Knowledge of historical fire activity tends to be focused at local to landscape scales with few attempts to examine how local patterns of fire activity scale to global patterns. Generally, fire activity varied globally and continuously since the last glacial maximum (LGM) in response to long-term changes in global climate and shorter-term regional changes in climate, vegetation, and human land use. We have synthesised sedimentary charcoal records of biomass burning since the LGM and present global maps showing changes in fire activity for time slices during the past 21,000 years (as differences in charcoal accumulation values compared to pre-industrial). There is strong broad-scale coherence in fire activity after the LGM, but spatial heterogeneity in the signals increases thereafter. In eastern and western North America and western Europe and southern South America, charcoal records indicate less-than-present fire activity from 21,000 to ~11,000 cal yr BP. In contrast, the tropical latitudes of South America and Africa show greaterthan-present fire activity from ~19,000 to ~17,000 cal yr BP whereas most sites from Indochina and Australia show greater-than-present fire activity from 16,000 to ~13,000 cal yr BP. Many sites indicate greater-than-present or near-present activity during the Holocene with the exception of eastern North America and eastern Asia from 8000 to ~2000 cal yr BP, Indonesia from 11,000 to 4000 cal yr BP, and southern South America from 6000 to 3000 cal yr BP where fire activity was less than present. Regional coherence in the patterns of change in fire activity was evident throughout the postglacial period. These complex patterns can be explained in terms of large-scale climate controls modulated by local changes in vegetation and fuel load.

It is widely accepted, based on data from the last few decades and on model simulations, that anthropogenic climate change will cause increased fire activity. However, less attention has been paid to the relationship between abrupt climate changes and heightened fire activity in the paleorecord. We use 35 charcoal and pollen records to assess how fire regimes in North America changed during the last glacial-interglacial transition (15 to 10 ka), a time of large and rapid climate changes. We also test the hypothesis that a comet impact initiated continental-scale wildfires at 12.9 ka; the data do not support this idea, nor are continent-wide fires indicated at any time during deglaciation. There are, however, clear links between large climate changes and fire activity. Biomass burning gradually increased from the glacial period to the beginning of the Younger Dryas. Although there are changes in biomass burning during the Younger Dryas, there is no systematic trend. There is a further increase in biomass burning after the Younger Dryas. Intervals of rapid climate change at 13.9, 13.2, and 11.7 ka are marked by large increases in fire activity. The timing of changes in fire is not coincident with changes in human population density or the timing of the extinction of the megafauna. Although these factors could have contributed to fire-regime changes at individual sites or at specific times, the charcoal data indicate an important role for climate, and particularly rapid climate change, in determining broad-scale levels of fire activity.biomass burning ͉ charcoal ͉ comet ͉ Younger Dryas

Study of microscopic charcoal from lake sediments has led to a greater understanding of past veg etation, climate and fire ecology. We investigated the potential of charcoal morphology as an indicator of vegetation type. Grasses, leaves and wood were burned under controlled conditions in the laboratory, and we used a dissecting scope, video camera, and image-capture software to image-sieved (125-μm screen) micro-scopic charcoal. Charcoal from grasses was significantly longer (562 μm) and had a greater length:width ratio (3.62) than charcoal derived from leaves (380 μm; 1.91) or wood (348 μm; 2.13). Length:width ratios of mixtures of grass and leaf charcoal were intermediate (50:50 mixture; 2.36) between ratios for grass or leaf charcoal alone, and charcoal yield (on a weight basis) declined as a function of combustion temperature. While a number of issues may complicate the application of these results to the field, the results do suggest that length:width ratios can be used as an indicator of vegetation type.

Summary 1Treeline ecotones, such as the prairie-forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2 In this study, we examined the prairie-forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3 Sediment magnetic and LOI data suggest four broad climatic periods occurring c . 11 350 -8250 BP (cool / humid), c . 8250-4250 BP (warm /dry), c . 4250-2450 BP (warm / humid), and c . 2450-0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4 The area surrounding both lakes was characterized by boreal forest c . 12 500-10 000 BP, changing to an Ulmus-Ostrya forest c . 10 000-9000 BP, changing to a community dominated by prairie ( Poaceae-Ambrosia-Artemisia ) and deciduous forest taxa c . 8000-4250 BP, and finally shifting to a Quercus -dominated woodland/savanna beginning c . 4250-3000 BP. 5 Charcoal influx increased from an average of 0.11-0.62 mm 2 cm − 2 year − 1 during the early Holocene forest period ( c . 11 350-8250 BP) to 1.71-3.36 mm 2 cm − 2 year − 1 during the period of prairie expansion ( c . 8250-4250 BP) and again increased to 4.18-4.90 mm 2 cm − 2 year − 1 at the start of the woodland/savanna period ( c . 4250 BP). 6 As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation.

