Time Since Fire Research Articles

Time since fire as a driver of taxonomic and phylogenetic patterns of grassland plant communities

Fire plays a key role in grasslands, determining the distribution and evolution of species and boundaries with neighboring ecosystems. Evidence of community-wide responses to fire is largely based on taxonomic and functional descriptors, while the phylogenetic dimension is overlooked. Here we evaluated how the taxonomic and phylogenetic structure of grassland plant communities responded to a time since fire (TSF) gradient. We sampled 12 communities in Southern Brazil under varying TSF and calculated taxonomic species richness (S) and dominance (D), phylogenetic diversity (PD), and mean phylogenetic distances (MPD). We used Structural Equation Models to test the relationships between the environmental gradient and community descriptors. Communities with longer TSF presented higher PD and MPD but lower species richness and increased taxonomic dominance. These sites were dominated by monocots, specifically C4 grasses, but also presented exclusive clades, whereas recently-burned sites presented lower taxonomic dominance and more species distributed in a wider variety of clades. Our results indicate that these scenarios are interchangeable and dependent on fire management. Fire adaptation was not constrained by phylogenetic relatedness, contrasting with previous findings for tropical savannahs and indicating that temperate and tropical non-forest ecosystems from South America respond differently to fire, possibly due to different evolutionary histories.

The effectiveness of past wildfire at limiting reburning is short-lived in a Mediterranean humid climate

BackgroundThe study of wildfire interactions (i.e., spread limitation and reburns) is gaining traction as a means of describing the self-limiting process of fire spread in the landscape and has important management implications but has scarcely been attempted in Europe. We examined to what extent previously burned areas restricted the development of individual large wildfires (> 500 ha) in mainland Portugal.ResultsFor the 1984–2021 period, we (1) modeled the proportion of large wildfire perimeters coinciding with transitions to shorter time since fire (TSF), i.e., locations where fire spread ceased upon encountering assumedly less flammable fuels, and (2) characterized the prevalence of different TSF in the composition of the area burned by large wildfires in relation to available TSF. Only 4% of the large wildfires did not comprise edges intersecting past wildfires. Low TSF (especially up to 8 years) resulted in large-wildfire perimeter limitation at TSF transitions. This effect was further enhanced by high historical burn probability and proximity to roadways and watercourses. Perimeter limitation did also increase under high (but not very high or extreme) fire danger, benefiting from maximum seasonal firefighting preparedness. TSF prevalence in the composition of large-wildfire area was extremely variable and thus an overall weak pattern emerged, with minimum and maximum prevalence respectively at TSF < 2 years and TSF ≥ 6 years.ConclusionsLarge wildfire limitation in Portugal is hampered by fast fuel build-up after fire, indicating a short-lived fire-hazard reduction effect under the prevailing Mediterranean humid climate of the study region. Nonetheless, such effect should be considered when planning fuel-reduction treatments and can be used opportunistically during large-wildfire suppression operations.

Genetic Diversity of Florida Scrub Lizards (Sceloporus woodi) Varies with Wildfire History

Fire shapes habitats and therefore influences the genetic characteristics of populations. Florida scrub is a fire-dependent habitat with several precinctive species, including Florida Scrub Lizards (Sceloporus woodi). Fire history of scrub patches could affect the movement patterns of Florida Scrub Lizards, thereby altering the genetic characteristics of local populations. We characterized the effect of time since fire (TSF) on genetic diversity and differentiation at 6 microsatellite loci in the Florida Scrub Lizard (n = 413) collected from 17 sites in Highlands County, Florida. Private allelic richness was positively correlated with TSF (r = 0.56, P = 0.009). In sites with a TSF of 3–17 yr, TSF was negatively correlated with expected heterozygosity (r = -0.90, P = 0.009), inbreeding (r = -0.77, P = 0.04), allelic richness (r = -0.79, P = 0.03), and private allelic richness (r = -0.80, P = 0.03); TSF was positively correlated with mean pairwise relatedness (r = 0.85, P = 0.02). Therefore, a consequence of TSF is short-term change to local population genetics that is likely precipitated by responses of Florida Scrub Lizards to habitat modification. At a TSF of >20 yr, TSF and genetic diversity were not correlated, indicating that factors other than fire shape genetic diversity in long-unburned locations. We detected genetic differentiation using Bayesian clustering and estimates of F-statistics. Our results highlight the importance of consistent fire regimes in the Florida scrub on the genetic diversity of Florida Scrub Lizards. The presence of Florida Scrub Lizard populations in long-unburned sites, however, warrants further investigation.

