•Live fuels were the main driver of the severity of crown-convective fires in pine forests.•Vegetation vertical structure, fire history and weather also affected fire severity.•Physical properties ...had a minor influence on the severity of crown-convective fires.•Low-density LiDAR had a high potential for evaluating pre-fire fuel structure.
The increasing occurrence of large and severe fires in Mediterranean forest ecosystems produces major ecological and socio-economic damage. In this study, we aim to identify the main environmental factors driving fire severity in extreme fire events in Pinus fire prone ecosystems, providing management recommendations for reducing fire effects. The study case was a megafire (11,891 ha) that occurred in a Mediterranean ecosystem dominated by Pinus pinaster Aiton in NW Spain. Fire severity was estimated on the basis of the differenced Normalized Burn Ratio from Landsat 7 ETM +, validated by the field Composite Burn Index. Model predictors included pre-fire vegetation greenness (normalized difference vegetation index and normalized difference water index), pre-fire vegetation structure (canopy cover and vertical complexity estimated from LiDAR), weather conditions (spring cumulative rainfall and mean temperature in August), fire history (fire-free interval) and physical variables (topographic complexity, actual evapotranspiration and water deficit). We applied the Random Forest machine learning algorithm to assess the influence of these environmental factors on fire severity. Models explained 42% of the variance using a parsimonious set of five predictors: NDWI, NDVI, time since the last fire, spring cumulative rainfall, and pre-fire vegetation vertical complexity. The results indicated that fire severity was mostly influenced by pre-fire vegetation greenness. Nevertheless, the effect of pre-fire vegetation greenness was strongly dependent on interactions with the pre-fire vertical structural arrangement of vegetation, fire history and weather conditions (i.e. cumulative rainfall over spring season). Models using only physical variables exhibited a notable association with fire severity. However, results suggested that the control exerted by the physical properties may be partially overcome by the availability and structural characteristics of fuel biomass. Furthermore, our findings highlighted the potential of low-density LiDAR for evaluating fuel structure throughout the coefficient of variation of heights. This study provides relevant keys for decision-making on pre-fire management such as fuel treatment, which help to reduce fire severity.
Interannual climate variations have been important drivers of wildfire occurrence in ponderosa pine forests across western North America for at least 400 years, but at finer scales of mountain ranges ...and landscapes human land uses sometimes over-rode climate influences. We reconstruct and analyse effects of high human population densities in forests of the Jemez Mountains, New Mexico from ca 1300 CE to Present. Prior to the 1680 Pueblo Revolt, human land uses reduced the occurrence of widespread fires while simultaneously adding more ignitions resulting in many small-extent fires. During the 18th and 19th centuries, wet/dry oscillations and their effects on fuels dynamics controlled widespread fire occurrence. In the late 19th century, intensive livestock grazing disrupted fuels continuity and fire spread and then active fire suppression maintained the absence of widespread surface fires during most of the 20th century. The abundance and continuity of fuels is the most important controlling variable in fire regimes of these semi-arid forests. Reduction of widespread fires owing to reduction of fuel continuity emerges as a hallmark of extensive human impacts on past forests and fire regimes.
This article is part of the themed issue ‘The interaction of fire and mankind’.
Tree mortality events across the globe have been associated with rising temperatures and altered disturbance regimes. Using fire scars, stand age structure, and tree-ring data, we evaluate changes in ...fire regimes, stand conditions, and climate to identify causes of mortality of a keystone species, whitebark pine, (WBP, Pinus albicaulis Engelm.) in a mixed subalpine forest in the Soldier Mountains, Idaho, at the lower elevation, low precipitation end, or trailing edge, of its range. Historically, the mean fire return interval was 66 years (±SD 34.1) until the late 19th century. Most fires were small, patchy, and of low to moderate severity consistent with grassy surface fuels. Climate synchronized fire disturbance, and larger fires burned across lower elevations and into WBP subalpine forests during very dry years. This occurred last in 1889 during a multi-year drought with fires documented across northwestern US. In the decades following, WBP establishment peaked and then declined, while subalpine fir (Abies labsiocarpa (Hook.) Nutt.) establishment peaked thirty years later and is ongoing. During global change-type or megadrought in the 2000s, WBP deaths per decade were 28-fold higher than the average since the early 20th century. Annual WBP mortality was positively associated with maximum summer temperature and negatively with summer precipitation. Ten years prior to the period of high WBP mortality, WBP annual radial growth fell below average and drought stress may have increased susceptibility to mountain pine beetle (Dendroctonus ponderosae Hopkins). Most dead WBP (90 %) showed signs of beetle attack. WBP mortality increased with fir basal area suggesting cumulative competition with subalpine fir likely contributed to mortality. Subalpine fir regeneration is three-fold more abundant than WBP and increased in areas with greater WBP mortality. We found a late 19th century drought concluded with a fire and subsequent WBP regeneration, whereas a 21st century fire-free megadrought accelerated WBP mortality and a forest shift towards subalpine fir dominance. Restoration using fire or thinning is imperative in trailing edge WBP forest given predictions of more frequent global-change-type megadrought.
