Holocene climate–fire–vegetation interactions at a subalpine watershed in southeastern British Columbia,Canada

Vegetation assemblages and associated disturbance regimes are spatially heterogeneous in mountain ecosystems throughout the world due to the complex terrain and strong environmental gradients. Given this complexity, numerous sites describing postglacial vegetation and fire histories are needed to adequately understand forest development and ecosystem responses to varying climate and disturbance regimes. To gain insight into long-term historical climate–fire–vegetation interactions in southeastern British Columbia, Canada, sedimentological and paleoecological analyses were performed on a sediment core recovered from a small subalpine lake. The pollen assemblages, stomata, and macroremains indicate that from 9500 to 7500 cal yr BP, Pinus-dominated forests occurred within the catchment and Alnus was also present. Climate was an important control of fire and fire frequency was highest at this time, peaking at 8 fires 1000 yr^− 1, yet charcoal accumulation rates were low, indicative of low terrestrial biomass abundance. From 7500 to 4600 cal yr BP, Pinus decreased as Picea, Abies and Larix increased and fire frequencies decreased to 3–6 fires 1000 yr^− 1. Since 7500 cal yr BP the fire regime varied at a millennial scale, driven by forest biomass abundance and fuel accumulation changes. Local scale (bottom-up) controls of fire increased in relative importance since at least 6000 cal yr BP.     

Quantifying the source area of macroscopic charcoal with a particle dispersal model

To aid interpreting the source area of charcoal in lake-sediment records, we compare charcoal deposition from an experimental fire to predictions from a particle dispersal model. This provides both a theoretical framework for understanding how lake sediments reflect fire history and a foundation for simulating sediment-charcoal records. The dispersal model captures the two-dimensional patterns in the empirical data (predicted vs. observed r² = 0.67, p < 0.001). We further develop the model to calculate the potential charcoal source area (PCSA) for several classes of fires. Results suggest that (1) variations in airborne charcoal deposition can be explained largely by the size of PCSAs relative to fire sizes and (2) macroscopic charcoal travels many kilometers, longer than suggested by dispersal data from experimental fires but consistent with dispersal data from uncontrolled fires.     

Climatic controls of Holocene fire patterns in southern South America

Holocene fire-climate-vegetation linkages are mostly understood at individual sites by comparing charcoal and pollen records with other paleoenvironmental proxy and model simulations. This scale of reconstruction often obscures detection of large-scale patterns in past fire activity that are related to changes in regional climate and vegetation. A network of 31 charcoal records from southern South America was examined to assess fire history along a transect from subtropic to subantarctic biomes. The charcoal data indicate that fire activity was greater than present at ca. 12,000 cal yr BP and increased further and was widespread at 9500 cal yr BP. Fire activity decreased and became more spatially variable by 6000 cal yr BP, and this trend continued to present. Atmospheric circulation anomalies during recent high-fire years show a southward shift in westerlies, and paleoclimate model simulations and data syntheses suggest that such conditions may have prevailed for millennia in the early Holocene when the pole-to-equator temperature gradients were weaker and annual temperatures were higher than present, in response to orbital-time-scale insolation changes.     

Millennial‐scale fire and vegetation history from a mesic hardwood forest of southeastern Wisconsin,USA

High‐resolution charcoal analysis of lake sediment cores was used to reconstruct the fire history from two sites in a mesic hardwood forest of south‐eastern Wisconsin located in the Kettle Moraine State Forest. Pollen data from the region indicate that the sites, which lie within 5 km of each other, have had a consistent presence of mesic hardwood forest for the last 6500 years. A pollen record from one of the sites confirmed the regional vegetation history and the charcoal analysis indicated that fire frequency at each site was temporally linked to regional drought. Periods of high fire occurrence occurred in connection with a region‐wide drought 4200 years ago and, over the last 2000 years, shorter‐scale regional droughts were centred at 1800, 1650, 1100, 1000, 800, 700 and 600 cal a BP. The fire histories indicate that the last 1000 years have had lower fire frequencies than the previous 6500 years and suggest that the mesic hardwood forests may be resilient to increases in fire that may result from future climate change. Copyright © 2011 John Wiley & Sons, Ltd.     

A signal-to-noise index to quantify the potential for peak detection in sediment-charcoal records

Charcoal peaks in lake-sediment records are commonly used to reconstruct fire histories spanning thousands of years, but quantitative methods for evaluating the suitability of records for peak detection are largely lacking. We present a signal-to-noise index (SNI) that quantifies the separation of charcoal peaks (signal) from other variability in a record (noise). We validate the SNI with simulated and empirical charcoal records and show that an SNI > 3 consistently identifies records appropriate for peak detection. The SNI thus offers a means to evaluate the suitability of sediment-charcoal records for reconstructing local fires. MATLAB and R functions for calculating SNI are provided.     

