Charcoal Peaks Research Articles

Stepwise deforestation during the Permian-Triassic boundary crisis linked to rising temperatures

Although the trajectory of the marine mass extinction at the ∼252-Ma Permian-Triassic (P-Tr) boundary has been well studied, details of the coeval collapse of terrestrial ecosystems remain murky. Here, we use hydrocarbon biomarker compositions and other geological records (i.e., organic carbon isotopes (δ13Corg), charcoal abundance, and Hg content) from a tropical peatland succession in southwestern China to reconstruct in detail the history of terrestrial ecosystem collapse during the P-Tr crisis. Our high-resolution hydrocarbon biomarker records reveal that this collapse proceeded in a stepwise manner with increasing intensity as the crisis unfolded. We recognize three discrete crisis stages: Stage I within the uppermost Xuanwei Formation and Stages II and III within the lowermost Kayitou Formation. Stage I, the early crisis stage, is marked by a significant decline in terrestrial biomass (continuing into the later stages), as recorded by reduced C29 steranes relative to total steranes and a concomitant reduction in the ratio of pristane to phytane (Pr/Ph). Stage II, the main crisis stage, records intensified soil erosion and sediment flux as revealed by rising dibenzofuran (DBF) content and high hopane/sterane ratios, the disappearance of coal seams, a sharp negative shift in δ13Corg, and peak concentrations of charcoal reflecting increased wildfire incidence. Stage III, the late crisis stage characterized by enhanced soil erosion, corresponds to peak values of Hg and Hg/TOC but no charcoal peak, suggesting intensified volcanism and a return to a humid climate. These stages closely follow temperature records, which show a stepwise rise during the crisis interval, implying that the deforestation process was strongly influenced by punctuated rises in temperature and/or its attendant effects (e.g., climate aridification). This P-Tr transition scenario suggests that global warming can trigger deforestation and reduce terrestrial carbon storage, thus serving as a positive climate feedback, with important implications for present-day climate change.

Holocene fire regimes, fire‐related plant functional types, and climate in south‐coastal British Columbia forests

AbstractPaleoecological records of past fire events and forest composition provide long‐term ecological context for modern changes in fire regimes and forest dynamics. Here, we use pollen and contiguous macroscopic charcoal analyses of lake sediments from Pender Island, British Columbia, Canada to reconstruct changes in fire regimes over the last 10,000 years and investigate how these interact with changes in climate and forest composition with a focus on fire‐related plant functional types. The relatively warm and dry early Holocene was characterized by high charcoal accumulation rates, fire episodes of moderate severity, and a mean fire return interval of 100 ± 27 years. Forests at the time were open‐canopy Pseudotsuga menziesii forests with abundant fire endurer taxa (e.g., Pteridium aquilinum) that have a competitive advantage in regimes of frequent fire. Fire continued to occur every ~100 years, on average, during the establishment of Quercus garryana savanna communities; however, a decrease in charcoal peak magnitudes suggests the fire regime shifted to one characterized by smaller and/or lower intensity surface fires. As temperature and moisture deficits decreased in the mid‐ and late Holocene (i.e., after ~6000 calendar years before present), mean fire return intervals lengthened to 176 ± 54 years and increased variability in charcoal peak magnitudes suggests a mixed fire regime of low‐moderate‐intensity fires combined with infrequent crown or stand‐replacing fires. Relatively stable and moderate climate, longer fire return intervals, and mixed‐severity fires allowed P. menziesii (a fire resister) to dominate closed‐canopy forests and for fire avoiders to gradually become more common forest constituents. Millennial‐scale climate change has acted as the dominant driver of changes in both fire regimes and forest composition over the last 10,000 years; however, changes in fire‐related plant functional types highlight the important role that interactions between vegetation and fire play in long‐term fire regimes and forest dynamics.

