soil-stored seed bank) and in one genus suggest that modelling landscape response to fire regimes will be complex in these arid ecosystems. We predict, however, that the dominant resprouting acacias in hummock grasslands of central Australia are highly resilient to a range of fire regimes.