Christy E. Briles

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 perspective on wildfires in the western USA

Understanding the causes and consequences of wildfires in forests of the western United States requires integrated information about fire, climate changes, and human activity on multiple temporal scales. We use sedimentary charcoal accumulation rates to construct long-term variations in fire during the past 3,000 y in the American West and compare this record to independent fire-history data from historical records and fire scars. There has been a slight decline in burning over the past 3,000 y, with the lowest levels attained during the 20th century and during the Little Ice Age (LIA, ca. 1400–1700 CE [Common Era]). Prominent peaks in forest fires occurred during the Medieval Climate Anomaly (ca. 950–1250 CE) and during the 1800s. Analysis of climate reconstructions beginning from 500 CE and population data show that temperature and drought predict changes in biomass burning up to the late 1800s CE. Since the late 1800s , human activities and the ecological effects of recent high fire activity caused a large, abrupt decline in burning similar to the LIA fire decline. Consequently, there is now a forest “fire deficit” in the western United States attributable to the combined effects of human activities, ecological, and climate changes. Large fires in the late 20th and 21st century fires have begun to address the fire deficit, but it is continuing to grow.

Paleoecological Perspectives on Fire Ecology: Revisiting the Fire-Regime Concept~!2009-09-02~!2009-11-09~!2010-03-05~!

Fire is well recognized as a key Earth system process, but its causes and influences vary greatly across spatial and temporal scales. The controls of fire are often portrayed as a set of superimposed triangles, with processes ranging from oxygen to weather to climate, combustion to fuel to vegetation, and local to landscape to regional drivers over broadening spatial and lengthening temporal scale. Most ecological studies and fire management plans consider the effects of fire-weather and fuels on local to sub-regional scales and time frames of years to decades. Fire reconstructions developed from high-resolution tree-ring records and lake-sediment data that span centuries to millennia offer unique insights about fires role that cannot otherwise be obtained. Such records help disclose the historical range of variability in fire activity over the duration of a vegetation type; the role of large-scale changes of climate, such as seasonal changes in summer insolation; the consequences of major reorganizations in vegetation; and the influence of prehistoric human activity in different ecological settings. This paleoecological perspective suggests that fire-regime definitions, which focus on the characteristic frequency, size and intensity of fire and particular fuel types, should be reconceptualized to better include the controls of fire regimes over the duration of a particular biome. We suggest that approaches currently used to analyze fire regimes across multiple spatial scales should be employed to examine fire occurrence across multiple temporal scales. Such cross-scale patterns would better reveal the full variability of particular fire regimes and their controls, and provide relevant information for the types of fire regimes likely to occur in the future with projected climate and land-use change.

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.

Biogeochemical impacts of wildfires over four millennia in a Rocky Mountain subalpine watershed

Wildfires can significantly alter forest carbon (C) storage and nitrogen (N) availability, but the long-term biogeochemical legacy of wildfires is poorly understood. We obtained a lake-sediment record of fire and biogeochemistry from a subalpine forest in Colorado, USA, to examine the nature, magnitude, and duration of decadal-scale, fire-induced ecosystem change over the past c. 4250 yr. The high-resolution record contained 34 fires, including 13 high-severity events within the watershed. High-severity fires were followed by increased sedimentary N stable isotope ratios (δ15N) and bulk density, and decreased C and N concentrations--reflecting forest floor destruction, terrestrial C and N losses, and erosion. Sustained low sediment C : N c. 20-50 yr post-fire indicates reduced terrestrial organic matter subsidies to the lake. Low sedimentary δ15N c. 50-70 yr post-fire, coincident with C and N recovery, suggests diminishing terrestrial N availability during stand development. The magnitude of post-fire changes generally scaled directly with inferred fire severity. Our results support modern studies of forest successional C and N accumulation and indicate pronounced, long-lasting biogeochemical impacts of wildfires in subalpine forests. However, even repeated high-severity fires over millennia probably did not deplete C or N stocks, because centuries between high-severity fires allowed for sufficient biomass recovery.

Fire‐regime complacency and sensitivity to centennial‐through millennial‐scale climate change in Rocky Mountain subalpine forests, Colorado, USA