Holocene fire occurrence and alluvial responses at the leading edge of pinyon–juniper migration in the Northern Great Basin,USA

Fire and vegetation records at the City of Rocks National Reserve (CIRO), south-central Idaho, display the interaction of changing climate, fire and vegetation along the migrating front of single-leaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma). Radiocarbon dating of alluvial charcoal reconstructed local fire occurrence and geomorphic response, and fossil woodrat (Neotoma) middens revealed pinyon and juniper arrivals. Fire peaks occurred ~ 10,700–9500, 7200–6700, 2400–2000, 850–700, and 550–400 cal yr BP, whereas ~ 9500–7200, 6700–4700 and ~ 1500–1000 cal yr BP are fire-free. Wetter climates and denser vegetation fueled episodic fires and debris flows during the early and late Holocene, whereas drier climates and reduced vegetation caused frequent sheetflooding during the mid-Holocene. Increased fires during the wetter and more variable late Holocene suggest variable climate and adequate fuels augment fires at CIRO. Utah juniper and single-leaf pinyon colonized CIRO by 3800 and 2800 cal yr BP, respectively, though pinyon did not expand broadly until ~ 700 cal yr BP. Increased fire-related deposition coincided with regional droughts and pinyon infilling ~ 850–700 and 550–400 cal yr BP. Early and late Holocene vegetation change probably played a major role in accelerated fire activity, which may be sustained into the future due to pinyon–juniper densification and cheatgrass invasion.     

The role of climate variability and fire in early and mid Holocene forest dynamics of southern Sweden

The objective of this study was to investigate the possible links between regional climate, fire and vegetation at the small spatial scale during the early and mid Holocene in southern Sweden using pollen, plant macrofossil and charcoal records from a small bog. The fire history was compared with climate reconstructions inferred from various proxy records in the study region. High fire activity is related to dry and warm climate around 8550, 7600, 5500–5100 and 4500 cal. a BC. Low fire activity ca. 6500–6000 and 4750 BC may correspond to the widespread ‘8.2 k event’ (ca. 6200 BC) recorded across the North Atlantic region, and a later, brief period of increased precipitation, respectively. The decrease in broadleaved trees culminating ca. 6500–6000 BC correlates with the ‘8.2 k event’. A long mid Holocene period with low fire activity (ca. 4350–1000 BC) agrees with the pattern emerging for Europe from the global charcoal database, and may correspond to generally wetter and cooler conditions. High fire activity ca. 8550 BC probably triggered the local establishment of Corylus. Warmer and drier conditions (and high fire activity) ca. 7600 BC might have favoured the establishment of Alnus, Quercus and Tilia. The fire‐adapted Pinus maintained important populations throughout the early and mid Holocene. Copyright © 2008 John Wiley & Sons, Ltd.     

The effects of fire and tephra deposition on forest vegetation in the Central Cascades, Oregon

High-resolution charcoal and pollen analyses were used to reconstruct a 12,000-yr-long fire and vegetation history of the Tumalo Lake watershed and to examine the short-term effects that tephra deposition have on forest composition and fire regime. The record suggests that, from 12,000 to 9200 cal yr BP, the watershed was dominated by an open Pinus forest with Artemisia as a common understory species. Fire episodes occurred on average every 115 yr. Beginning around 9200 cal yr BP, and continuing to the present, Abies became more common while Artemisia declined, suggesting the development of a closed forest structure and a decrease in the frequency of fire episodes, occurring on average every 160 yr. High-resolution pollen analyses before and after the emplacement of three distinct tephra deposits in the watershed suggest that nonarboreal species were most affected by tephra events and that recovery of the vegetation community to previous conditions took between 40 and 100 yr. Changes in forest composition were not associated with tephra depositional events or changes in fire-episode frequency, implying that the regional climate is the more important control on long-term forest composition and structure of the vegetation in the Cascade Range.     

Re‐evaluation of late Holocene fire histories of three boreal bogs suggest a link between bog fire and climate

We re‐evaluated mid‐ and late Holocene fire records at three bog sites in southern Finland and Estonia through quantitative and qualitative analysis of sedimentary charcoal records. Our analysis suggests synchrony amongst the Kontolanrahka, Männikjärve and Lakkasuo bog records. Over the last 5000 years, three episodes of elevated fire occurrence were evident in these bog records, taking place from 4500 to 4000, 3500 to 2000 and 1000 to 500 cal. a BP. These episodes were probably influenced primarily by climate, but also possibly by expansion/retreat of (pre)historic human populations. Although previous studies have proposed that during the late Holocene only weak fire?climate linkages exist, the presented analysis suggests millennial‐ and centennial‐scale synchronous burning episodes.     

