Reconstruction Of Fire History Research Articles

Integrating charcoal morphology and stable carbon isotope analysis to identify non-grass elongate charcoal in tropical savannas

Fire is inextricably linked to the vegetation that provides the fuel load. For palaeofire records to contribute meaningfully to the reconstruction of past landscape fire history, it is helpful to identify the vegetation that has been burnt, for example, grassy versus woody vegetation in tropical savannas. The morphological characteristics of charcoal particles can provide useful information on source vegetation type, and the aspect ratio of charcoal particles has been proposed to identify the contribution of grasses to environmental records. Stable carbon isotope analysis of pyrogenic carbon can also chemically identify the proportion of C3 and C4 biomass in charcoal samples but has yet to be widely applied alongside charcoal morphological analysis. Using carbon isotope analysis we demonstrate that C3 sedges contribute elongate charcoal to a fire record where C4 grasses are absent. These results challenge the widespread assumption that elongate charcoal is primarily or exclusively derived from grass, as most experimental studies demonstrating this relationship were conducted in environments where graminoids (grass-like forms) did not significantly contribute to available fuels. In turn, this complicates the simple interpretation of elongate aspect ratios for charcoal in fire records as direct proxies for the proportion of grasses in an environment, beyond differentiating temperate forests from grasslands. Minimal work to date has been done on separating charcoal derived from different graminoid types and future studies would benefit from the ability to differentiate graminoids including Poaceae and Cyperaceae in fire records. These results highlight the benefits of a multi-proxy approach to the interpretation of fire records in tropical savannas.

Holocene fire history reconstruction of a mid-elevation mixed-conifer forest in the Eastern Cascades, Washington, USA

Fire histories of mid-elevation mixed-conifer forests are uncommon in the eastern Cascades, limiting our understanding of long-term fire dynamics in these environments. The purpose of this study was to reconstruct the fire and vegetation history for a moist mid-elevation mixed-conifer site, and to determine whether Holocene fire activity in this watershed was intermediate to fire regimes observed at higher and lower elevations in the eastern Cascades. Fire activity and vegetation change was reconstructed using macroscopic charcoal and pollen analysis of sediment core from Long Lake. This site is located ~45 km west of Yakima, WA, and exists in a grand fir-dominated, mixed-conifer forest. Results show low fire activity from ca. 9870 to 6000 cal yr BP, after which time fire increased and remained frequent until ca. 500 cal yr BP. A woodland environment existed at the site in the early Holocene, with the modern coniferous forest establishing ca. 6000–5500 cal yr BP. A mixed-severity fire regime has existed at the site for the past ~6000 years, with both higher- and lower-severity fire episodes occurring on average every ~80–100 years. However, only one fire episode occurred in the Long Lake watershed during the past 500 years, and none within the past ~150 years. Based on a comparison with other eastern Cascade sites, Holocene fire regimes at Long Lake, particularly during the late Holocene, appear to be intermediate between those observed at higher- and lower elevation sites, both in terms of fire severity and frequency.

Evaluating Best Practices for Macroscopic Charcoal-Based Fire History Reconstructions through a Research Experience for Undergraduates

Evaluating Best Practices for Macroscopic Charcoal-Based Fire History Reconstructions through a Research Experience for Undergraduates

The history of human land use activities in the Northern Alps since the Neolithic Age. A reconstruction of vegetation and fire history in the Mangfall Mountains (Bavaria, Germany)

Millenia of sustainable, low intensity land use have formed the cultural landscapes of central Europe. Studies from the Central Alps show that mountain pastures also look back onto many thousand years of land use history. In this palynological and pedoanthracological study in the border region between Germany and Austria in the Mangfall Mountains, we aim to close the knowledge gap that exists for the German part of the Northern Alps, where no conclusive evidence for the onset of pastoral activities has been presented so far. Our results reveal strong evidence, that mountain pasture use in this region reaches back to the Iron Age at least. However, the reconstruction of vegetation and fire history indicates human interaction with the environment much earlier, starting in the Neolithic Age, where we found evidence of slash and burn activities and first occurrences of pasture indicator pollen. A rising number of mega charcoal pieces dated to the Bronze Age suggests increased slash and burn activities, possibly linked to the creation of open space for pasturing. Therefore, our results provide profound evidence of human interaction with the mountain environment, beginning in the Neolithic Age and clear evidence of mountain pasture use beginning during the Iron Age at 750 BC. Based on palynology and pedoanthracology it is, however, difficult to clearly differentiate between pasturing, hunting and other human interactions with the environment. Further archaeological studies in this area could add valuable information to our findings and shed more light onto the early history of farming activities in the Northern Alps.