Exploring Ecoacoustic Trajectories in a Giant Sequoia Forest After Wildfire

Forest management strategies that create spatially diverse fire-caused disturbance outcomes, consistent with historic fire regimes, are a desired condition for fire adapted western United States forests. In this context, the temporal dynamics of forest response to fire can inform the tempo and scale of forest management, including prescribed burning. Here, we investigated the use of ecoacoustic methods to assess ecological condition in a four-year period (2016–2019) after wildfire in a giant sequoia forest landscape within Kings Canyon National Park, California, United States. Audio recorders at nine sites were deployed soon after the 2015 Rough Fire subsided. The monitoring sites were located in regions with different fire histories, representing five fire history categories. We used the Acoustic Complexity Index (ACI) to document biotic chorus complexity. This previously tested ecoacoustic index provided a daily indicator of biotic sound activity in frequencies dominated by avian calls. Patterns in ACI were evaluated using generalized additive mixed models to understand the relationship with time-since-fire and covariates that accounted for season, fire history category, and weather conditions. We showed that time-since fire and fire-history influenced patterns in ACI after accounting for season and air temperature effects. Monitoring sites where prescribed fire preceded the Rough Fire showed the highest predicted ACI and evidence for a relatively consistent seasonal pattern in ecoacoustic activity across subsequent seasons. Sites without prescribed fire and burned by the Rough Fire exhibited the most pronounced successive decreases in ACI in the first and second years after the fire. The daily temporal resolution of the ecoacoustic index also revealed phenological shifts related to time-since-fire and fire history. Sites unburned by the Rough Fire offered some context for how fire changed ecoacoustic activity post-wildfire, however evidence suggested they were also impacted by the presence of the nearby Rough Fire. The patterns in the ecoacoustic index when combined with vegetation surveys offered complementary insights into ecological dynamics of regeneration after fire. Our exploratory analysis showed that using ecoacoustic methods in wildfire monitoring offers a scalable approach to remote sensing of ecological trends. Archived recordings from the monitoring effort afford future opportunities for new or more detailed insights.

Wildfire contribution to streamflow variability across Australian temperate zone

As wildfires become more frequent and severe, there are concerns regarding their impacts on water yield from forested catchments. While there are many studies in Australia about the effects of individual wildfires on streamflow at fine scales (<1 km2) in specific geographic settings, the effects of wildfire regimes on streamflow at broad spatial scales across temperate forests in Australia are not well understood. In this paper, we combined climate, wildfire, streamflow records (1975 to 2018), topographic, and landcover data from 92 catchments (74 – 4740 km2) in the Australian temperate zone to quantify the contribution of wildfire regimes over time to streamflow variability in different hydroclimatic settings (humid, dry sub-humid, and semi-arid) and geographic regions (Southeast Australia (SEA), Southwest Western Australia (SWWA), and Tasmania (TAS)). Wildfire regimes were represented by two metrics: the burnt area to drainage area (BDA) ratio for wildfire events in each year and a spatially averaged metric of Time Since Fire (TSF), which is a spatial average of time since the last wildfire in the catchment. By comparing prefire and postfire runoff ratios our study found that on average there was a short-term increase in runoff ratio (∼3% in year 1 and ∼6% in year 2 post-fire) after wildfires with BDA > 25%. No influence of fire was found in the long-term (15–20 years after the wildfire) runoff ratio. We found that wildfire regime, measured by TSF, explained ∼8.8% of the variation in annual streamflow across the Australian temperate zone, and that with decreasing TSF (i.e., increased wildfire impact), the average streamflow increased. Streamflow variation explained by wildfire regimes varied with hydroclimate. The explained variance of streamflow by wildfire regimes in semi-arid catchments (23%) and dry sub-humid catchments (13%) were higher than humid catchments (5%). Our results provide a broad-scale understanding of how wildfire regimes influence streamflow variability at broad temporal and spatial scales, and provided important context and baseline information for determining the implications of changes in climate and fire regimes for regional water availability.