The mid-Holocene (ca. 8000–4000 cal yr BP) was a time of marked aridity throughout much of Minnesota, and the changes due to mid-Holocene aridity are seen as an analog for future responses to global warming. In this study, we compare the transition into (ca. 9000–7000 yr ago) and out of (ca. 5000–2500 yr ago) the mid-Holocene (MH) period at Kimble Pond and Sharkey Lake, located along the prairie forest ecotone in south-central Minnesota, using high resolution (∼ 5–36 yr) sampling of pollen, charcoal, sediment magnetic and loss-on-ignition properties. Changes in vegetation were asymmetrical with increasing aridity being marked by a pronounced shift from woodland/forest-dominated landscape to a more open mix of grassland and woodland/savanna. In contrast, at the end of the MH, grassland remained an important component of the landscape despite increasing effective moisture, and high charcoal influxes (median 2.7–4.0 vs. 0.6–1.7 mm2 cm− 2 yr− 1 at start of MH) suggest the role of fire in limiting woodland expansion. Asymmetric vegetation responses, variation among and within proxies, and the near-absence of fire today suggest caution in using changes associated with mid-Holocene aridity at the prairie forest boundary as an analog for future responses to global warming.

The Big Woods region of Minnesota is on the prairie-forest border and is a much studied model for the interaction of climate, fire and vegetation. The purpose of this study was (a) to document the extent and timing of changes in vegetation and fire over the past 2000 years and (b) to examine the link between charcoal influx and vegetation during the eighteenth and early nineteenth centuries. To reconstruct changes in vegetation, fire and climate, sieve charcoal (120 gm), pollen, environmental magnetism and loss-on-ignition were used. Charcoal deposition at prairie lakes was 3-4 times greater than for woodland or forest lakes, consistent with current understanding of the fire dependence of prairies, but charcoal influxes to forests were considerably higher than reported elsewhere, suggesting that ground fires were more common than assumed. Regionally, charcoal deposition at a subset of 17 lakes began to change at cal. AD 1100, dating to the period of the 'Mediaeval Climate Anomaly' (MCA; cal. AD 900-1250) but A preceding the 'Little Ice Age' (LIA; cal. AD 1250). For seven of 14 lakes the MCA was characterized HOLOCENE by a significant increase in charcoal deposition. This was followed by a decrease with the onset of the RESEARCH LIA, which for most sites continued into the nineteenth century. Relative timing of changes in charcoal PAPER and pollen and the other proxies differed from site to site, suggesting no one single response to climatic change. This variation most likely derives from local differences in fuels and topography, as compounded by strong positive feedbacks between fuels and fire.

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Abstract. Many ecosystem processes that influence Earth system feedbacks -vegetation growth, water and nutrient cycling, disturbance regimes -are strongly influenced by multidecadal-to millennial-scale climate variations that cannot be directly observed. Paleoclimate records provide information about these variations, forming the basis of our understanding and modeling of them. Fossil pollen records are abundant in the NE US, but cannot simultaneously provide information about paleoclimate and past vegetation in a modeling context because this leads to circular logic. If pollen data are used to constrain past vegetation changes, then the remaining paleoclimate archives in the northeastern US (NE US) are quite limited. Nonetheless, a growing number of diverse reconstructions have been developed but have not yet been examined together. Here we conduct a systematic review, assessment, and comparison of paleotemperature and paleohydrological proxies from the NE US for the last 3000 years. Regional temperature reconstructions (primarily summer) show a long-term cooling trend (1000 BCE-1700 CE) consistent with hemispheric-scale reconstructions, while hydroclimate data show gradually wetter conditions through the present day. Multiple proxies suggest that a prolonged, widespread drought occurred between 550 and 750 CE. Dry conditions are also evident during the Medieval Climate Anomaly, which was warmer and drier than the Little Ice Age and drier than today. There is some evidence for an acceleration of the longer-term wetting trend in the NE US during the past century; coupled with an abrupt shift from decreasing to increasing temperatures in the past century, these changes could have wide-ranging implications for species distributions, ecosystem dynamics, and extreme weather events. More work is needed to gather paleoclimate data in the NE US to make inter-proxy comparisons and to improve estimates of uncertainty in reconstructions.