Utility and optimization of LANDSAT-derived burned area maps for southern California

ABSTRACT Maps of burned area derived from Land Remote-Sensing Satellite (LANDSAT) system imagery may be more reliable than fire perimeter records based on ground observation or manual cartographic delineation. This study evaluates accuracy of LANDSAT-derived burned area maps associated with 19 fires (65 to 86,776 ha) within shrublands of southern California in the period 1996–2018. High spatial resolution aerial images collected soon after these 19 fires were used to verify burned fractions within LANDSAT ground resolution elements. Validated burned fractions were used to optimize classification thresholds applied to burn severity metrics based on LANDSAT Normalized Burn Ratio (NBR), Normalized Burn Ratio 2 (NBR2), and Normalized Difference Vegetation Index (NDVI). The burn severity metrics included post-fire spectral index data (e.g., NBR), pre-fire to post-fire differences (e.g., dNBR), and relative differences (e.g., RdNBR). Optimized classifications of dNBR had the greatest overall accuracy (94%) compared to dNBR2 (93%) and dNDVI (92%). Classifications based on standard difference metrics yielded burn area maps that were 1% more accurate than those based on relative-difference metrics for each spectral index. Classification products based solely on post-fire imagery were 85% to 90% accurate, depending on the spectral index utilized. In comparison, perimeter data from the California Fire and Resource Assessment Programme (FRAP) and Monitoring Trends in Burn Severity (MTBS) substantially overestimated burned extent (by about 44%). The MTBS severity maps excluding the ‘unburned/low severity’ class slightly overestimated burned extent (11%). Commission error in the FRAP data set was attributed to low cartographic detail and inclusion of internal unburned patches. Site-specific differences in unburned soil and vegetation fractions (within partly-burned areas) correlated strongly with overestimation of burned area in the optimized LANDSAT-derived maps; areal overestimates of 10%–15% resulted from data grain size rather than map commission error. The main innovation presented in this study is an empirical method to predict dNBR thresholds based on local (pre-fire) NDVI mean and variance, which produced burned area maps of 90% median accuracy. This LANDSAT-based method could support an efficient reconstruction of fire history in recent decades, which would be more comprehensive and accurate than available fire records for southern California.

A meta-analytical approach to understanding the charcoal source area problem

The sedimentary charcoal proxy is plagued by the source area problem: estimates of the spatial scales for which charcoal serves as a reliable recorder of fire history span several orders of magnitude, thereby precluding higher-order fire-climate and fire-vegetation analyses requiring spatially explicit reconstructions. This problem is tackled using a meta-analytical approach: (1) published estimates of charcoal source area are compiled to assess all available data and constraints, (2) the limitations and implications of these source area estimates are discussed, and (3) the parameter space of the relationship between charcoal size and dispersal distance is modelled using the published estimates as constraints and with the goal of estimating the functional charcoal source area (FCSA), or the globally applicable source area necessary to facilitate spatially-explicit fire history reconstructions and data-model comparisons. Source area estimates exist in numerous forms and flavors including direct assessments of source area (both relevant (RSAC) and potential charcoal source area (PCSA)) as well as indirect and/or opportunistic observations of dispersal distance. These estimates span several orders of magnitude and do not exhibit size dependent dispersal which may be due to differences in the flavor of source area constrained by each study, limitations of the inferences by the observed fire history at each site in time and space, the physical inability of size class differences examined in paleofire research (e.g. micron scale), to result in observable size-dependence of dispersal, or the effects of other unconstrained influences affecting dispersal and source area. The model results show the FCSA of all size fractions is limited to 50 km. This estimate of the FCSA enables spatially explicit paleofire reconstructions and climate model comparisons. However, the meta-analysis results and broad strokes approach of the model in this paper highlight the need for further research characterizing the parameters affecting charcoal dispersal as well as the limitations of fire history in delineating charcoal source area.