•Recent whitebark pine (WBP) mortality accelerated succession towards subalpine fir•Fire was a recurring disturbance in edge WBP forest over 5 centuries, until 1889•Fire favored WBP regeneration and fire exclusion favors subalpine fir•In dry years, fires in lower elevations burned into WBP forests•During a 21st century long hot drought, WBP growth declined, and beetle kill spiked
A 600 cm peat sequence was extracted from the ombrotrophic peat bog Wildseemoor in the northern Black Forest, covering the last ca. 10,000 years thus allowing for identification of potential changes ...in dust input during the Holocene. Such information is critical to the understanding of past environmental change at the local to supra-regional scale and helps with interpreting, in particular, fire history and its links to both climate change and anthropogenic impacts. Dust composition change was investigated using X-ray fluorescence core scanning (XRF-CS) to establish bulk chemical composition of lithogenic input to the peatland, using element signatures, and in particular the ratios of Ca/Ti and Ti/Zr. Two main different sources of dust could be differentiated, a local and a distal (long-distance) dust source, which show variations through time. Distal dust input is elevated during the early Holocene (ca. 8,800 - 8,300 cal BP), as well as around 5,000 cal BP, 3,000 cal BP, 2,300 cal BP and the last ca. 200 years. Distal dust might originate from remobilized loess deposits in the Upper Rhine Graben or, periodically, more distant sources such as the Sahara or the Massif Central. Local dust input roughly correlates to peaks in charcoal influx after ca. 3,850 cal BP, potentially recording phases of increased anthropogenic influence in the surroundings of Wildseemoor. Additional research in nearby bogs (from the Black Forest and the Vosges), as well as more precise geochemical fingerprinting of the different dust sources, is needed to explore the regional extent and significance of Holocene environmental changes recorded at Wildseemoor.
Fire is an essential component of the landscapes and forests of the Pacific Northwest, including the temperate rainforests of the Olympic Peninsula. Previous fire history reconstructions from the ...peninsula show that fire return intervals varied throughout the postglacial period, primarily in response to climatic changes and corresponding shifts in vegetation. However, much less is known about the fire history of the low-elevation forests of the Olympic Peninsula and the role of cultural fire regimes in these environments. The purpose of this study was to reconstruct the paleoenvironmental history of a low-elevation study site, Beaver Lake, located in the northwestern part of the peninsula. Using macroscopic charcoal, pollen, and sedimentological analyses of a ca. 3440-year-long record, we reconstructed a high-resolution record that shows fire activity was remarkably high during the late Holocene for an area considered part of a temperate rainforest. However, patterns of burning varied throughout the record, with the first third (ca. 3440 to 2350 cal yr BP) and last third (ca. 800 cal yr BP to the present day) recording much higher amounts of fire activity compared to the middle portion (ca. 2350 to 800 cal yr BP), which recorded very little fire activity. Larger and/or more severe fires that likely burned during multi-year droughts correlate with peaks in magnetic susceptibility and pollen trends that indicate substantial geomorphic responses and successional changes in forest structure following these events. Cooler temperatures and a wetter climate, indicated by nearby glacial advancements, may have staved off fire activity during the period of low fire activity. Cultural burning by the Indigenous tribes of the Olympic Peninsula likely contributed to the observed fire activity at Beaver Lake as nearby prairies were managed for resource procurement, indicated in particular by an increase in herbaceous morphotype charcoal during the past 800 years.