Late Pleistocene montane forest fire return interval estimates from Mount Kenya

AbstractPast forest fire events and fire frequencies are reconstructed with sediment–charcoal records at lake catchment spatial scales. Few quantitative palaeofire analyses exist in tropical montane forests, where fire return intervals are long (decadal and centennial scales) because of the infrequency of fire weather and fuel conditions. Fire return intervals are a key characteristic of fire regimes and changing fire frequencies rapidly alter land cover compositions and vegetation structure. Charcoal records from small lakes with relatively small catchments covered with dense forest provide an opportunity to reconstruct low‐frequency, high‐severity fires through a time series decomposition approach to identify charcoal peaks above a varying background rate as a proxy for palaeofire events. The sediment core from Rumuiku wetland on Mount Kenya, equatorial eastern Africa, accumulated a nearly linear age–depth model and provided a high temporal resolution (10 years cm–1) sieved charcoal count record (>125 µm). Pollen analysis showed a significant change in montane forest assemblage occurred at 21 200 cal abpfrom a montane forest with abundantPodocarpusandJuniperusto a forest with more abundantHagenia. This change in forest altered the vegetation composition and structure with concomitant changes to the fire regime. Forest biomass in theHageniaforests decreased and it is likely that fire activity qualitatively changed toward lower intensity and lower severity fires. The quantitative fire event reconstruction focuses on the interval from 27 000 to 16 500 cal a bp and the older montane forest that experienced higher severity fires from 27 000 to 21 200 cal abp,which reconstructed a temporally heterogeneous fire regime with fire return intervals that ranged from 30–430 years and a mean of 120 years (median 160 years) in the catchment. These are the first estimates of fire return intervals of mountain forests in eastern Africa. We then explore the potential for further comparative research and incremental research contributions to improve quantitative and qualitative palaeofire research in tropical forest ecosystems. We discuss the potential to use these types of data for characterizing variables of fire regimes prior to ostensibly significant modification by anthropogenic activity as well as during the recent past as human land use pressures increased within Afromontane forests.

Föhn, fire and grazing in Southern Tibet? A 20,000-year multi-proxy record in an alpine ecotonal ecosystem

The lithology, ostracods, palynomorphs and black carbon of a sediment core from a saline wetland in the southern Tibetan highlands (29°14′33.40″N/87°13′09.10″E, 4480 m a.s.l., A.R. Xizang, China) is analyzed and climate modelling applied with respect to core questions in Quaternary research: (1) Do meso-climatic effects of regional landscape structures like high mountains overrule the effects of global climates? (2) Did life persist during the LGM? (3) Which shifts in ecosystems can be detected? (4) Does the fire-record testify for the presence of humans and their environmental impact? (5) How old is Neolithisation in the world’s largest highlands?The results of this integrated multi-disciplinary investigation revealed (1) the meso-climatic effect of the Himalayan arc’s relief channeled westerlies with West-Himalayan forest-pollen types into the southern Tibetan highlands, with (2) higher winter-precipitation, slightly lower summer precipitation, and the persistence of life for plants, and herbivores and therefore probably hunters. (3) The most arid period occurred between 17.8 and 15 cal kyr B.P., possibly synchronous with the Heinrich 1 event. Around 14.8 cal kyr B.P. humidity increased with torrential rains of high erosive power on sparsely plant-covered slopes and lasted until 4 cal kyr B.P. (4) The continuous fire-record proved the availability of combustible matter and a constant fire source which is believed to be human-attributed. Charcoal peaks around 17, 16 and 13 cal kyr B.P. remained sporadic until 11.5 cal kyr B.P. and attained the highest values since 8.7 cal kyr B.P. Black carbon increased steadily until 11 cal kyr B.P. and sharply peaked with high fluctuations around 10 cal kyr B.P., indicating high frequency intensive fires at the beginning of the Holocene. Between 10 and 6 cal kyr B.P. fire frequency decreases, with a slight peak around 4 cal kyr B.P. (5) The dawn of pastoralism in the early Mid Holocene Climatic Optimum dated back to 8.7 cal kyr B.P. with a decline of grass-pollen and the synchronous emergence of grazing weeds, leaving a perception gap of 5000 years between the palaeo-ecological and the archaeological state of the art.

Late-Holocene climate changes linked to ecosystem shifts in the Northwest Wisconsin Sand Plain, USA