Fire, drought and flooding rains: The effect of climatic extremes on bird species’ responses to time since fire

AbstractAimClimatic extremes and fire affect ecosystems across the globe, yet our understanding of how species are influenced by the interaction of these broadscale ecological drivers is poorly understood. Using a ten‐year dataset, we tested how extreme drought and rainfall interacted with time since fire (TSF) to shape bird species’ distributions.LocationSemi‐arid mallee woodlands of south‐eastern Australia.MethodsWe quantified the effects of climatic extremes on bird species’ occurrence, species richness and incidence at 180 sites across three climatic periods—an El Niño‐associated drought (the “Big Dry”), immediately after La Niña drought‐breaking rainfall (“Big Wet”) and three years following the La Niña event (“Post‐Big Wet”). We then compared species’ responses with TSF across the three climatic periods using a chronosequence of sites from 1 to 117 years post‐fire.ResultsLa Niña rainfall had sustained impacts on species’ occurrence. Over half of species increased significantly during the Big Wet. Despite three intervening years of below‐average rainfall, three quarters of these species remained comparably high, Post‐Big Wet. By contrast, less than half of threatened and declining species benefited from high rainfall. Responses of species to TSF were found to differ contingent on climatic conditions: almost twice as many species responded to TSF during the Big Wet and almost three times as many Post‐Big Wet, compared with the Big Dry. Across climatic periods, a majority of species showed preference for mid to older post‐fire vegetation.Main conclusionsVariation in responses to TSF is likely due to the effect of climatic variation on resources. We suggest that, at sites of different post‐fire age, interactions between TSF and climate may differentially influence both the availability and longevity of resources. Given climatic extremes are predicted to become increasingly severe with climate change, accounting for their influence on fauna–fire dynamics will require careful management of fire.

Impacts of prescribed burnings on litter production, nitrogen concentration, δ13C and δ15N in a suburban eucalypt natural forest of subtropical Australia

Prescribed burning is projected to be adopted more frequently with intensifying climate change; thus, a long-term study is necessary to understand the burning impacts on forest productivity and carbon (C) and nitrogen (N) cycling. Litter fall production rate can be used to indicate burning impacts on forest productivity, whereas N concentration, and C and N isotope composition (δ13C and δ15N) can be used to infer burning impacts on C and N cycling in plant-soil system. In this study, the impacts of low-intensity prescribed burning on litter production, N concentration, and C and N isotope compositions were continuously investigated for 6 years at five study sites in a natural eucalypt forest of subtropical Australia. Higher leaf litter production rate, N concentration and δ15N, and lower δ13C could be seen shortly after prescribed burning. The higher leaf litter N concentration and lower δ13C were likely due to the ease of competition for soil N and moisture from understory vegetation in the short term by prescribed burning. Leaf δ15N and N concentration were closely correlated, and seasonal changes in leaf litter production rate, δ13C and δ15N were observed. Burning season and related severity might determine the suppression degree of understory vegetation. Time since fire (TSF) was a significant impact factor influencing the litter fall production rate, N concentration, δ13C and δ15N of leaf litter fall for a decade following prescribed burning. However, monthly rainfall and temperature were less consistent in their impacts. Nitrogen limitation was enhanced by prescribed burning through the removal of litter and understory vegetation in the N poor forest and might be responsible for the long-term burning impacts. Low-intensity prescribed burning might have a long-lasting impact on forest litter productivity in nutrient poor forests in subtropical Australia.