What was burning in the past? Charcoal identifications supplement an early-Holocene fire-history reconstruction in Yellowstone National Park, USA

Analysis of charcoal particles preserved in lake sediments has been frequently used to reconstruct fire, vegetation and climate history. Larger macroscopic particles (>125 μm) are attributed to local fires, whereas microscopic charcoal particles (<125 μm), observed on pollen slides, are a signal of regional fire. Taxonomic identification of the charcoal particles adds to the fire-history reconstruction by providing information about fuel composition and past fire conditions. Identification of charcoal can unravel one of the longstanding questions regarding past fire regimes, namely what was burning. This paper describes an analysis of charcoal particles preserved in the sediments of a small closed-basin lake in Yellowstone National Park. Blacktail Pond (44.954°N, 110.604°W; 2012 m elev) is located in steppe vegetation surrounded by conifer forest. Previous studies of the site have presented a multi-proxy environmental history of the last 14,650 years. The focus of this investigation was on the fire history of the early-Holocene (11,500–9900 cal yr BP). Thirty-nine sediment samples were taken at 0.5 cm intervals to examine macroscopic charcoal particles, with attention on two large charcoal peaks at 10,970 and 10,200 cal yr BP that registered large or near-site fires. Twenty-eight samples contained 243 large-enough particles (180–250 μm) to be examined with scanning electron microscopy. We were able to identify charcoal of conifers (Pinus, Picea, and Abies or Juniperus), Artemisia and unidentified deciduous shrubs, and unidentified monocotyledons. The first fire burned a mixture of fuels, whereas the second episode burned Artemisia, shrubs and herbs but no conifers. The results are consistent with previous pollen interpretations at Blacktail Pond that indicate an open mixed-conifer forest in the early Holocene prior to 10,750 cal yr BP, replaced by Pinus contorta forest on the slopes and steppe in the valley. The relative dominance of Pinus contorta-type charcoal points to the importance of fire as a catalyst for the spread of this species.

Evaluating fossil charcoal representation in small peat bogs: Detailed Holocene fire records from southern Sweden

In this study, we assess how representative a single charcoal record from a peat profile in small bogs (1.5–2 ha in area) is for the reconstruction of Holocene fire history. We use high-resolution macrocharcoal (&gt;250 μm) analysis of continuous series of 2 cm3 samples from two small bogs in southern Sweden. We compare (1) duplicate charcoal records from the same core, (2) duplicate charcoal records from profiles in the same site (10 m apart), and (3) charcoal records from two sites within the same region (15 km apart). Comparisons are made for charcoal counts and area expressed as accumulation rates. The results suggest that (a) charcoal counts and area are highly correlated in all records; (b) duplicate charcoal records within the same core are very similar, although some charcoal peaks are found in only one of the two records; (c) although long-term trends in fire regimes are similar between duplicate charcoal records from nearby profiles within the same site and between charcoal records from sites within the same region, some individual charcoal peaks/fire events are asynchronous between records. The known historical fires of the town of Växjö (1570 and 1612 CE) are recorded at the two study sites, which indicates a macrocharcoal source area of minimum 15 km in diameter. The 2 cm3 peat samples contained relatively low amounts of macrocharcoal; we therefore recommend to analyse larger samples from small peat bogs with comparable peat accumulation rates. This will improve the reliability of the macrocharcoal record and its interpretation.

Landscape–lake interactions in the Beartooth Mountains, Wyoming: a 350-year fire history reconstruction