Records of century-scale climate variability in the Upper Midwest generally agree that moisture availability increased between 4000 and 3000 cal. yr BP (calendar years before present = 1950 CE), and that there were large, frequent droughts 1000–700 cal. yr BP followed by wetter/cooler conditions. Variability among regional sites, however, remains problematic. In this study we reconstruct climate on the Northwest Wisconsin Sand Plain (NWSP), USA, to identify potential climatic drivers of previously documented changes in vegetation and fire regimes. Oak pollen was replaced by pollen from xeric pine taxa at several sites on the NWSP ~1425 cal. yr BP, accompanied by a change to larger, less frequent charcoal peaks. Another major vegetation change occurred ~700 cal. yr BP, when pollen of the more mesic P. strobus L. (white pine) increased and charcoal influx decreased. We used a vegetation-independent lake-level record to determine whether long-term changes in moisture availability were associated with these ecosystem changes. Decreases in percent organic matter in shallow-water sediment cores from Cheney Lake indicate that the lake level decreased sharply ~1500 cal. yr BP, consistent with the interpretation that the changes in vegetation and fire regime were driven by a severe and previously undocumented drought. The lake level rose again, reaching approximately modern levels by 800–700 cal. yr BP, consistent with the hypothesis of cooler/wetter conditions in the Upper Midwest in the past ~700 years and with the expansion of mesic taxa on the NWSP 700 cal. yr BP.

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 (>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.

Lacustrine charcoal peaks provide an accurate record of surface wildfires in a North European boreal forest

We evaluated the skills of different palaeofire reconstruction techniques to reconstruct the fire history of a boreal landscape (Russian Karelia) affected by surface fires. The analysis of dated lacustrine sediments from two nearby lakes was compared with independent dendrochronological dating of fire scars, methods which have rarely been used in context of surface fires. We used two sediment sub-sampling volumes (1 and 3.5 cm3, wet volumes) and three methods of calculating the Charcoal Accumulation Rate to reconstruct fire histories: CHAR number, charcoal surface area and estimated charcoal volume. The results show that palaeofire reconstructions obtained with fossil charcoal data from lake sediments and dendrochronology are similar and complementary. Dendrochronological reconstruction of fire scars established 12 fire dates over the past 500 years, and paleo-data from lake sediments identified between 7 and 13 fire events. Several ‘false fire events’ were also recorded in the charcoal chronologies, likely because of errors associated with the estimation of the sediment accumulation rate in the unconsolidated part of the sediment. The number of replicates, that is, number of sub-samples and lakes analyzed, had an effect on the number of identified fire events, whereas no effect was seen in the variation in the analyzed sediment volume or the choice of the charcoal-based metric. Whenever possible, we suggest the use of the dendrochronological data as an independent control for the calibration of charcoal peak series, which helps provide more realistic millennia-long reconstruction of past fire activity. We also argue for the use of 1 cm3sample volume, a sampling protocol involving sampling of more than one lake, and sufficient number of intra-sample replicates to achieve skilful reconstructions of past fire activity.

Polylepiswoodland dynamics during the last 20,000 years

AbstractAimTo determine the palaeoecological influences of climate change and human land use on the spatial distribution patterns ofPolylepiswoodlands in the Andes.LocationTropical Andes above 2,900 m between 2°S and 18°S of latitude.MethodsPollen and charcoal data were gathered from 13 Andean lake sediment records and were rescaled by the maximum value in each site. The rescaled pollen data were used to estimate a mean abundance and coefficient of variation to show woodland expansions/contractions and woodland fragmentation over the last 20,000 years. The rescaled charcoal was displayed as a 200‐year moving median using 500‐year bins to infer the influence of fire on woodland dynamics at landscape scale. Pollen and charcoal were compared with speleothem, clastic flux and archaeological data to assess the influence of moisture balance, glacial activity and human impact on the spatial distribution ofPolylepiswoodlands.ResultsWoodland expansion and fire were correlated with precipitation changes and glacier dynamics fromc. 20 to 6 kcalbp(thousands of calibrated years before present). Charcoal abundances between 20 and 12 kcalbpwere less common than from 12 kcalbpto modern. However, human‐induced fires were unlikely to be the main cause of a woodland decline centred at 11 kcalbp, as woodlands recovered from 10.5 to 9.5 kcalbp(about twofold increase). Charcoal peaks analogous to those that induced the woodland decline at 11 kcalbpwere commonplace post‐9.5 kcalbpbut did not trigger an equivalent woodland contraction. An increase in the coefficient of variation afterc. 5.5 kcalbpsuggests enhanced fragmentation and coincided with the shift from logistic to exponential growth of human populations. Over the last 1,000 years,Polylepisbecame hyper‐fragmented with over half of sites losingPolylepisfrom the record and with coefficients of variation paralleling those of glacial times.Main conclusionsPolylepiswoodlands formed naturally patchy woodlands, rather than a continuous vegetation belt, prior to human occupation in the Andes. The main factors controlling pre‐human woodland dynamics were precipitation and landscape heterogeneity. Human activity led to hyper‐fragmentation during the lastc. 1,000 years.