Time-since fire and cynipid gall wasp assemblages on oaks

Fires are ubiquitous features of many terrestrial ecosystems and can greatly impact the structure and evolution of plant communities. However, much less is known about how fire history impacts higher trophic levels. Using detailed records on the history and intensity of fires at the Archbold Biological Station (ABS) in central Florida, USA, we examined how time-since-fire affects the cynipid gall wasp assemblage (Hymenoptera: Cynipidae) associated with four oak species (Quercus) that are dominant components of the plant community in this region. Cynipid abundance, richness and diversity were quantified from 1249 oak trees/shrubs in 20 sites that varied in time-since-fire from 1.5 to 91 years. Among all sites and oaks, we found 24 species of cynipids and there was very little species overlap among oak species, even within the same site. Gall abundance increased with time-since-fire and was correlated with tree height, suggesting that available host material or plant architecture may be a primary driver of cynipid recovery. Within 3 years of a fire, 14 of the 23 cynipid species were detected among the sites, and by seven years since fire, all but two species could be detected. Overall, species richness and diversity reached an asymptote within ≈ 7 years. Given how quickly the cynipid assemblage recovers after a fire, frequent fires at ABS are unlikely to negatively impact these insects. However, in smaller or more isolated scrub-oak fragments, recovery could be much slower.

Time since fire is an over-simplified measure of habitat suitability for the New Holland mouse

Abstract Fire has shaped much of the Australian landscape, and alterations to natural or historical fire regimes are implicated in the decline of many native mammal species. Time since fire (TSF) is a common metric used to understand vegetation and faunal responses to fire but is unlikely to capture the complexity of successional changes following fire. The New Holland mouse (Pseudomys novaehollandiae), a threatened and declining rodent species native to southeastern Australia, is traditionally considered an early post-fire successional species. Here, we use a 48-year dataset to test whether this posited association with early TSF is upheld, and whether the species’ occurrence and abundance are governed by TSF. We find support for a minimal influence of TSF on the species’ occurrence, and that while abundance of P. novaehollandiae is partly explained by TSF, considerable uncertainty and variation among fire events and locations limit the usefulness of TSF in informing conservation management strategies. We suggest that it is not helpful to consider the species as early successional and that fire planning for P. novaehollandiae conservation is best considered at a local scale. Additionally, we provide guidelines for maximizing individual survival and persistence during and after planned burns.

Non-additive effects of alternative stable states on landscape flammability in NW Patagonia: fire history and simulation modelling evidence

Understanding the relationship between flammability and time since fire (TSF) is crucial for predicting ecosystem responses to changes in fire regimes. Landscapes composed of alternative stable states displaying positive fire–vegetation feedbacks are especially sensitive to these changes. We derived TSF–flammability functions (Logistic, Olson, Moisture, Weibull) from survival analysis applied to north-west Patagonian landscapes and simulated landscapes composed of different proportions of alternative stable states (shrublands and forest) and fire frequencies. We expected that landscapes dominated by shrublands would show an asymptotic growth (Logistic or Olson) and those dominated by forest would show a hump-shaped growth (Moisture). Additionally, we expected that the landscape-level flammability functions would resemble the pattern of the most abundant community. We found that shrublands tended to dominate the TSF–flammability relationship (Logistic) even when they were less abundant in the landscape (non-additive effects). The flammability function followed a hump-shaped growth (Moisture) only when the forest cover was &amp;gt;80%. Our results highlight that alternative stable states occur not only because of positive fire–vegetation feedbacks, but also thanks to the non-additivity of the flammability of the different states in the landscape. Non-additive effects could have an important role in accelerating landscape transformations towards more flammable states.

The disproportionate importance of long-unburned forests and woodlands for reptiles.