Ancient charcoal as a natural archive for paleofire regime and vegetation change in the Mayumbe,Democratic Republic of the Congo

Charcoal was sampled in four soil profiles at the Mayumbe forest boundary (DRC). Five fire events were recorded and 44 charcoal types were identified. One stratified profile yielded charcoal assemblages around 530 cal yr BP and > 43.5 cal ka BP in age. The oldest assemblage precedes the period of recorded anthropogenic burning, illustrating occasional long-term absence of fire but also natural wildfire occurrences within tropical rainforest. No other charcoal assemblages older than 2500 cal yr BP were recorded, perhaps due to bioturbation and colluvial reworking. The recorded paleofires were possibly associated with short-lived climate anomalies. Progressively dry climatic conditions since ca. 4000 cal yr BP onward did not promote paleofire occurrence until increasing seasonality affected vegetation at the end of the third millennium BP, as illustrated by a fire occurring in mature rainforest that persisted until around 2050 cal yr BP. During a drought episode coinciding with the ‘Medieval Climate Anomaly’, mature rainforest was locally replaced by woodland savanna. Charcoal remains from pioneer forest indicate that fire hampered forest regeneration after climatic drought episodes. The presence of pottery shards and oil-palm endocarps associated with two relatively recent paleofires suggests that the effects of climate variability were amplified by human activities.     

Comparing fire-history interpretations based on area, number and estimated volume of macroscopic charcoal in lake sediments

Sedimentary charcoal particles from lakes are commonly used to investigate fire history. Fire-history reconstructions are based on measuring the surface area or counting the number of charcoal fragments in adjacent samples. Recently, the volume of charcoal particles was advised as a more accurate method for quantifying past charcoal production. Large charcoal datasets, used to synthesize global fire history, include these different types of charcoal measurements and implicitly assume that they provide comparable fire-history information. However, no study has demonstrated that this assumption is valid. Here we compare fire-frequency reconstructions based on measurements of charcoal area and number, and estimates of charcoal volume from two lake sediment records from the eastern Canadian boreal forest. Results indicate that the three proxies provide comparable fire-history interpretations when using a locally defined threshold to identify fire events.     

Postglacial fire,vegetation, and climate history across an elevational gradient in the Northern Rocky Mountains,USA and Canada

A 13,100-year-long high-resolution pollen and charcoal record from Foy Lake in western Montana is compared with a network of vegetation and fire-history records from the Northern Rocky Mountains. New and previously published results were stratified by elevation into upper and lower and tree line to explore the role of Holocene climate variability on vegetation dynamics and fire regimes. During the cooler and drier Lateglacial period, ca 13,000 cal yr BP, sparsely vegetated Picea parkland occupied Foy Lake as well as other low- and high-elevations with a low incidence of fire. During the warmer early Holocene, from ca 11,000–7500 cal yr BP, low-elevation records, including Foy, indicate significant restructuring of regional vegetation as Lateglacial Picea parkland gave way to a mixed forest of Pinus-Pseudotsuga-Larix. In contrast, upper tree line sites (ca >2000 m) supported Pinus albicaulis and/or P. monticola-Abies-Picea forests in the Lateglacial and early Holocene. Regionally, biomass burning gradually increased from the Lateglacial times through the middle Holocene. However, upper tree line fire-history records suggest several climate-driven decreases in biomass burning centered at 11,500, 8500, 4000, 1600 and 500 cal yr BP. In contrast, lower tree line records generally experienced a gradual increase in biomass burning from the Lateglacial to ca 8000 cal yr BP, then reduced fire activity until a late Holocene maximum at 1800 cal yr BP, as structurally complex mesophytic forests at Foy Lake and other sites supported mixed-severity fire regimes. During the last two millennia, fire activity decreased at low elevations as modern forests developed and the climate became cooler and wetter than before. Embedded within these long-term trends are high amplitude variations in both vegetation dynamics and biomass burning. High-elevation paleoecological reconstructions tend to be more responsive to long-term changes in climate forcing related to growing-season temperature. Low-elevation records in the NRM have responded more abruptly to changes in effective precipitation during the late Holocene. Prolonged droughts, including those between 1200 and 800 cal yr BP, and climatic cooling during the last few centuries continues to influence vegetation and fire regimes at low elevation while increasing temperature has increased biomass burning in high elevations.     