Florida wildfires during the Holocene Climatic Optimum (9000–5000 cal yr BP)

Fire is a key ecological driver in terrestrial ecosystems of the southeastern United States. The combination of plant species composition and fire poses the possibility of positive feedback loops in a landscape. How fire regimes will change in the future with global warming is uncertain. To better understand the main factors that control fire, Holocene fire history was studied using macroscopic charcoal (as a fire proxy) in sediment cores from two shallow lakes in north Florida, USA. Following the onset of lacustrine sedimentation in the early Holocene, there was a 95% decline in average charcoal concentration, from 28.3 to 1.4 charcoal particles cm−3, at ~ 7500 cal yr BP in a 3.9-m core from Newnans Lake (basal age: 8870 cal yr BP). At ~ 7000 cal yr BP in a 5.4-m core from nearby Lochloosa Lake (basal age: 9280 cal yr BP), particle density declined by 99%, from 113.8 to 1.4 charcoal particles cm−3. These declines in charcoal concentration are not artifacts of changes in bulk sedimentation, i.e. dilution, but instead reflect lower charcoal production. Newnans Lake and Lochloosa Lake averaged ~ 8.93 and ~ 40.42 charcoal particles cm−3, respectively over their entire records. Pollen records from the two study lakes do not display the Quercus to Pinus shift during the Holocene Climatic Optimum (9000–5000 cal yr BP) observed previously in cores from many studied lakes in the region. Instead, pine pollen, as a percentage of total oak and pine pollen, was high (> 50%) through the entire Holocene. Pine and oak pollen counts and the variety of arboreal pollen, however, were lower in early Holocene deposits. No relation was found between pine and oak percentages and macroscopic charcoal quantity, i.e. before, during, or after charcoal peaks. There were, however, qualitative differences between pollen assemblages within charcoal-rich and charcoal-poor depth intervals. Early to middle Holocene sediments contained pollen of Quercus spp., Pinus spp., Ambrosia sp., Amaranthaceae, Cyperaceae and Poaceae. Late Holocene sediments contained hydric and mesic pollen types including Taxodium spp., Carya sp., Ilex sp., and Liquidambar styraciflua. The flat, poorly drained topography around both lakes played a role in shaping Holocene plant communities and probably explains why a Quercus-to-Pinus shift seen elsewhere in Florida did not occur in the study region.

Fire activity and hydrological dynamics in the past 5700 years reconstructed from Sphagnum peatlands along the oceanic–continental climatic gradient in northern Poland

Fire is a critical component of many ecosystems and, as predicted by various climate models, fire activity may increase significantly in the following years due to climate change. Therefore, knowledge about the past fire activity of various ecosystems is highly important for future nature conservation purposes. We present results of high-resolution investigation of fire activity and hydrological changes in northern Poland. We analyzed microscopic charcoal from three Sphagnum-dominated peatlands located on the south of Baltic, on the oceanic–continental (west–east) climatic gradient, and reconstructed the history of fire in the last 5700 years. We hypothesize that air circulation patterns are highly important for local fire activity, and that fire activity is more intensive in peatlands influenced by continental air masses. We have found out that forest fires have been occurring regularly since the past millennia and were linked to climatic conditions. We show that fire activity (related to climate and fuel availability) was significantly higher in sites dominated by continental climate (northeastern Poland) than in the site located under oceanic conditions (northwestern Poland)—microscopic charcoal influx was 13.3 times higher in the eastern study site of the gradient, compared to the western study site. Recorded fire activity patterns were different between the sites in a long timescale. Moreover, most of the recorded charcoal peaks occurred during high water tables. Rising human pressure has caused droughts and water table instability, and substantial increase in fire activity in the last 400 years.

Holocene vegetation dynamics on the Apakará summit of the neotropical Guayana Highlands and potential environmental drivers

The Guayana Highlands (GH) are natural laboratories to study the influence of environmental drivers on neotropical ecosystems. The GH summits have been characterized by constant vegetation patterns during the Holocene, except for a few sites close to altitudinal ecotones. Here, we report a new pollen record showing two significant vegetation shifts, from Myrica forests to tepuian meadows (5340calyrBP) and then to Chimantaea shrublands (2720calyrBP). These changes are analyzed in terms of potential forcing factors such as regional climate changes, fire and autogenic succession. The first shift occurred shortly after the Holocene Thermal Maximum and the onset of a cooling/drying trend. A charcoal peak was coeval with this change suggesting some potential influence of fire. Autogenic processes related to lake infilling might have also been involved. The second vegetation shift took place during a phase of increased precipitation variability due to an ENSO intensification. The establishment of Chimantaea shrublands started at the end of this phase coinciding with another charcoal peak (2300calyrBP). These results support the idea of relevant ecological changes in the GH during the Holocene, the apparent vegetation constancy recorded in other GH summits could be due to site insensitivity.