Our understanding of the impacts of time since fire on reptiles remains limited, partly because there are relatively few locations where long‐term, spatially explicit fire histories are available. Such information is important given the large proportion of some landscapes that are managed with frequent prescribed fire to meet fuel management objectives. We conducted a space‐for‐time study across a landscape in southeastern Australia where the known fire history spanned 6 months to at least 96 years. Four methods were used to survey reptiles in 81 forest and woodland sites to investigate how time since fire (TSF), habitat, and environmental variables affect reptile richness, abundance, and composition. We used generalized linear models, generalized linear mixed‐effects models, PERMANOVA, and SIMPER to identify relationships between the reptile assemblage (richness, abundance, and composition, respectively) and TSF, habitat, and environmental variables. All three reptile metrics were associated with TSF. Reptile richness and abundance were significantly higher in sites >96 years postfire than younger fire ages (0.5–12 years). Reptile composition at long‐unburned sites was dissimilar to sites burned more recently but was similar between sites burned 0.5–2 and 6–12 years prior to sampling. Synthesis and applications. Long‐unburned forests and woodlands were disproportionately more important for reptile richness and abundance than areas burned 6 months to 12 years prior to sampling. This is important given that long‐unburned areas represent <8% of our study area. Our results therefore suggest that reptiles would benefit from protecting remaining long‐unburned areas from fire and transitioning a greater proportion of the study area to long‐unburned. However, some compositional differences between the long‐unburned sites and sites 0.5–12 years postfire indicate that maintaining a diversity in fire ages is important for conserving reptile diversity.

Effects of Fire and Pollinator Visitation on the Reproductive Success ofAsimina reticulata(Annonaceae), the Netted Pawpaw

ABSTRACT The netted pawpaw (Annonaceae, Asimina reticulata) is a widespread Florida endemic plant that produces relatively few fruits. To understand its distribution and reproductive success, we posed three questions: (a) how do time-since-fire (TSF) and pollinator visitation influence reproductive success, (b) is it pollinator dependent, and (c) does outcrossing increase reproductive success? In 2016, we studied 157 plants in three flatwoods burn units (last burned, 2015, 2012, or 2001) and one grazed pasture at Archbold Biological Station in south-central Florida. Plants in pasture and recently burned flatwoods exhibited greater flower, fruit, and seed output, suggesting that habitat openness may promote competitive release or increased pollinator visitation. Pollinator visitation was greatest on plants within the 1-yr TSF, and the hairy flower scarab (Trichiotinus viridans) was the most prevalent visitor across all sites. Autogamously crossed flowers produced no fruit, confirming that seed production ...

Drivers of postfire soil organic carbon accumulation in the boreal forest.

The accumulation of soil carbon (C) is regulated by a complex interplay between abiotic and biotic factors. Our study aimed to identify the main drivers of soil C accumulation in the boreal forest of eastern North America. Ecosystem C pools were measured in 72 sites of fire origin that burned 2-314years ago over a vast region with a range of ∆ mean annual temperature of 3°C and one of ∆ 500mm total precipitation. We used a set of multivariate a priori causal hypotheses to test the influence of time since fire (TSF), climate, soil physico-chemistry and bryophyte dominance on forest soil organic C accumulation. Integrating the direct and indirect effects among abiotic and biotic variables explained as much as 50% of the full model variability. The main direct drivers of soil C stocks were: TSF >bryophyte dominance of the FH layer and metal oxide content >pH of the mineral soil. Only climate parameters related to water availability contributed significantly to explaining soil C stock variation. Importantly, climate was found to affect FH layer and mineral soil C stocks indirectly through its effects on bryophyte dominance and organo-metal complexation, respectively. Soil texture had no influence on soil C stocks. Soil C stocks increased both in the FH layer and mineral soil with TSF and this effect was linked to a decrease in pH with TSF in mineral soil. TSF thus appears to be an important factor of soil development and of C sequestration in mineral soil through its influence on soil chemistry. Overall, this work highlights that integrating the complex interplay between the main drivers of soil C stocks into mechanistic models of C dynamics could improve our ability to assess C stocks and better anticipate the response of the boreal forest to global change.

