Andrea Brunelle

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.

Long-term relations among fire, fuel, and climate in the north-western US based on lake-sediment studies

Pollen and high-resolution charcoal records from the north-western USA provide an opportunity to examine the linkages among fire, climate, and fuels on multiple temporal and spatial scales. The data suggest that general charcoal levels were low in the late-glacial period and increased steadily through the last 11 000 years with increasing fuel biomass. At local scales, fire occurrence is governed by the interaction of site controls, including vegetation, local climate and fire weather, and topography. At subregional scales, patterns in the long term fire-episode frequency data are apparent: The Coast Range had relatively few fires in the Holocene, whereas the Klamath–Siskiyou region experienced frequent fire episodes. Fire regimes in the northern Rocky Mountains have been strongly governed by millennial- and centennial-scale climate variability and regional differences in summer moisture. At regional scales, sites in present-day summer-dry areas show a period of protracted high fire activity within the early Holocene that is attributed to intensified summer drought in the summer-dry region. Sites in summer-wet areas show the opposite pattern, that fire was lower in frequency than present in the early Holocene as result of strengthened monsoonal circulation then. Higher fire-episode frequency at many sites in the last 2000 years is attributed to greater drought during the Medieval Climate Anomaly and possibly anthropogenic burning. The association between drought, increased fire occurrence, and available fuels evident on several time scales suggests that long-term fire history patterns should be considered in current assessments of historical fire regimes and fuel conditions.

Pollen accumulation in lake sediments during historic spruce beetle disturbances in subalpine forests of southern Utah, USA

Paleoecological reconstructions using lake sediments provide important information about ecological dynamics and forest disturbance processes that occurred prior to the historic period of scientific observation. In high-altitude and high-latitude ecosystems where landscape-scale disturbances recur at time intervals exceeding observation in many regions, e.g. western North America, reconstructed environmental data are essential in providing context for land managers. During the most recent two decades eruptive populations of bark beetles (Dendroctonus spp.) have rapidly and profoundly altered subalpine forest ecosystems across western North America. Outbreaks of these insects are unprecedented in scale and severity, at least historically. Currently, little information exists about these destructive outbreaks and in general, the information that exists, does not extend beyond the most recent few centuries. The research presented here examines sedimentary pollen records from six subalpine basins affected by severe spruce beetle (D. rufipennis) epidemics during the 20th century in the high-elevation plateaus and mountain ranges of south-central Utah. Reciprocal exchanges in dominance between pollen abundance of host spruce (Picea engelmannii) and non-host subalpine fir (Abies lasiocarpa) associated with historic outbreaks are conspicuous. Calculating simple ratios of host and non-host pollen accumulations offers a useful metric to visually identify spruce beetle outbreaks using sedimentary records. However, supporting lines of evidence may be required to identify these disturbances with greater certainty over the Holocene. Our data and findings provide a platform with which to begin exploration of other paleoecological proxy methods for the ultimate purpose of generating more temporally extensive reconstructions of bark beetle disturbances using sedimentary records.

Pollen Evidence of Historical Forest Disturbance on the Wasatch Plateau, Utah

ABSTRACT. Environmental indicators from lake sediments provide excellent opportunities to improve understanding of forest disturbance processes and corresponding changes in forest composition. Our research provides a methodology for assessing recent, historic, and prehistoric disturbances using lacustrine sediment records, We collected sediment cores from Blue Lake, a small subalpine lake on the Wasatch Plateau in central Utah. These cores record environmental changes caused by both spruce beetle (Dendroctonus rufipennis Kirby) and human (logging and livestock grazing) modification, We observed deteriorated insect remains in the lake sediments, These remains correspond temporally with a historic spruce beetle outbreak, though alkaline conditions in the lake water may have inhibited preservation of bark beetle remains. Pollen data reveal that despite the unprecedented level of mortality among Engelmann spruce (Picea engelmannii Parry ex Engelm.) resulting from the spruce beetle epidemic, logging activities subsequent to Euro-American settlement appear to be the most severe disturbance to the Blue Lake watershed over the last 750 years.

Isotopic analysis of wetland development in the American Southwest.

The analysis of stable isotope and elemental fractions of organic material collected from San Bernardino Ciénega was used to understand the history of vegetation composition and climate change within this desert wetland. A 4000-yr record of sediment buildup, based on four 14C measurements, provides unique opportunities for the study of environmental conditions within an arid landscape and documents climate shifts from drier to wetter conditions in the late Holocene. δ13C, δ15N, and C:N values were measured from a 3.8 m deep sedimentary section to understand the dynamics of vegetation and hydrology in desert wetlands. Through this section we observe δ 13C and C:N values indicating a shift in the dominant source of organic matter within the section: prior to 850 cal. yr BP (below 60 cm), aquatic vascular plants and occasionally terrestrial vegetation were the primary organic sources, whereas freshwater algae were the dominant organic matter source above this level. These values indicate that while conditions remained arid at this locality, the amount of standing water on the ciénega has increased over time. These results document both climate change and vegetation evolution on the ecotone of the Sonoran and Chihuahuan deserts and demonstrate how the study of local sediment accumulation in ciénegas can provide critical information on changing conditions within arid environments.