Identifying past fire regimes throughout the Holocene in Ireland using new and established methods of charcoal analysis

Globally, in recent years there has been an increase in the scale, intensity and level of destruction caused by wildfires. This can be seen in Ireland where significant changes in vegetation, land use, agriculture and policy, have promoted an increase in fires in the Irish landscape. This study looks at wildfire throughout the Holocene and draws on lacustrine charcoal records from seven study sites spread across Ireland, to reconstruct the past fire regimes recorded at each site. This work utilises new and accepted methods of fire history reconstruction to provide a recommended analytical procedure for statistical charcoal analysis. Digital charcoal counting was used and fire regime reconstructions carried out via the CharAnalysis programme. To verify this record new techniques are employed; an Ensemble-Member strategy to remove the objectivity associated with parameter selection, a Signal to Noise Index to determine if the charcoal record is appropriate for peak detection, and a charcoal peak screening procedure to validate the identified fire events based on bootstrapped samples. This analysis represents the first study of its kind in Ireland, examining the past record of fire on a multi-site and paleoecological timescale, and will provide a baseline level of data which can be built on in the future when the frequency and intensity of fire is predicted to increase.

Spatiotemporal patterns of tundra fires: late-Quaternary charcoal records from Alaska

Abstract. Anthropogenic climate change has altered many ecosystem processes in the Arctic tundra and may have resulted in unprecedented fire activity. Evaluating the significance of recent fires requires knowledge from the paleofire record because observational data in the Arctic span only several decades, much shorter than the natural fire rotation in Arctic tundra regions. Here we report results of charcoal analysis on lake sediments from four Alaskan lakes to infer the broad spatial and temporal patterns of tundra-fire occurrence over the past 35 000 years. Background charcoal accumulation rates are low in all records (range is 0–0.05 pieces cm−2 yr−1), suggesting minimal biomass burning across our study areas. Charcoal peak analysis reveals that the mean fire-return interval (FRI; years between consecutive fire events) ranged from ca. 1650 to 6050 years at our sites, and that the most recent fire events occurred from ca. 880 to 7030 years ago, except for the CE 2007 Anaktuvuk River Fire. These mean FRI estimates are longer than the fire rotation periods estimated for the past 63 years in the areas surrounding three of the four study lakes. This result suggests that the frequency of tundra burning was higher over the recent past compared to the late Quaternary in some tundra regions. However, the ranges of FRI estimates from our paleofire records overlap with the expected values based on fire-rotation-period estimates from the observational fire data, and the differences are statistically insignificant. Together with previous tundra-fire reconstructions, these data suggest that the rate of tundra burning was spatially variable and that fires were extremely rare in our study areas throughout the late Quaternary. Given the rarity of tundra burning over multiple millennia in our study areas and the pronounced effects of fire on tundra ecosystem processes such as carbon cycling, dramatic tundra ecosystem changes are expected if anthropogenic climate change leads to more frequent tundra fires.

Holocene vegetation change in relation to fire and volcanic events in Jilin, Northeastern China

Volatiles erupted from large-scale explosive volcanic activities have a significant impact on climate and environmental changes. As an important ecological factor, the occurrence of fire is affected by vegetation cover, and fire can feed back into both vegetation and climatic change. The causes of fire events are diverse; and can include volcanic eruptions. The amount of charcoal in sediment sequences is related to the frequency and intensity of fire, and hence under good preservation conditions fire history can be reconstructed from fossil charcoal abundance. Until now, little research on the role of fire has been carried out in northeastern China. In this study, through research on charcoal and tephra shards from Gushantun and Hanlongwan, Holocene vegetation change in relation to fire and volcanic events in Jilin, Northeastern China, was investigated. Where tephra shards are present in Gushantun it is associated with low level of both conifers and broadleaved trees, and is also associated with a pronounced charcoal peak. This suggests forest cover was greatly reduced from a fire caused by an eruption of the Tianchi volcano. We also detected one tephra layer in Hanlongwan, which also has the almost same depth with low level forest pollen values and one charcoal peak. This was caused probably by an eruption of the Jinlongdingzi volcano.