Fire intensity drives post-fire temporal pattern of soil carbon accumulation in Australian fire-prone forests

The impact of fire on global C cycles is considerable but complex. Nevertheless, studies on patterns of soil C accumulation following fires of differing intensity over time are lacking. Our study utilised 15 locations last burnt by prescribed fire (inferred low intensity) and 18 locations last burnt by wildfire (inferred high intensity), with time since fire (TSF) up to 43years, in a homogenous forest type in south eastern Australia. Following a stratified approach to mineral soil sampling, the soil % total C (% CTot) and % recalcitrant pyrogenic C (% RPC), were estimated. Generalised additive models indicated increases in % CTot at TSF >30years in sites last burnt by wildfire. Estimates in sites last subjected to prescribed fire however, remained constant across the TSF chronosequence. There was no significant difference in % CTot between the different fire types for the first 20years after fire. In the first 10years after wildfires, % RPC was elevated, declining to a minimum at ca. TSF 25years. After prescribed fires, % RPC was unaffected by TSF. Differences in response of % CTot and % RPC to fire type may reflect the strength of stimulation of early successional processes and extent of charring. The divergent response to fire type in % CTot was apparent at TSF longer than the landscape average fire return interval (i.e., 15 to 20years). Thus, any attempt to increase C sequestration in soils would require long-term exclusion of fire. Conversely, increased fire frequency is likely to have negligible impact on soil C stocks in these forests. Further investigation of the effects of fire frequency, fire intensity combinations and interaction of fire with other disturbances will enhance prediction of the likely impact of imposed or climatically induced changes to fire regimes on soil C.

Bayesian networks elucidate interactions between fire and other drivers of terrestrial fauna distributions

AbstractFire is a major driver of community composition and habitat structure and is extensively used as an ecological management tool in flammable landscapes. Interactions between fire and other processes that affect animal distributions, however, cause variation in faunal responses to fire and limit our ability to identify appropriate fire management regimes for biodiversity conservation. Bayesian networks (BNs) have not previously been used to examine terrestrial faunal distributions in relation to fire, but offer an alternative statistical framework for modeling complex environmental relationships as they explicitly capture interactions between predictor variables. We developed a conceptual model of the interactions between drivers of faunal distributions in fire‐affected landscapes, and then used a non‐parametric BN modeling approach to describe and quantify these relationships for a suite of terrestrial native mammal species. We also tested whether BNs could be used to predict these species’ distributions using only remote‐sensed or mapped variables. Data were collected at 113 sites across 47,000 ha of continuous eucalypt forest in the Otway Ranges, southeastern Australia; time‐since‐fire (TSF) ranged from six months to 74 yr. Habitat complexity increased with TSF and forest wetness. Critical‐weight‐range (35–5500 g) marsupials and rodents were generally more likely to occur at long unburnt sites with high habitat complexity, and in wetter forest types. In contrast, large grazers and browsers preferred less complex habitats and younger or drier forest. Species occurrences were more strongly affected by habitat complexity than TSF, coarse woody debris cover, or invasive predator (Vulpes vulpes or Felis catus) occurrence. Bayesian network models effectively discriminated between the presence and absence of most native mammal species, even when only provided with data on remote‐sensed or mapped variables (i.e., without field‐assessed data such as habitat complexity). Non‐parametric BNs are an effective technique for explicitly modeling the complex and context‐dependent influence of fire history on faunal distributions, and may reduce the need to collect extensive field data on habitat structure and other proximate drivers.

Time since disturbance affects colonization dynamics in a metapopulation.