•We developed the 553-year-long fire history of black pine forests in western Anatolia.•The period of 1853–1934 CE was found as critical fire regime shift period.•A decrease in fire frequency ...observed in the late 19th and early 20th centuries.•Drought and prior wet conditions were main drivers of fires over the past 553 years.
In this study, we aimed to use tree-ring based fire reconstruction to understand the spatiotemporal patterns of past fires in different climate types of western Anatolia. We collected fire scarred wood samples from living trees as wedges and remnant woods from ten sites along a transect that represents a continental to Mediterranean climate gradient. We determined fire years and assigned seasonality of fires based on the intraring position of the fire scars. We calculated fire statistics and analysed fire-climate relationships. Breakpoints in our Anatolian regional fire chronology were estimated to determine the regime shifts. A decrease in fire frequency was recorded at most of the sites after the end of the 19th and the beginning of the 20th century. We observed two critical fire regime shift periods. The period between 1853 and 1934 is characterized by highly frequent (a total of 82 fires) and simultaneous fires occurring in multiple sites and this period overlapped with the longest and most severe drought period of the past 550 years. The fire frequency decline after 1934 coincided with the period of the first forest protection law in 1937. Dry, as well as prior wet conditions were main drivers of fires in the black pine forests in western Anatolia. We observed a decrease in fire frequency in the late 19th and early 20th centuries due to fire suppression activities. Continued fire suppression activities may cause fuel accumulation and pose a risk for more intense fires and thus a paradox for forests in the future. Based on future climate projections, we will face prolonged fire seasons as a consequence of increasing drought frequency, which may shift the fire regime from surface to crown fires with the accumulation of combustible material in the understory in black pine forests.
•We assessed fire history in southeastern pine savannas using tree-rings methods.•We documented a frequent, low-severity fire regime at all three sites.•Most fire intervals were biennial or ...annual.•Fires documented occurred during the post-settlement period and were human-set.•Our sites lack fire exclusion and are valuable as reference sites for restoration.
The reintroduction and maintenance of historical surface fire regimes are primary goals of ecological restoration across many open, pine-dominated ecosystems in North America. In the United States, most of these ecosystems experienced long periods of fire exclusion in the 20th century, leaving few locations to serve as reference sites for ecological conditions associated with a continuous history of recurrent, low-severity fire. Here, we present a tree-ring perspective of uninterrupted surface fire activity from three pine savanna sites in the Red Hills Region of northern Florida and southwestern Georgia, USA. Our sites include two old-growth stands of longleaf pine (Pinus palustris): the Wade Tract on Arcadia Plantation and the Larkin Tract on Millpond Plantation. We also sampled the largely second-growth mixed pine savannas of Tall Timbers Research Station. Documentary records for burning at these sites are limited to recent decades and are often incomplete, although regional land-use traditions and scattered historical records indicate frequent fire may have persisted through the 20th century to present day. Fire-scarred cross sections from externally-scarred stumps, dead trees, and live trees provided tree-ring evidence of frequent fires occurring from the beginning of our fire-scar record in the late 19th century onward. Both fire frequency and seasonality were relatively consistent throughout time and among sites. Biennial and annual fire intervals were the most common. Most fire scars occurred in the dormant and early-earlywood portions of the rings, indicating that these fires were human-set fires during the months of January to mid-April, before the main lightning-fire season. Our findings regarding post-settlement fire frequency are consistent with previous estimates of fire frequency during earlier centuries, resulting from lightning and Native American ignitions. We recommend that our sites be used as reference sites for restoration as they are among the relatively few areas in the United States with a continuous history of frequent low-severity fire without 20th century fire exclusion.
