Kendrick J. Brown

Wildfire responses to abrupt climate change in North America

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.

Forest fire and climate change in western North America: insights from sediment charcoal records

Millennial-scale records of forest fire provide important baseline information for ecosystem management, especially in regions with too few recent fires to describe the historical range of variability. Charcoal records from lake sediments and soil profiles are well suited for reconstructing the incidence of past fire and its relationship to changing climate and vegetation. We highlight several records from western North America and their relevance in reconstructing historical forest dynamics, fire-climate relationships, and feedbacks between vegetation and fire under climate change. Climatic effects on fire regimes are evident in many regions, but comparisons of paleo-fire records sometimes show a lack of synchrony, indicating that local factors substantially affect fire occurrence, even over long periods. Furthermore, the specific impacts of vegetation change on fire regimes differ among regions with different vegetation histories. By documenting the effects on fire patterns of major changes in climate and vegetation, paleo-fire records can be used to test the mechanistic models required for the prediction of future variations in fire.

Ancient Fires on Southern Vancouver Island, British Columbia, Canada: A Change in Causal Mechanisms at about 2,000 ybp

Abstract Charcoal records were examined from seven sediment cores and two stratigraphic sections on southern Vancouver Island, Canada. Charcoal influx and climate trend regressions were established using high order polynomial functions. During the late-glacial (ca. 13,000–10,000 ybp), variations in the charcoal record suggest that fires likely responded to changes in fuel availability and climate. The high incidence of early-Holocene (ca. 10,000–7,000ybp) fires may have been partly modified by human activity, though it seems more likely that climate exerted the greatest control. A decrease in fires during the mid- and early late-Holocene from 7,000–4,000 and 4,000–2,000 ybp respectively is consistent with a regional moistening trend, implying that fires were climatically limited. In the late late-Holocene from 2,000 ybp–present, several sites record an increase in charcoal influx even though climate was continuing to moisten and cool, suggesting that non-climatic factors were responsible for the observed increase in fire activity. Estimates of native populations range up to thousands of people for southern Vancouver Island before the arrival of Europeans. These people were knowledgeable of fire, suggesting that humans were responsible for the increase in fires during the late late-Holocene cool, moist interval.

A spatio-temporal reconstruction of Holocene temperature change in southern Scandinavia

Holocene mean July (TJul) and mean January (TJan) temperatures were reconstructed at Lake Trehörningen in southwest Sweden using pollen–climate calibration functions and converted into anomalies relative to modern temperatures. The anomalies were applied to 28 meteorological stations in Denmark, with each station adjusted to account for spatial variability in both TJul and TJan. The resulting reconstructions were merged to create composite TJul and TJan records. The individual reconstructions were also used to establish the parameterization of the TJul and TJan values, resulting in Holocene temperature maps for all of Denmark. Low TJul values characterised southern Sweden and Denmark at the start of the Holocene, followed by an increase during the early Holocene. Likewise, TJan was initially low at the start of the Holocene, followed by an increase between 10 500 and 9900 cal. BP. Thereafter, TJan was relatively stable between 9900 and 8000 cal. BP. The general increase in temperature during the early Holocene was accompanied by a decrease in the amplitude of the mean monthly temperature record, indicating that a maritime climate was established at this time. Maximum TJul and TJan are recorded during the mid-Holocene interval from 8000 to 4500 cal. BP, with consistently high July temperatures (>18°C) between 6700 and 5400 cal. BP. During the late Holocene, both TJul and TJan gradually decreased. The Holocene temperature maps of Denmark reveal spatial patterns in both summer and winter temperature. In the summer, an east–west decreasing temperature gradient, related to a maritime effect, has persisted in Denmark throughout the Holocene. In contrast, the temperature in winter has varied spatially along subtle coastal–inland and topographical gradients. Various mechanisms have been invoked to explain the Holocene trends in temperature, including the melting of the Scandinavian Ice Sheet, changes in insolation, and changes in the dominant type of atmospheric circulation affecting northern Europe.

Postglacial evolution and spatial differentiation of seasonal temperate rainforest in western Canada

Surface samples from Vancouver Island, Canada, were used to assess the relationship between discrete seasonal temperate rainforest (STR) plant communities and their corresponding pollen signatures. Pollen from ten sediment cores was further used to evaluate the postglacial development of these communities. Principal components analysis (PCA) of the surface data revealed the distinctiveness of the modern pollen rain, with samples from the Coastal Douglas Fir (CDF) zone, the dry Coastal Western Hemlock (CWH) zone, the wet CWH subzones and the Mountain Hemlock (MH) zone clustering distinctly. PCA of the fossil data revealed early-seral open canopy, wet rainforest, subalpine rainforest and Lateglacial plant associations and showed that the STR has changed markedly through time. Pinus woodlands with low palynological richness prevailed in the early Lateglacial period, only to be supplanted by mixed conifer forest with increased pollen richness. In the early Holocene, STR vegetation differentiated spatially as early-seral open canopy forests expanded, though a non-analogue Picea-dominated forest persisted on the moist outer coast. Generally high pollen richness is attributed to the expansion of dryland habitat coupled with the development of a fire-maintained vegetation mosaic. In the mid- and late-Holocene intervals, open canopy communities persisted in eastern areas, eventually developing into modern CDF and dry CWH forest. In contrast, moist and oceanic CWH rainforest developed on central and western Vancouver Island, whereas subalpine forest established at high elevation. Pollen richness declined in the mid Holocene concomitant with increased precipitation and a general reduction in the incidence of fire, though this trend was offset somewhat in the late Holocene by paludification.

Quantifying holocene climate change in Denmark

Starting approximately 6,000 years ago, the introduction of agriculture in Denmark had a marked effect on the landscape as the natural virgin forest that characterised the country was cleared and replaced by farmland. Since then, continued human action has further modified the natural state of the Danish landscape and ecosystems, culminating in a culturally-modified landscape. One result of this historical human activity was a decoupling between landscape and climate in Danmark. Consequently, while it remains possible to use fossil records to reconstruct how the Danish landscape and ecosystems have changed through time, it is extremely difficult to reconstruct how the climate has changed due to the human factor. In response to this limitation, there are no long-term quantitative climate reconstructions available for Denmark. Here, an interdisciplinary approach that combines analysis of modern climate data with the development of climate transfer functions and geospatial analysis is used to quantitatively reconstruct how climate has varied through time in Denmark. The particular climate variables to be examined include mean annual temperature, July temperature and mean annual precipitation. In addition to producing the first long-term quantitative reconstructions of climate in Denmark, geospatial analysis is also used to generate the first Holocene climate maps for Denmark. Climate Change: Global Risks, Challenges and Decisions IOP Publishing IOP Conf. Series: Earth and Environmental Science 6 (2009) 072015 doi:10.1088/1755-1307/6/7/072015