Paleofire reconstruction for high-elevation forests in the Sierra Nevada, California, with implications for wildfire synchrony and climate variability in the late Holocene

Here, we present two high-resolution records of macroscopic charcoal from high-elevation lake sites in the Sierra Nevada, California, and evaluate the synchroneity of fire response for east- and west-side subalpine forests during the past 9200 yr. Charcoal influx was low between 11,200 and 8000 cal yr BP when vegetation consisted of sparse Pinus-dominated forest and montane chaparral shrubs. High charcoal influx after ∼ 8000 cal yr BP marks the arrival of Tsuga mertensiana and Abies magnifica, and a higher-than-present treeline that persisted into the mid-Holocene. Coeval decreases in fire episode frequency coincide with neoglacial advances and lower treeline in the Sierra Nevada after 3800 cal yr BP. Independent fire response occurs between 9200 and 5000 cal yr BP, and significant synchrony at 100- to 1000-yr timescales emerges between 5000 cal yr BP and the present, especially during the last 2500 yr. Indistinguishable fire-return interval distributions and synchronous fires show that climatic control of fire became increasingly important during the late Holocene. Fires after 1200 cal yr BP are often synchronous and corroborate with inferred droughts. Holocene fire activity in the high Sierra Nevada is driven by changes in climate linked to insolation and appears to be sensitive to the dynamics of the El Niño-Southern Oscillation.     

古火灾历史重建的研究进展

火是地球系统的重要组成部分,与气候、植被、生物地球化学循环和人类活动密切相关。火对全球气候和生态系统的影响已成为目前全球变化研究的一个热点。火灾发生后会在周围的环境中留下许多燃烧产物,如黑碳、木炭屑、多环芳烃、左旋葡萄糖等,它们广泛存在于海洋、湖泊、河流、土壤和陆地风成沉积物中;还会留下一些火灾痕迹,如树木火疤、土壤磁...     

Holocene fire and vegetation dynamics in a montane forest, North Cascade Range, Washington, USA

We reconstructed a 10,500-yr fire and vegetation history of a montane site in the North Cascade Range, Washington State based on lake sediment charcoal, macrofossil and pollen records. High-resolution sampling and abundant macrofossils made it possible to analyze relationships between fire and vegetation. During the early Holocene (> 10,500 to ca. 8000 cal yr BP) forests were subalpine woodlands dominated by Pinus contorta. Around 8000 cal yr BP, P. contorta sharply declined in the macrofossil record. Shade tolerant, mesic species first appeared ca. 4500 cal yr BP. Cupressus nootkatensis appeared most recently at 2000 cal yr BP. Fire frequency varies throughout the record, with significantly shorter mean fire return intervals in the early Holocene than the mid and late Holocene. Charcoal peaks are significantly correlated with an initial increase in macrofossil accumulation rates followed by a decrease, likely corresponding to tree mortality following fire. Climate appears to be a key driver in vegetation and fire regimes over millennial time scales. Fire and other disturbances altered forest vegetation at shorter time scales, and vegetation may have mediated local fire regimes. For example, dominance of P. contorta in the early Holocene forests may have been reinforced by its susceptibility to frequent, stand-replacing fire events.     

Microscopic charcoal in sediments: Quaternary fire history of the grassland and savanna regions in South Africa

The microscopic charcoal content of several Quaternary pollen sequences is used to investigate fire history in South Africa both during the Holocene and the Late Pleistocene. Although fluctuations in charcoal composition are recorded, it is difficult to link them directly to either human‐made or natural fires. Strong long‐term variations in microscopic charcoal of Middle and Upper Pleistocene layers are independent of pollen indications of past temperature and moisture conditions. Holocene charcoal sequences from different areas show no correlation, so no phases of regional burning are found. Some fluctuations in charcoal probably are the result of local burning in the various regions. The arrival of Iron Age people some 2000 yr ago apparently did not coincide with widespread wild fires, as these events do not consistently appear in regional microscopic charcoal records. The only exception appears to be the Wonderkrater spring deposit. Relatively open savanna environments, which are implied by pollen analysis at some sites during this period and the generally low microscopic charcoal contents, were either caused by climate change or controlled burning by Iron Age people. Copyright © 2002 John Wiley & Sons, Ltd.     