Athrotaxis cupressoides is a slow‐growing and long‐lived conifer that occurs in the subalpine temperate forests of Tasmania, a continental island to the south of Australia. In 1960–1961, ...human‐ignited wildfires occurred during an extremely dry summer that killed many A. cupressoides stands on the high plateau in the center of Tasmania. That fire year, coupled with subsequent regeneration failure, caused a loss of ca. 10% of the geographic extent of this endemic Tasmanian forest type. To provide historical context for these large‐scale fire events, we (i) collected dendroecological, floristic, and structural data, (ii) documented the postfire survival and regeneration of A. cupressoides and co‐occurring understory species, and (iii) assessed postfire understory plant community composition and flammability. We found that fire frequency did not vary following the arrival of European settlers, and that A. cupressoides populations were able to persist under a regime of low‐to‐mid severity fires prior to the 1960 fires. Our data indicate that the 1960 fires were (i) of greater severity than previous fires, (ii) herbivory by native marsupials may limit seedling survival in both burned and unburned A. cupressoides stands, and (iii) the loss of A. cupressoides populations is largely irreversible given the relatively high fuel loads of postfire vegetation communities that are dominated by resprouting shrubs. We suggest that the feedback between regeneration failure and increased flammability will be further exacerbated by a warmer and drier climate causing A. cupressoides to contract to the most fire‐proof landscape settings.
•Examined importance of controls on mixed-severity burn patterns.•Biophysical environment and fire history best explained burn severity patterns.•Spatial autocorrelation important factor when ...modeling burn severity patterns.•Pre-fire airborne LiDAR measurements of forest structure did not improve results.•Methods provide means to estimate likely burn severity patterns across landscapes.
Recent and projected increases in the frequency and severity of large wildfires in the western U.S. makes understanding the factors that strongly affect landscape fire patterns a management priority for optimizing treatment location. We compared the influence of variations in the local environment on burn severity patterns on the large 2013 Rim fire that burned under extreme drought with those of previous smaller fires for a study area in the Sierra Nevada, California, USA. Although much of the Rim fire burned during plume-dominated conditions resulting in large high-severity patches, our study area burned under milder fire weather resulting in a mix of fire severities. In our study area the Rim fire produced a higher proportion of moderate- and high-severity effects than occurred in previous fires. Random forest modeling explained up to 63% of the Rim fire burn variance using seven predictors: time since previous fire, actual evapotranspiration (AET), climatic water deficit (Deficit), previous maximum burn severity, burning index, slope position, and solar radiation. Models using only a subset of biophysical predictors (AET, Deficit, slope position and steepness, and solar radiation) explained 55% of the Rim fire and 58% of the maximum fire burn severity of previous fires. The relationship of burn severity to patterns of AET, however, reversed for the Rim fire (positive) compared to earlier fires (negative). Measurements of pre-Rim fire forest structure from LiDAR did not improve our ability to explain burn severity patterns. We found that accounting for spatial autocorrelation in burn severity and biophysical environment was important to model quality and stability. Our results suggest water balance and topography can help predict likely burn severity patterns under moderate climate and fire weather conditions, providing managers with general guidance for prioritizing fuel treatments and identifying where fire is less likely to burn with higher severities even for locations with higher forest density and canopy cover.
Wildfire has profound and pervasive consequences for forest ecosystems via directly altering soil physicochemical properties and modulating microbial community. In this study, we examined the changes ...in soil properties and microbial community composition and structure at different periods after highly severe wildfire events (44 plots, 113 samples) in the Chinese Great Khingan Mountains. We also separated charcoal from burnt soils to establish the relationship between microbial community structures in soils and charcoal. We found that wildfire only significantly altered bacterial and fungal β-diversity, but had no effect on microbial α-diversity across a 29-year chronosequence. The network analysis revealed that the complexity and connectivity of bacterial and fungal communities were significantly increased from 17 years after fire, compared with either unburnt soils or soils with recent fires (0–4 years after fire). Differential abundance analysis suggested that bacterial and fungal OTUs were enriched or depleted only during 0–4 years after fire compared with the unburnt soils. In addition, soil pH, dissolved organic C and dissolved organic N were key determinants of soil bacterial and fungal communities during 17–29 years after fire. The fire-derived charcoal provided a new niche for microbial colonization, and microbes colonized in the charcoal had a significantly different community structure from those of burnt soils. Our data suggest that soil bacterial and fungal communities changed significantly during the recovery from fire events in terms of the abundance and co-occurrence networks in the boreal forest ecosystems.
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•Wildfire significantly alters bacterial and fungal community structures.•Fire history significantly changes the average degree of microbial networks.•More differentially abundant OTUs are found in charcoal than in burnt soil.•Soil factors are key drivers of post-fire soil microbial communities.
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