Wildfire is the dominant natural disturbance agent in forests of the Greater Yellowstone Ecosystem, and can have long-lasting impacts on local landscapes and limnologic processes. We applied a multi-proxy paleolimnological approach to infer the impact of forest fires on the catchment of Island Lake, Wyoming (USA), over the last ~ 350 years. Specifically, we examined whether fossil diatom assemblages recorded indirect, limnological impacts from fire, including changes in water clarity, nutrient input, and lake mixing. The objective of this study was to determine if post-fire lake changes could be reconstructed effectively using fossil diatom assemblages from Island Lake, Wyoming, by comparing the diatom record to dendrochronological and macroscopic charcoal records. Local and regional fire years AD 1747 and 1900 were determined from tree-ring records, which were corroborated with sediment accumulation rates and magnetic susceptibility in Island Lake sediments. Inferred lacustrine conditions that were associated with fire events, particularly regional, mixed-severity fire year 1900, included increased turbidity, greater water column mixing, and possible augmented influx of total phosphorus to the system. Thus, the fossil diatom assemblage in Island Lake appears to have recorded forest fire activity indirectly, particularly when fire activity was severe or widespread within the catchment. An increase in small colonial Fragilaria (sensu lato) species since 1975, in association with the rise in abundance of Asterionella formosa and Tetracyclus glans, suggests recent changes in thermal stratification patterns, epilimnion temperatures, and available nutrients.

Unprecedented long-distance transport of macroscopic charcoal from a large, intense forest fire in eastern Australia: Implications for fire history reconstruction

Macroscopic charcoal records from wetland sediment cores are used to reconstruct long-term records of fire frequency. A central premise for the use of this tool is that macroscopic charcoal (&gt;125 μm) represents local fires involving local vegetation. Several records reveal that there may often be exceptions to these guidelines. Previous studies have shown that particles larger than 1 cm long can travel at least 20 km from the location of a fire. We present observations of unprecedented long-distance transport of large (⩽5 cm long) charcoal particles at least 50 km from a fire west of Sydney, Australia. Factors that contribute to long-distance transport of large charcoal particles are fire intensity, upper level wind speed and landscape topography. The fires west of Sydney were large and intense, upper level (~10 km) winds exceeded 90 km h-1, and the topography east of the fire was flat or undulating. Smoke plumes from intense fires like this can reach an altitude of at least 15 km. Charcoal morphology also contributed to long-distance transport in this case. Eucalyptus trees can produce large quantities of aerodynamically efficient particles; from paper thin, smooth, decorticating bark and large sclerophyllous leaves. The presence of large macroscopic charcoal particles in wetland sediments does not automatically indicate local fires and could result from distant, large, intense fires. Large, intense fires can occur in Australia, the grasslands of Kazakhstan, Namibia, the Sahel and Patagonia. High intensity fires also occur in the forested areas of the western United States and Boreal North America. Fires in these regions could result in long-distance transport of large macroscopic charcoal particles under the right circumstances. Local charcoal flux studies are therefore critical for the interpretation of macroscopic charcoal records. We cannot rely on information from areas with different fire regimes, fire intensities or vegetation types.

Population collapse and retreat to fire refugia of the Tasmanian endemic conifer Athrotaxis selaginoides following the transition from Aboriginal to European fire management.

Untangling the nuanced relationships between landscape, fire disturbance, human agency, and climate is key to understanding rapid population declines of fire-sensitive plant species. Using multiple lines of evidence across temporal and spatial scales (vegetation survey, stand structure analysis, dendrochronology, and fire history reconstruction), we document landscape-scale population collapse of the long-lived, endemic Tasmanian conifer Athrotaxis selaginoides in remote montane catchments in southern Tasmania. We contextualized the findings of this field-based study with a Tasmanian-wide geospatial analysis of fire-killed and unburned populations of the species. Population declines followed European colonization commencing in 1802ad that disrupted Aboriginal landscape burning. Prior to European colonization, fire events were infrequent but frequency sharply increased afterwards. Dendrochronological analysis revealed that reconstructed fire years were associated with abnormally warm/dry conditions, with below-average streamflow, and were strongly teleconnected to the Southern Annular Mode. The multiple fires that followed European colonization caused near total mortality of A. selaginoides and resulted in pronounced floristic, structural vegetation, and fuel load changes. Burned stands have very few regenerating A. selaginoides juveniles yet tree-establishment reconstruction of fire-killed adults exhibited persistent recruitment in the period prior to European colonization. Collectively, our findings indicate that this fire-sensitive Gondwanan conifer was able to persist with burning by Aboriginal Tasmanians, despite episodic widespread forest fires. By contrast, European burning led to the restriction of A. selaginoides to prime topographic fire refugia. Increasingly, frequent fires caused by regional dry and warming trends and increased ignitions by humans and lightning are breaching fire refugia; hence, the survival Tasmanian Gondwanan species demands sustained and targeted fire management.