Presence of lakes and wetlands decreases resilience of jack pine ecosystems to late-Holocene climatic changes

We reconstructed vegetation and fire histories from four sites located on a sandy outwash plain in northwestern Wisconsin (USA) to test whether lakes and wetlands have influenced how vegetation and fire regimes in pine–oak forests responded to late-Holocene climatic changes. Because of positive feedbacks between jack pine (Pinus banksiana Lamb.) and fire, communities with few fire breaks should be more resilient to changing climatic conditions. Pollen and charcoal from lake-sediment cores were used to reconstruct vegetation changes at 50- to 100-year intervals and forest fire history at decadal time scales for the past 2500 years. The presence of fire breaks affected both fire regimes and the response of vegetation to climatic changes. Areas with more fire breaks had smaller charcoal peaks and the vegetation was more responsive to climate changes. The vegetation in areas with few fire breaks was more resilient, maintaining higher amounts of jack pine and (or) red pine than the more protected sites. We interpret these findings as evidence that positive feedbacks between fire and jack pine forests stabilized vegetation at sites where fire breaks were absent, and that such sites may be relatively resilient to future climate changes, until jack pine is no longer able to regenerate under the regional climatic conditions.

Charcoal and pollen analysis: Examples of Holocene fire dynamics in Mediterranean Iberian Peninsula

The present study focuses on charcoal particles (a component of vegetal ashes, Bodí et al., 2014) counted during palynological analysis. The absolute frequencies of this microcharcoal have been correlated with pollen percentages of resprouts, mesic trees, Pinus or AP (arboreal pollen), in order to observe patterns in the spread of wildfires, dynamic changes in plant succession, and possible cyclicities in Holocene microclimatic changes. The sites selected for this work are located within the Mediterranean area of the Iberian Peninsula and in the Eivissa Balearic Islands, between 42°16′ N and 36°47′ N. Results show two brief changes in vegetation dynamics during the Holocene. The first coincides with the 8.2calkyr event and marks both the beginning of Holocene dynamic changes in plant succession and the Climatic Optimum. During this first phase, charcoal peaks coincide with minimum percentages of mesic taxa (low rainfall) and occur just before an increase of resprout taxa (taxa which grow after fires). The second change happens at the ~4.2calBPkyr event, when fires are most frequent and resprout taxa increase their values compared to mesic taxa and forest cover (AP). We have also observed some differences in vegetal dynamics studied at different latitudes, which appear to be punctuated by climate or insularity. In short, (i) actual succession of Mediterranean vegetation can be recognized in the past; (ii) resilience in vegetation is more common during the first half of the Holocene than in the recent Holocene: A trend to aridity, reduced biomass and anthropization has caused vegetation resilience to decline; (iii) synchronicity between fire peaks of different sequences studied is very rare; and (iv) coincidence of many peaks of charcoal with Bond events inclines us to think that fires are more conditioned by the climate than by anthropogenic impact.

A Paleolimnological Record of Late Holocene Vegetation Change from the Central California Coast

In this paper, we present results of the analysis of the pollen and microscopic charcoal content of a sediment core from Skylark Pond near Point Año Nuevo in Santa Cruz County, California. The core covers approximately the last 3,000 years and is of interest because Skylark Pond is located only 1.8 km from Quiroste Valley State Cultural Preserve, an important ceremonial and habitation location for the historic Quiroste tribe containing numerous late Holocene archaeological sites. The results show an increase in fire activity from the fifteenth century to the present. Peaks in charcoal at ca. AD 1425 along with subsequent high charcoal abundance indicate either small, frequent fires ignited by humans or large natural conflagrations. Significant changes after European colonization include increases in grass, oak, and bracken fern, all of which indicate the opening of the redwood forest by logging and ranching. Historic period charcoal peaks can be attributed to the burning of logging slash.