A multi-proxy record of hydroclimate, vegetation, fire, and post-settlement impacts for a subalpine plateau, central Rocky Mountains, U.S.A

Apparent changes in vegetation distribution, fire, and other disturbance regimes throughout western North America have prompted investigations of the relative importance of human activities and climate change as potential causal mechanisms. Assessing the effects of Euro-American settlement is difficult because climate changes occur on multi-decadal to centennial time scales and require longer time perspectives than historic observations can provide. Here, we report vegetation and environmental changes over the past ~13,000 years as recorded in a sediment record from Bison Lake, a subalpine lake on a high plateau in northwestern Colorado. Results are based on multiple independent proxies, which include pollen, charcoal, and elemental geochemistry, and are compared with previously reported interpretations of hydroclimatic changes from oxygen isotope ratios. The pollen data indicate a slowly changing vegetation sequence from sagebrush steppe during the late glacial to coniferous forest through the late Holocene. The most dramatic vegetation changes of the Holocene occurred during the ‘Medieval Climate Anomaly’ (MCA) and ‘Little Ice Age’ (LIA) with rapid replacement of conifer forest by grassland followed by an equally rapid return to conifer forest. Late Holocene vegetation responses are mirrored by changes in fire, lake biological productivity, and watershed erosion. These combined records indicate that subsequent disturbance related to Euro-American settlement, although perhaps significant, had acted upon a landscape that was already responding to MCA-LIA hydroclimatic change. Results document both rapid and long-term subalpine grassland ecosystem dynamics driven by agents of change that can be anticipated in the future and simulated by ecosystem models.

A 14,000-year record of fire, climate, and vegetation from the Bear River Range, southeast Idaho, USA

The vegetation and fire history of the Bear River Range (BRR), Southeast Idaho has been reconstructed from pollen, plant macrofossils, and macroscopic charcoal from lacustrine sediments. Overall, the BRR record shows independent responses of vegetation and fire regime to climate variation. The reconstructions suggest strong seasonal bias from the proxies evaluated, with the pollen record most sensitive to insolation-driven summer temperature trends, and the charcoal-based fire record more sensitive to winter snowpack variability. Together, the proxies suggest that the early Holocene experienced larger than average snowpacks but very warm summers. Warmer than modern summer temperatures were maintained through much of the mid-Holocene, but snowpacks decreased dramatically, creating the most extreme xeric conditions in the Holocene between ~7100 and 6000 BP. After 6000 BP, summers began to show a consistent cooling trend. Winter precipitation remained low until ~4400 BP, after which higher than average snowpacks are indicated until 2000 BP. Pollen and charcoal data relationships at ~8800 BP and from 1800 to 800 BP suggest periods with anomalously wet summers that created a unique fire regime during those intervals.

Reconstruction of the mid-Holocene paleoclimate of the Ecuadorian Andean páramo at Tres Lagunas, Ecuador

Previous paleoclimate studies have suggested a warm/dry event during the mid-Holocene in páramo vegetation of the tropical Andes of South America. However, the timing of the mid-Holocene warm/dry event in Tres Lagunas, Ecuador, remains uncertain, since a previously reported bog core record characterized the warm/dry event during a hiatus in sediment deposition. In order to understand the timing of the warm/dry event in Tres Lagunas in relation to regional records, a lake sediment core was collected. Subsamples from the core were analyzed for radiocarbon dates, pollen, magnetic susceptibility, and charcoal to reconstruct the depositional, vegetational, and fire history of the area. A near-constant sedimentation rate in the lake core indicated that the lake did not dry, in contrast to the adjacent previously reported bog core. Increases in lower elevation pollen types suggest a warm period between 2700 and 2200 cal. yr BP, with the peak of warming at 2200 cal. yr BP co-occurring with the end of hiatus in the bog core record. Statistically significant increases in charcoal influx and magnetic susceptibility from 3900 to 800 cal. yr BP also suggest a dry climate during this period. While the lake record corroborates the presence of a warm/dry period in Tres Lagunas during the mid-Holocene, this record suggests a shorter period of warm/dry climate where the intensity was not sufficient to cause the lake to dry. However, anthropogenic alteration of the landscape, either vegetation or burning, must also be considered when interpreting Holocene records from this region.

Using Paleoecology to Inform Land Management as Climates Change: An Example from an Oak Savanna Ecosystem

Oak savanna, a transitional ecosystem between open prairie and dense oak forest, was once widespread in Minnesota. Upon European settlement much of the oak savanna was destroyed. Recently, efforts to restore this ecosystem have increased and often include the reintroduction of fire. Though fire is known to serve an important role within oak savannas, there are currently few studies which address fire regimes on timescales longer than the last century. This research presents a paleoecological history of Sherburne National Wildlife Refuge (SNWR) in MN, USA, spanning the last ~8000 years. The objectives of this study were to use charcoal, pollen, and magnetic susceptibility of lake sediments collected from Johnson Slough (JS) within the refuge to evaluate the natural range of variability and disturbance history of the oak savanna within the refuge, assess the success of current restoration strategies, and add to the regional paleoecological history. The mid/late Holocene period of the JS record shows a period of high fire activity from ca. 6500 to 2600 cal year BP, with a shift from prairie to oak savanna occurring over this same period. A (possibly agricultural) disturbance to JS sediments affected the period from ca. 2600 cal year BP to 1963 AD, which includes the time of Euro–American settlement. However, the destruction and subsequent restoration of the oak savanna is evident in a pollen ratio of Quercus:Poaceae, indicating that current restoration efforts have been successful at restoring the oak savanna to within the natural range of variability seen just prior to destruction.