Widespread fire years in the US–Mexico Sky Islands are contingent on both winter and monsoon precipitation

The climate of the south-western United States and northern Mexico borderlands is marked by a bimodal precipitation regime with the majority of moisture arriving during the cool season via Pacific frontal storm systems, and intense convective storms during the North American Monsoon (NAM). The fire season occurs primarily during the arid foresummer in May and June, before the development of the NAM. Most tree-ring studies of fire climatology in the region have evaluated only the role of winter precipitation. We used tree-ring-width-based reconstructions of both winter and monsoon precipitation, coupled with fire scar reconstructions of fire history from mountain ranges in the US and Mexico, to quantify the historical role and interactions of both seasons of precipitation in modulating widespread fire years. Winter precipitation was the primary driver of widespread fire years in the region, but years with drought in both seasons had the highest fire frequency and most widespread fires. These relationships define a unique monsoon fire regime, in which the timing and amount of monsoon precipitation are important factors in limiting the length of fire season and regulating widespread fire years.

Mesophication in temperate Europe: A dendrochronological reconstruction of tree succession and fires in a mixed deciduous stand in Białowieża Forest.

The shift from shade‐intolerant species to shade‐tolerant mesophytic species in deciduous and mixed forests of the temperate zone is well described in studies from North America. This process has been termed mesophication and it has been linked to changes in fire regime. Fire suppression results in the cessation of establishment of heliophytic, fire‐dependent tree species such as oak (Quercus) and pine (Pinus). Due to the scarcity of old‐growth forests in Europe, data on long‐term compositional changes in mixed forests are very limited, as is the number of studies exploring whether fire played a role in shaping the dynamics.The aim of this study was to reconstruct tree succession in a 43‐ha natural mixed deciduous forest stand in Białowieża Forest (BF), Poland using dendrochronological methods. In addition, the presence of aboveground fire legacies (charred and fire‐scarred deadwood) enabled the fire history reconstruction.Dendrochronological data revealed tree establishment (Quercus) back to the end of the 1500s and fires back to 1659. Under a regime of frequent fires until the end of the 18th century, only oak and pine regenerated, sporadically. A shift in the fire regime in the first half of the 19th century triggered oak and pine cohort regeneration, then gradually spruce (Picea) encroached. Under an increasingly dense canopy and less flammable conditions, regeneration of shade‐tolerant Carpinus, Tilia, and Acer began simultaneously with the cessation of oak and pine recruitment. Synthesis. The study reports the first evidence of mesophication in temperate Europe and proves that fire was involved in shaping the long‐term dynamics of mixed deciduous forest ecosystems. Our data suggest that fire exclusion promoted a gradual recruitment of fire‐sensitive, shade‐tolerant species that inhibited the regeneration of oak and pine in BF.

Fire history and climate-growth response of Abies spectabilis : A case from Langtang National Park, Nepal Himalaya

This study presents the potential of a conifer species (Abies spectabilis D. Don) to reconstruct fire history by using dendro chronological technique along with thedendroclimatic response in Langtang National Park, Central Himalaya of Nepal. For the fire history reconstruction, altogether eight cross-sections samples from fire affected eight trees and another 20 tree-cores from 10 trees with visible fire scars were taken. In the case of dendroclimatic study, 24 healthy cores of A. spectabilis were selected from the 40 cores extracted from 19 trees. The standard dendro chronological methodology was used for sample preparation and analysis. A 199-year long ring-width chronology of A. spectabilis spanning from 1818 to 2016 AD was developed. In spite of visible fire burn in near bark-surface, no potential fire scars are seen in inner parts in the cross-section samples. However, 12 cores showed that three fire burns occurred simultaneously in the forest area in the years 1917−1918, 1969−1970 and 2009−2010, respectively. Tree-ring-based fire event-record is found to be concurrent to the local people's perceptions/experience about the past fire history in the area. Tree growth climate relationship showed sensitive responses to both growing and non-growing season’s temperature and precipitation variability. Summer temperature had positive influence on growth of the species. Precipitation of monsoon and autumn were found to have negative influence on radial growth whereas pre-monsoon precipitation had positive association with tree radial-growth. This preliminary assessment shows that there is a huge potential of tree-ring research for long-term fire history in the region and helps us to better understand the role of fire in the ecology and management in the Himalayan region. The study can also be replicated in other fire-affected areas of the Himalayan region by using fire sensitive species in the sampling.