Postglacial climate and fire‐mediated vegetation change on the western Olympic Peninsula, Washington (USA)

The mode and tempo of forest compositional change during periods of rapid climate change, including the potential for the fire regime to produce nonlinear relationships between climate and vegetation, is a long‐standing theme of forest ecological research. In the old conifer forests of the coastal Pacific Northwest, fire disturbances are sufficiently rare that their relation to climate and their ecological effects are poorly understood. We used a 14 700‐year high‐resolution sediment record from Yahoo Lake on the Olympic Peninsula, Washington, USA, to examine vegetation (landscape vegetation from pollen and local vegetation from macrofossils) and fire (landscape fire from total charcoal and local fire from charcoal peaks) in conjunction with independent records of climate. We hypothesized that the successional stage of the local forest will exhibit alternate stable states over a range of fire activity, that species turnover will increase abruptly above a certain level of fire activity and that both responses would be more gradual at the landscape scale than the local scale. Supporting these hypotheses, at the local scale, we found strong evidence for alternate stable states of late vs. early successional communities and inertia of species turnover to changing fire activity. At the landscape scale, vegetation responded more gradually to changing fire activity. From 14 700 to 7000 years ago, high landscape vegetation turnover occurred along with high landscape fire activity, especially during the warm summers of the early Holocene. In several instances, local species turned over completely following fire events but several centuries after climate change. In contrast, during the last 7000 years, the local forest composition was dominated by late‐successional species with little species turnover, despite periods of moderate fire activity. We suggest that the relatively minor climate fluctuations of the past 7000 years were not sufficient to cause large‐scale species turnover after fire. The Yahoo Lake fire and vegetation record of the early Holocene provides a model for dramatic ecosystem change following an anticipated shift to warmer summer temperatures.

Climatic and land cover influences on the spatiotemporal dynamics of Holocene boreal fire regimes

Although recent climatic warming has markedly increased fire activity in many biomes, this trend is spatially heterogeneous. Understanding the patterns and controls of this heterogeneity is important for anticipating future fire regime shifts at regional scales and for developing land management policies. To assess climatic and land cover controls on boreal forest fire regimes, we conducted macroscopic-charcoal analysis of sediment cores and GIS analysis of landscape variation in south-central Alaska, USA. Results reveal that fire occurrence was highly variable both spatially and temporally over the past seven millennia. At two of four sites, the lack of distinct charcoal peaks throughout much of this period suggests the absence of large local fires, attributed to abundant water bodies in the surrounding landscape that have likely functioned as firebreaks to limit fire spread. In contrast, distinct charcoal peaks suggest numerous local fires at the other two sites where water bodies are less abundant. In periods of the records where robust charcoal peaks allow identification of local-fire events over the past 7000 years, mean fire return intervals varied widely with a range of 138-453 years. Furthermore, the temporal trajectories of local-fire frequency differed greatly among sites and were statistically independent. Inferred biomass burning and mean summer temperature in the region were not significantly correlated prior to 3000 years ago but became positively related subsequently with varying correlation strengths. Climatic variability associated with the Medieval Climate Anomaly and the Little Ice Age, along with the expansion of flammable Picea mariana forests, probably have heightened the sensitivity of forest burning to summer temperature variations over the past three millennia. These results elucidate the patterns and controls of boreal fire regime dynamics over a broad range of spatiotemporal scales, and they imply that anthropogenic climatic warming and associated land cover changes, in particular lake drying, will interact to affect boreal forest burning over the coming decades.

Did fires drive Holocene carbon sequestration in boreal ombrotrophic peatlands of eastern Canada?

Wildfire is an important factor on carbon sequestration in the North American boreal biomes. Being globally important stocks of organic carbon, peatlands may be less sensitive to burning in comparison with upland forests, especially wet unforested ombrotrophic ecosystems as found in northeastern Canada. We aimed to determine if peatland fires have driven carbon accumulation patterns during the Holocene. To cover spatial variability, six cores from three peatlands in the Eastmain region of Quebec were analyzed for stratigraphic charcoal accumulation. Results show that regional Holocene peatland fire frequency was ~2.4fires1000yr−1, showing a gradually declining trend since 4000cal yr BP, although inter- and intra-peatland variability was very high. Charcoal peak magnitudes, however, were significantly higher between 1400 and 400cal yr BP, possibly reflecting higher charcoal production driven by differential climatic forcing aspects. Carbon accumulation rates generally declined towards the late-Holocene with minimum values of ~10gm−2yr−1 around 1500cal yr BP. The absence of a clear correlation between peatland fire regimes and carbon accumulation indicates that fire regimes have not been a driving factor on carbon sequestration at the millennial time scale.