Summary 1. Key uncertainties in anticipating future fire regimes are their sensitivity to climate change, and the degree to which climate will impact fire regimes directly, through increasing the probability of fire, versus indirectly, through changes in vegetation and landscape flammability. 2. We studied the sensitivity of subalpine forest fire regimes (i.e. fire frequency, fire severity) to previously documented climate variability over the past 6000 years, utilizing pollen and macroscopic charcoal from high-resolution lake-sediment records in Rocky Mountain National Park, Colorado. We combined data from the four lakes to provide composite records of vegetation and fire history within a 200 km 2 study area. 3. Rates of forest burning were relatively complacent to millennial-scale summer cooling and decreased effective moisture. Mean return intervals between fire episodes, defined over 500-year periods, generally varied between 150 and 250 years, consistent with tree-ring-based estimates spanning recent centuries. Variability around these long-term means, however, was significantly correlated with variability in summer moisture (i.e. more burning with drier summers), inferred from existing lake-level and supporting palaeoenvironmental records. 4. The most pronounced change in fire regimes was in response to decreased subalpine forest density ca. 2400 cal. year BP, itself a response to regional cooling. This indirect impact of climate was followed by a decrease in charcoal production per fire, a proxy for crown-fire severity, while the long-term rate of burning remained unchanged. Over the last 1500 years, increased summer evaporation and drought frequency were associated with increased fire severity, highlighting a direct link between fire and climate. 5. Synthesis. Subalpine forest fire history reveals complacency and sensitivity of fire regimes to changing vegetation and hydroclimate over the past 6000 years. Complacency is highlighted by non-varying fire frequency over millennia. Sensitivity is evident through changes in biomass burned per fire (and inferred fire severity), in response to climate-induced changes in forest density and, more recently, increased summer drought. Overall, the palaeo record suggests that (i) fire severity may be more responsive to climate change than fire frequency in Rocky Mountain subalpine forests and (ii) the indirect impacts of climate on vegetation and fuels are important mechanisms determining fire-regime response to climate change.

Holocene forest development and maintenance on different substrates in the Klamath Mountains, northern California, USA

The influence of substrate on long-term vegetation dynamics has received little attention, and yet nutrient-limited ecosystems have some of the highest levels of endemism in the world. The diverse geology of the Klamath Mountains of northern California (USA) allows examination of the long-term influence of edaphic constraints in subalpine forests through a comparison of vegetation histories between nutrient-limited ultramafic substrates and terrain that is more fertile. Pollen and charcoal records spanning up to 15000 years from ultramafic settings reveal a distinctly different vegetation history compared to other soil types. In non-ultramafic settings, the dominant trees and shrubs shifted in elevation in response to Holocene climate variations resulting in compositional and structural changes, whereas on ultramafic substrates changes were primarily structural, not compositional. Fire activity was similar through most of the Holocene with the exception of declines over the last 4000 years on ultramafic substrates, likely due to the reduction of understory fuels and cooler wetter conditions than in the middle Holocene. These results suggest that the tree and shrub distributions were more responsive to past climate changes on non-ultramafic substrates compared to those on ultramafic substrates. The combination of these dynamics may help explain high levels of plant diversity in the Klamath Mountains and provide insights for managing these complex ecosystems.

The fate of sediment, wood and organic carbon eroded during an extreme flood, Colorado Front Range, USA

Identifying and quantifying the dominant processes of erosion and tracking the fate of sediment, wood, and carbon eroded during floods is important for understanding channel response to floods, downstream sediment and carbon loading, and the influence of extreme events on landscapes and the terrestrial carbon cycle. We quantify sediment, wood, and organic carbon (OC) from source to local sink following an extreme flood in the tectonically quiescent, semi-arid Colorado (USA) Front Range. Erosion of >500,000 m3 or as much as ~115 yr of weathering products occurred through landsliding and channel erosion during September 2013 flooding. More than half of the eroded sediment was deposited at the inlet and delta of a water supply reservoir, resulting in the equivalent of 100 yr of reservoir sedimentation and 2% loss in water storage capacity. The flood discharged 28 Mg C/km2, producing an event OC flux equivalent to humid, tectonically active areas. Post-flood remobilization resulted in a further ~100 yr of reservoir sedimentation plus export of an additional 1.3 Mg C/km2 of wood, demonstrating the ongoing impact of the flood on reservoir capacity and carbon cycling. Pronounced channel widening during the flood created accommodation space for 40% of flood sediment and storage of wood and eroded carbon. We conclude that confined channels, normally dismissed as transport reaches, can store and export substantial amounts of flood constituents.

Controls on Mountain Plant Diversity in Northern California: A 14,000-Year Overview

A network of eight Holocene paleoenvironmental records from lakes in the Klamath Mountains of Northern California provides insights on how diverse coniferous forests are maintained in the face of climate change. Pollen data suggest that in most cases plants kept pace with climate change. The steep costal-to-inland precipitation gradient resulted in asynchronous responses to climate change with coastal forests responding before inland sites. This was likely due to the proximity to oceans, warm valleys, and the differential responses to changes in ocean upwelling. Plants growing on soils with heavy metals showed little response to Holocene climate variability, suggesting that they experienced stability during the Holocene, which helps explain the localized plant diversity on the harsh soils. Plant communities on soils without heavy metals adjusted their ranges along elevational gradients in response to climate change, however. Fires were a common occurrence at all sites and tracked climate; however, sites that were more coastal experienced fewer fires than inland sites. Fire severity remained similar through the Holocene at individual sites; however, it was low to moderate at southern locations and higher at more northern locations. The article highlights historical factors that help explain the diversity of plant species in the forests of Northern California and provides insights for managing these complex ecosystems.