Dwarf Blackgum (Nyssa sylvatica) Contains Datable Fire Scars that Complement an Existing Fire History

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.

Macro-charcoal accumulation in floodplain wetlands: Problems and prospects for reconstruction of fire regimes and environmental conditions.

Floodplain wetland ecosystems respond dynamically to flooding, fire and geomorphological processes. We employed a combined geomorphological and environmental proxy approach to assess allochthonous and autochthonous macro-charcoal accumulation in the Macquarie Marshes, Australia, with implications for the reconstruction of fire regimes and environmental conditions in large, open-system wetlands. After accounting for fluvial macro-charcoal flux (1.05 ± 0.32 no. cm-2 a-1), autochthonous macro-charcoal in ~1 m deep sediment profiles spanning ~1.7 ka were highly variable and inconsistent between cores and wetlands (concentrations from 0 to 438 no. cm-3, mean accumulation rates from 0 to 3.86 no. cm-2 a-1). A positive correlation existed between the number of recent fires, satellite-observed ignition points, and macro-charcoal concentrations at the surface of the wetlands. Sedimentology, geochemistry, and carbon stable isotopes (δ13C range -15 to -25 ‰) were similar in all cores from both wetlands and varied little with depth. Application of macro-charcoal and other environmental proxy techniques is inherently difficult in large, dynamic wetland systems due to variations in charcoal sources, sediment and charcoal deposition rates, and taphonomic processes. Major problems facing fire history reconstruction using macro-charcoal records in these wetlands include: (1) spatial and temporal variations in fire activity and ash and charcoal products within the wetlands, (2) variations in allochthonous inputs of charcoal from upstream sources, (3) tendency for geomorphic dynamism to affect flow dispersal and sediment and charcoal accumulation, and (4) propensity for post-depositional modification and/or destruction of macro-charcoal by flooding and taphonomic processes. Recognition of complex fire-climate-hydrology-vegetation interactions is essential. High-resolution, multifaceted approaches with reliable geochronologies are required to assess spatial and temporal patterns of fire and to reconstruct in order to interpret wetland fire regimes.

Paleoclimate reconstruction and mire development in the Eastern Great Hungarian Plain for the last 20,000 years

We present the reconstruction of mire vegetation changes and fire history recorded in a continuous sediment profile that spans the last 20,000 cal yr BP from the Late Pleniglacial to Holocene in North-eastern Hungary. We also aimed to reveal past climate changes by using ecological requirements of specific aquatic plants as summer temperature indicators. Our results suggest the formation of a mesotrophic mire around 20,000 cal yr BP with brown moss, Betula sp. and Selaginella selaginoides cover beside the occurrence of Phragmites australis, Typha latifolia and T. angustifolia suggesting base-rich fen and tundra-like wet-ground habitats on the lakeshore. This community shifted to reed dominated swamp at c. 18,300 cal yr BP with inferred min. July temperatures of 12–15.7 °C. pinus sp., Betula nana, B. pendula/pubescens, Hippuris vulgaris and P. australis dominated until 16,600 cal yr BP pointing to shallow muddy stagnant water and colder climatic conditions than in the preceding interval. The most warmth-demanding species, T. latifolia and T. angustifolia indicated July mean temperatures >14–15.7 °C soon after the LGM. The formation of biogenic carbonate also started at an early stage; major accumulation occurred between 15,200 and 10,000 cal yr BP. In the Early Holocene, environmental indicator species (e.g., Phragmites australis and T. latifolia) pointed to warmer and shallower conditions, while the late Holocene was characterized by strong eutrophication and reed swamp dominance on the lakeshore. Elevated macrocharcoal concentration, wood fragments and remains of Typha species suggested frequent local fires and dry mire surface conditions during the last 1700 years.