晚冰期月亮湖炭屑记录反映的古气候演化

位于大兴安岭中段的月亮湖地处季风/非季风影响过渡地带,其沉积岩心下部886～546 cm的炭屑记录揭示了末次冰期晚期到全新世早期(20.9～10.8 cal.ka B.P.)的古气候演化历史,反映了东亚季风对研究区气候的影响。研究区炭屑浓度的变化主要由可供燃烧的生物量决定,生长在气候温暖时期的森林草原能够提供更多可供燃烧的生物量。在同一植被类型的条件下,气候寒冷湿润时炭屑浓度低,气候温暖干旱时炭屑浓度高。20.9～18.0 cal.ka B.P.炭屑浓度较低, 气候寒冷偏干,18.0～15.3 cal.ka B.P.炭屑浓度最低,气候寒冷湿润,15.3～14.4 cal.ka B.P.炭屑浓度增高,气候开始向温暖的方向发展,14.4～11.8 cal.ka B.P.炭屑浓度快速变化,气候也经历了一系列的快速变化,11.8～10.8 cal.ka B.P.炭屑浓度总体较高,气候温暖湿润。<50 μm的炭屑浓度指示了区域火演化的历史, >50 μm的炭屑则反映了当地野火发生的状况。     

Changing fire regimes in the temperate rainforest region of southern Chile over the last 16,000 yr

A high-resolution macroscopic charcoal record from Lago Melli (42°46′S, 73°33′W) documents the occurrence of forest fires in the lowlands of Isla Grande de Chiloé, southern Chile, over the last 16,000 yr. Our data suggest that fire activity in this region was largely modulated by the position/intensity of the southern westerlies at multi-millennial time scales. Fire activity was infrequent or absent between 16,000-11,000 and 8500-7000 cal yr BP and was maximal between ∼ 11,000-8500 and 3000-0 cal yr BP. A mosaic of Valdivian/North Patagonian rainforest species started at ∼ 6000 cal yr BP, along with a moderate increase in fire activity which intensified subsequently at 3000 cal yr BP. The modern transition between these forest communities and the occurrence of fires are largely controlled by summer moisture stress and variability, suggesting the onset of high-frequency variability in summer precipitation regimes starting at ∼ 5500 cal yr BP. Because negative anomalies in summer precipitation in this region are teleconnected with modern El Niño events, we propose that the onset of El Niño-like variability at ∼ 5700-6200 cal yr BP led to a reshuffling of rainforest communities in the lowlands of Isla Grande de Chiloé and an increase in fire activity.     

Vegetation,fire, and climate history of the northwestern Great Basin during the last 14,000 years

The northwestern Great Basin lies in the transition zone between the mesic Pacific Northwest and xeric intermountain West. The paleoenvironmental history based on pollen, macroscopic charcoal, and plant macrofossils from three sites in the northwestern Great Basin was examined to understand the relationships among the modern vegetation, fire disturbance and climate. The vegetation history suggests that steppe and open forest communities were present at high elevations from ca 11,000 to 7000 cal yr BP, and were replaced by forests composed of white fir, western white pine, and whitebark pine in the late Holocene. Over the last 11,000 years, fires were more frequent in mid-elevation forests (10–25 fire episodes/1000 years) and rare in high-elevation forests (2–5 fire episodes/1000 years). Applying modern pollen–climate relationships to the fossil pollen spectra provided a means to interpret past climate changes in this region. In the past 9000 years summer temperatures decreased from 1 to 4 °C, and annual precipitation has increased 7–15%. These results indicate that the millennial-scale climate forcing driving vegetation changes can be quantified within the intermountain West in general and northwestern Great Basin in particular. In addition, fire can be considered an important component of these ecosystems, but it does not appear to be a forcing mechanism for vegetation change at the resolution of these records.     

Postglacial vegetation, climate, and fire history along the east side of the Andes (lat 41-42.5°S), Argentina

The history of the low-elevation forest and forest-steppe ecotone on the east side of the Andes is revealed in pollen and charcoal records obtained from mid-latitude lakes. Prior to 15,000 cal yr BP, the vegetation was characterized by steppe vegetation with isolated stands of Nothofagus. The climate was generally dry, and the sparse vegetation apparently lacked sufficient fuels to burn extensively. After 15,000 cal yr BP, a mixture of Nothofagus forest and shrubland/steppe developed. Fire activity increased between 13,250 and 11,400 cal yr BP, contemporaneous with a regionally defined cold dry period (Huelmo/Mascardi Cold Reversal). The early-Holocene period was characterized by an open Nothofagus forest/shrubland mosaic, and fire frequency was high in dry sites and low in wet sites; the data suggest a sharp decrease in moisture eastward from the Andes. A shift to a surface-fire regime occurred at 7500 cal yr BP at the wet site and at 4400 cal yr BP at the dry site, preceding the expansion of Austrocedrus by 1000-1500 yr. The spread of Austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late Holocene. The change to a surface-fire regime is consistent with increased interannual climate variability and the onset or strengthening of ENSO. The present-day mixed forest dominated by Nothofagus and Austrocedrus was established in the last few millennia.