Disturbances are widespread in nature and can have substantial population-level consequences. Most empirical studies on the effects of disturbance track population recovery within habitat patches, but have an incomplete representation of the recolonization process. In addition, recent metapopulation models represent post-disturbance colonization with a recovery state or time-lag for disturbed ("focal") patches, thus assuming that recolonization rates are uniform. However, the availability of colonists in neighbouring "source" patches can vary, especially in frequently disturbed landscapes such as fire-managed forests that have a mosaic of patches that differ in successional state and undergo frequent local extinctions. To determine how time since disturbance in both focal and neighbouring source patches might affect metapopulations, we studied the effects of time since fire (TSF) on abundances of a specialist palmetto beetle within and between fire management units in Apalachicola National Forest, Florida. We measured beetle abundances at three distances from the shared edge of paired units, with units ranging from 0 to 64months since fire and the difference in time since burning for a focal-source pair ranging from 3 to 58months. Soon after fire, beetle abundances within management units were highest near the unit edge, but this pattern changed with increasing TSF. Between paired units, the more recently disturbed ("focal") unit's beetle abundance was positively related to source unit abundance, but the shape of this relationship differed based on focal unit TSF and the units' difference in time since burning. Results suggest that both focal and source habitat history can influence recolonization of recently disturbed patches and that these effects may persist over years. Thus, when predicting metapopulation dynamics, variation in habitat characteristics should be considered not only for patches receiving colonists but for patches supplying colonists as well.

Fire Regime along Latitudinal Gradients of Continuous to Discontinuous Coniferous Boreal Forests in Eastern Canada

Fire is the main disturbance in North American coniferous boreal forests. In Northern Quebec, Canada, where forest management is not allowed, the landscape is gradually constituted of more opened lichen woodlands. Those forests are discontinuous and show a low regeneration potential resulting from the cumulative effects of harsh climatic conditions and very short fire intervals. In a climate change context, and because the forest industry is interested in opening new territories to forest management in the north, it is crucial to better understand how and why fire risk varies from the north to the south at the transition between the discontinuous and continuous boreal forest. We used time-since-fire (TSF) data from fire archives as well as a broad field campaign in Quebec’s coniferous boreal forests along four north-south transects in order to reconstruct the fire history of the past 150 to 300 years. We performed survival analyses in each transect in order to (1) determine if climate influences the fire risk along the latitudinal gradient; (2) fractionate the transects into different fire risk zones; and (3) quantify the fire cycle—defined as the time required to burn an area equivalent to the size of the study area—of each zone and compare its estimated value with current fire activity. Results suggest that drought conditions are moderately to highly responsible for the increasing fire risk from south to north in the three westernmost transects. No climate influence was observed in the last one, possibly because of its complex physical environment. Fire cycles are shortening from south to north, and from east to west. Limits between high and low fire risk zones are consistent with the limit between discontinuous and continuous forests, established based on recent fire activity. Compared to the last 40 years, fire cycles of the last 150–300 years are shorter. Our results suggest that as drought episodes are expected to become more frequent in the future, fire activity might increase significantly, possibly leading to greater openings within forests. However, if fire activity increases and yet remains within the range of variability of the last 150–300 years, the limit between open and closed forests should stay relatively stable.

Long-Term Recovery of Microbial Communities in the Boreal Bryosphere Following Fire Disturbance

Our study used a ∼360-year fire chronosequence in northern Sweden to investigate post-fire microbial community dynamics in the boreal bryosphere (the living and dead parts of the feather moss layer on the forest floor, along with the associated biota). We anticipated systematic changes in microbial community structure and growth strategy with increasing time since fire (TSF) and used amplicon pyrosequencing to establish microbial community structure. We also recorded edaphic factors (relating to pH, C and N accumulation) and the physical characteristics of the feather moss layer. The molecular analyses revealed an unexpectedly diverse microbial community. The structure of the community could be largely explained by just two factors, TSF and pH, although the importance of TSF diminished as the forest recovered from disturbance. The microbial communities on the youngest site (TSF = 14years) were clearly different from older locations (>100years), suggesting relatively rapid post-fire recovery. A shift towards Proteobacterial taxa on older sites, coupled with a decline in the relative abundance of Acidobacteria, suggested an increase in resource availability with TSF. Saprotrophs dominated the fungal community. Mycorrhizal fungi appeared to decline in abundance with TSF, possibly due to changing N status. Our study provided evidence for the decadal-scale legacy of burning, with implications for boreal forests that are expected to experience more frequent burns over the course of the next century.