Integrating fire-scar, charcoal and fungal spore data to study fire events in the boreal forest of northern Europe

Fire is a major disturbance agent in the boreal forest, influencing many current and future ecosystem conditions and services. Surprisingly few studies have attempted to improve the accuracy of fire-event reconstructions even though the estimates of the occurrence of past fires may be biased, influencing the reliability of the models employing those data (e.g. C stock, cycle). This study aimed to demonstrate how three types of fire proxies – fire scars from tree rings, sedimentary charcoal and, for the first time in this context, fungal spores of Neurospora – can be integrated to achieve a better understanding of past fire dynamics. By studying charcoal and Neurospora from sediment cores from forest hollows, and the fire scars from tree rings in their surroundings in the southern Fennoscandian and western Russian boreal forest, we produced composite fire-event data sets and fire-event frequencies, and estimated fire return intervals. Our estimates show that the fire return interval varied between 126 and 237 years during the last 11,000 years. The highest fire frequency during the 18th–19th century can be associated with the anthropogenic influence. Importantly, statistical tests revealed a positive relationship between other fire event indicators and Neurospora occurrence allowing us to pinpoint past fire events at times when the sedimentary charcoal was absent, but Neurospora were abundant. We demonstrated how fire proxies with different temporal resolution can be linked, providing potential improvements in the reliability of fire history reconstructions from multiple proxies.

Dendrochronological reconstruction of fire history in coniferous forests in the Monarch Butterfly Biosphere Reserve, Mexico

BackgroundSome have proposed that fire return intervals lengthen with elevation in montane tropical coniferous forests, such as those found in central Mexico. This would generate patterns of synchronous tree establishment in higher elevation forests dominated by Abies sp. Mill., and patterns of continuous tree establishment in lower elevation forests dominated by Pinus sp. L. However, it is common to find codominant stands of both genera at intermediate elevations. The Monarch Butterfly Biosphere Reserve (MBBR) is located in this region, and is covered by Abies religiosa (Kunth) Schltdl. & Cham-dominant forest, A. religiosa–Pinus pseudostrobus Lindley-codominant forest, and P. pseudostrobus-dominant forest. Despite the ecological importance of the MBBR’s coniferous forests, little is known about their fire return intervals, including their relationship with climate, and their effects on tree establishment patterns according to the elevational range proposed above. In this study, using dendrochronological techniques, we reconstructed the historical fire return intervals for each forest type, evaluated their relationship with droughts and with the El Niño Southern Oscillation (ENSO), and assessed the effect of fire return intervals on tree establishment patterns. A total of 110 cross-sections were obtained, from 23 sites across the MBBR.ResultsThe fire chronology covered the period between 1925 and 2015. Fire return intervals averaged two years in stands dominated by A. religiosa, by P. pseudostrobus, and codominated by both species, regardless of elevation. No fire-free periods were detected, even after the establishment of the reserve in 2000. Fire frequency was not associated with periods of drought and ENSO. There was no relationship between fire return intervals and patterns of tree establishment, which was continuous in all three forest types.ConclusionsOur results suggest that coniferous forests of the MBBR have historically experienced frequent fires that are unrelated to climate and tree establishment. This may be evidence that natural fire regimes in these coniferous forests have been drastically altered by human activities. Although these frequent low-severity fires could adversely affect tree regeneration, and eventually decrease the integrity of the overwintering habitat of the Monarch Butterfly, the continuous tree regeneration observed suggests that these effects have not yet taken place.

Stable or seral? Fire-driven alternative states in aspen forests of western North America

As important centres for biological diversity, aspen forests are essential to the function and aesthetics of montane ecosystems in western North America. Aspen stands are maintained by a nuanced relationship with wildfire, although in recent decades aspen mortality has increased. The need to understand the baseline environmental conditions that favour aspen is clear; however, long-term fire history reconstructions are rare due to the scarcity of natural archives in dry montane settings. Here, we analyse a high-resolution lake sediment record from southwestern, Utah, USA to quantify the compositional and burning conditions that promote stable (or seral) aspen forests. Our results show that aspen presence is negatively correlated with subalpine fir and that severe fires tend to promote persistent and diverse aspen ecosystems over centennial timescales. This information improves our understanding of aspen disturbance ecology and identifies the circumstances where critical transitions in montane forests may occur.