Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire-climate-vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.Canada | drought code | global circulation model | paleoclimate R ecent increases in wildfire frequency and biomass burning in boreal regions in response to ongoing climate warming threaten the carbon sink strength of native ecosystems and, by extension, further contribute to global warming (1). Up-to-date model-based fire predictions indicate that these trends will persist in the coming decades as atmospheric greenhouse gas concentrations will attain unprecedented levels by the end of this century (2, 3). However, model-based fire predictions depend on data collected over short periods-usually less than 100 y-that do not cover a wide range of fire-climate interactions and feedback processes arising from changes in vegetation features. This reduces the robustness of fire predictions, which must therefore be supplemented by paleoecological investigations (4). These investigations often integrate several scientific disciplines, datasets, approaches, and methodologies, thereby providing a robust assessment of how recent trends in fire activity fit into the long-term perspective.Until now, paleofire reconstructions based on charcoal lacustrine deposits have mostly focused on describing past fire activity in terms of frequency and biomass burning (5). Here we address an additional aspect of fire history and fire-climate relationships, namely, the change in fire size over periods of substantial climate change. To do this, we use sedimentary charcoal records extracted from nine kettle lakes located in the eastern North American boreal forest and model simulations of past climate. We introduce a new metric, developed from the combination of the fire frequency and biomass burning components, which allows us to assess the mean biomass burned per fi...

Vegetation, fire and climate history are investigated in the 10 000‐year record of Zarishat fen located today in the steppe grasslands of Armenia (Near East). Pollen‐based climate quantification provides a reconstruction of seasonal parameters. The development of in‐situ water‐dependant plants and of forests at lower altitude at 8200 cal a BP echoes the shift from an arid and cold [annual precipitation (Pann) = 452 mm; mean temperature of the coldest month (MTCO) = −11.1 °C)] Early Holocene to a more humid and warmer (Pann = 721 mm; MTCO = −6.8 °C) Mid–Late Holocene. This marks the onset of lower seasonality, in particular more effective precipitation brought during late spring by the Westerlies. Paralleling the Mediterranean precipitation pattern, precipitation in the Near East and Central Asia decreased during the Mid–Late Holocene in favour of higher seasonality controlled in winter/spring by the Siberian High. Fire history and sedge‐based fen development record drier phases at approximately 6400, 5300–4900, 3000, 2200–1500 and 400 cal a BP, which resemble the precipitation pattern of the South‐Western Mediterranean and contrast with the Holocene pattern in the South‐Central and South‐Eastern Mediterranean regions. Arid phases in Armenia are believed to be related to multi‐centennial‐scale variation of Westerly activity (North Atlantic Oscillation‐like).

The influence of climatic and local nonclimatic factors on the fire regime of the eastern Canadian boreal forest over the last 8000 years is investigated by examining charred particles preserved in four lacustrine deposits. Herein, we compare the distribution of fire-free intervals (FFIs) and the synchronicity of fire events among sites, using Ripley's K-function to determine the extent of the role of local-scale vs. large-scale processes with respect to fire control. Between 8000 and 5800 cal. BP (calibrated years before present) the climatic and ecological conditions were less conducive to fire events than after this date. After 5800 cal. BP, the number of fires per 1000 years (fire frequency) progressively increased, reaching a maximum ca. 3400 cal. BP. There was a sharp decrease in fire frequency during the last 800 years. Between 8000 and 4000 cal. BP, comparable FFIs and synchronous fire episodes were determined for the study sites. During this period, the fire frequency was predominantly controlled by climate. After 4000 cal. BP, two sites displayed independent fire histories (different FFI distributions or asynchronous fire events), underlining the important influence of local factors, including short-term fuel wetness, characteristics of the watershed and landscape connectivity, in determining fire occurrence. We conclude that climatic changes occurred during the last 4000 years that induced a rise in the water table; this may explain the high spatial heterogeneity in fire history. Current and projected global climatic changes may cause similar spatial variability in fire frequency.

Understanding the drivers of the boreal forest fire activity is challenging due to the complexity of the interactions driving fire regimes. We analyzed drivers of forest fire activity in Northern Scandinavia (above 60 N) by combining modern and proxy data over the Holocene. The results suggest that the cold climate in northern Scandinavia was generally characterized by dry conditions favourable to periods of regionally increased fire activity. We propose that the cold conditions over the northern North Atlantic, associated with low SSTs, expansion of sea ice cover, and the southward shift in the position of the subpolar gyre, redirect southward the precipitation over Scandinavia, associated with the westerlies. This dynamics strengthens high pressure systems over Scandinavia and results in increased regional fire activity. Our study reveals a previously undocumented teleconnection between large scale climate and ocean dynamics over the North Atlantic and regional boreal forest fire activity in Northern Scandinavia. Consistency of the pattern observed annually through millennium scales suggests that a strong link between Atlantic SST and fire activity on multiple temporal scales over the entire Holocene is relevant for understanding future fire activity across the European boreal zone.

We investigated changes in wildfire risk over the 1901À2002 (AD) period with an analysis of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested ecoregions of the North American and Eurasian continents. The MDC is an estimate of the net effect of changes in evapotranspiration and precipitation on cumulative moisture depletion in soils, and is well correlated with annual fire statistics across the circumboreal (explaining 25-61% of the variance in regional area burned). We used linear trend and regime shift analyses to investigate (multi-) decadal changes in MDC and percentage area affected by drought, and kernel function for analysis of temporal changes in the occurrence rates of extreme drought years. Our analyses did not reveal widespread patterns of linear increases in dryness through time as a response to rising Northern Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought severity changes that were inherent to the nonuniformly distributed impacts of climate change on dryness. Notably, significant trends toward increasing summer moisture in southeastern and southwestern boreal Canada were detected. The diminishing wildfire risk in these regions is coherent with widely reported decreases in area burned since about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely, we found evidence for increasing percentage area affected by extreme droughts in Eurasia ( 1 0.57% per decade; Po0.05) and occurrence rates of extreme drought years in Eurasian taiga (centered principally on the Okhotsk-Manchurian taiga, P 5 0.07). Although not statistically significant, temporal changes in occurrence rates are sufficiently important spatially to be paid further attention. The absence of a linear trend in MDC severity, in conjunction with the presence of an increase in the occurrence rate of extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could be shifting toward being increasingly pulse dependent.

SummaryStrategic introduction of less flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated.Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales.Lacustrine charcoals from northern landscapes of eastern boreal Canada indicate that RegFF during the mid-Holocene (6000-3000 yr ago) was significantly higher than pre-industrial RegFF (AD c. 1750). In southern landscapes, RegFF was not significantly higher than the preindustrial RegFF in spite of the declining drought severity. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component.Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.

No abstract

To evaluate the influence of long-distance transport of charcoal particles on the detection of local wildfires from lake sediment sequences, we tracked three consecutive years of charcoal deposition into traps set within seven boreal lakes in northeastern Canada. Peaks in macroscopic charcoal accumulation (>150 μm) were linked to both local (inside the watershed) and regional wildfires. However, regional fires were characterized by higher proportions of small particles (<0.1 mm 2 ) in charcoal assemblages. We conclude that the analysis of particle size distribution is useful to discriminate "true" local fires from regional wildfires.