Adsorption capacity of wildfire-produced charcoal from Pacific Northwest forests

This research evaluated the adsorption capacity of charcoal collected from O-horizon soils of mixed-severity fire regime forests in the Olympic Peninsula, Washington, USA. This property is important to consider in the context of post-fire soil carbon (C) recovery and nutrient cycling as charcoal has a high affinity for dissolved organic C compounds released by throughfall and organic matter decomposition. Charcoal samples produced in the laboratory at 300°C, 500°C, and 800°C provided a baseline for chemical composition and adsorption capacity using phenol as the sorbate. These samples were used to compare charcoal collected at wildfire chronosequence sites (3 – 115 years) where formation temperature was unknown and adsorption capacity is likely modified through weathering and adsorption of organic compounds over time. Wildfire-produced charcoal displayed no trend with time since fire (TSF) (p > 0.1) yet measured an average capacity of 29.70 (± 6.23) μg phenol. mg charcoal-1 across all sites. Increased formation temperature of laboratory charcoal correlated with a higher adsorption affinity of simple phenol in solution. Thermogravimetric analysis (TGA) was used to provide information on the composition of samples and quantified as the percent of total mass released by pyrolysis and combustion using discrete regions of the thermal curve. The black carbon (BC) fraction, defined in this study as the mass combustible at 1000°C under O2 (gas), was positively correlated to temperature in laboratory charcoal (r2=0.82, p<0.01) and negatively correlated to TSF in field-collected charcoal despite plot effects (r2=0.68; p<0.01). While the BC fraction of laboratory charcoal increased with increasing temperature, field-collected charcoal was found to have a higher BC fraction at more recent fire sites and suggests wildfire temperatures<300°C or oxidative decomposition with TSF. The mass of lignocellulose and adsorbed organic compounds volatilized between 200°C–750°C decreased with temperature in laboratory charcoal (r2=0.91; p<0.001) and increased with TSF in field-collected charcoal (r2=0.72; p<0.01). Results from TGA suggest that charcoal collected along the chronosequence sites display the BC fraction linked to the hypothesized trend of decreased adsorption capacity with TSF, which the adsorption experiment did not confirm.

13,000 years of fire history derived from soil charcoal in a British Columbia coastal temperate rain forest

AbstractLittle is known regarding the fire history of high‐latitude coastal temperate rain forests in the Pacific Northwest (PNW) of North America. While reconstructing historical fire regimes typically requires dendrochronological records from fire‐scarred trees or stratigraphically preserved lake sediment data, this type of information is virtually non‐existent in this region. To describe the long‐term fire history of a site on the central coast of British Columbia, Canada, we radiocarbon‐dated 52 pieces of charcoal. Charcoal ages ranged from 12,670 to 70 yr BP. Fires occurred regularly since 12,670 yr BP, with the exception of a distinct fire‐free period at 7500–5500 yr BP. Time since fire (TSF) estimates from soil charcoal and fire‐scarred trees ranged from 12,670 to 100 yr BP (median = 327 yr), and 70% of the sites examined had burned within the past 1000 yr. An increase in fire frequency in the late Holocene is consistent with the widely held hypothesis that anthropogenic fires were common across the PNW. We evaluate TSF distributions and discuss the difficulties in assigning actual fire dates from charcoal fragments with large inbuilt ages in a coastal temperate rain forest setting. We determine that a comprehensive approach using soil charcoal and fire scar analyses is necessary to reconstruct general trends in fire activity throughout the Holocene in this region.