Christopher I. Roos

Fire in the Earth system.

Burn, Baby, Burn Wildfires can have dramatic and devastating effects on landscapes and human structures and are important agents in environmental transformation. Their impacts on nonanthropocentric aspects of the environment, such as ecosystems, biodiversity, carbon reserves, and climate, are often overlooked. Bowman et al. (p. 481) review what is known and what is needed to develop a holistic understanding of the role of fire in the Earth system, particularly in view of the pervasive impact of fires and the likelihood that they will become increasingly difficult to control as climate changes. Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.

The human dimension of fire regimes on Earth

Humans and their ancestors are unique in being a fire-making species, but ‘natural’ (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from ‘natural’ background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research.

Multiscale perspectives of fire, climate and humans in Western North America and the Jemez Mountains, USA

Interannual climate variations have been important drivers of wildfire occurrence in ponderosa pine forests across western North America for at least 400 years, but at finer scales of mountain ranges and landscapes human land uses sometimes over-rode climate influences. We reconstruct and analyse effects of high human population densities in forests of the Jemez Mountains, New Mexico from ca 1300 CE to Present. Prior to the 1680 Pueblo Revolt, human land uses reduced the occurrence of widespread fires while simultaneously adding more ignitions resulting in many small-extent fires. During the 18th and 19th centuries, wet/dry oscillations and their effects on fuels dynamics controlled widespread fire occurrence. In the late 19th century, intensive livestock grazing disrupted fuels continuity and fire spread and then active fire suppression maintained the absence of widespread surface fires during most of the 20th century. The abundance and continuity of fuels is the most important controlling variable in fire regimes of these semi-arid forests. Reduction of widespread fires owing to reduction of fuel continuity emerges as a hallmark of extensive human impacts on past forests and fire regimes. This article is part of the themed issue ‘The interaction of fire and mankind’.

Native American depopulation, reforestation, and fire regimes in the Southwest United States, 1492-1900 CE.

Significance Debates about the magnitude, tempo, and ecological effects of Native American depopulation after 1492 CE constitute some of the most contentious issues in American Indian history. Was population decline rapid and catastrophic, with effects extensive enough to change even the earth’s atmosphere? Or was depopulation more moderate, with indigenous numbers declining slowly after European colonization? Through a study of archaeology and dendrochronology, we conclude that neither of these scenarios accurately characterizes Pueblo peoples in the Southwest United States. Among the Jemez pueblos of New Mexico, depopulation struck swiftly and irrevocably, but occurred nearly a century after first contact with Europeans. This population crash subsequently altered the local environment, spurring the growth of trees and facilitating the spread of frequent forest fires. Native American populations declined between 1492 and 1900 CE, instigated by the European colonization of the Americas. However, the magnitude, tempo, and ecological effects of this depopulation remain the source of enduring debates. Recently, scholars have linked indigenous demographic decline, Neotropical reforestation, and shifting fire regimes to global changes in climate, atmosphere, and the Early Anthropocene hypothesis. In light of these studies, we assess these processes in conifer-dominated forests of the Southwest United States. We compare light detection and ranging data, archaeology, dendrochronology, and historical records from the Jemez Province of New Mexico to quantify population losses, establish dates of depopulation events, and determine the extent and timing of forest regrowth and fire regimes between 1492 and 1900. We present a new formula for the estimation of Pueblo population based on architectural remains and apply this formula to 18 archaeological sites in the Jemez Province. A dendrochronological study of remnant wood establishes dates of terminal occupation at these sites. By combining our results with historical records, we report a model of pre- and post-Columbian population dynamics in the Jemez Province. Our results indicate that the indigenous population of the Jemez Province declined by 87% following European colonization but that this reduction occurred nearly a century after initial contact. Depopulation also triggered an increase in the frequency of extensive surface fires between 1640 and 1900. Ultimately, this study illustrates the quality of integrated archaeological and paleoecological data needed to assess the links between Native American population decline and ecological change after European contact.

A 1416-year reconstruction of annual, multidecadal, and centennial variability in area burned for ponderosa pine forests of the southern Colorado Plateau region, Southwest USA

Fire history reconstructions from fire scars in tree rings have been valuable for assessing fire regime changes and their climatic controls. It has been asserted, however, that these two- to four-century long records from the western USA are unrepresentative of longer periods of the Holocene and are of limited use for understanding current or future fire regimes. The Medieval Climate Anomaly (800–1300 ce) is often suggested as a better analog for future Southwestern US climates but is beyond the chronological range of most fire-scar studies in this region. To evaluate fire regime changes over the past millennium, we build on centennial-length fire–climate studies to generate a 1416 year long reconstruction of fire activity in ponderosa pine forests of the Southern Colorado Plateau region of Arizona and New Mexico. We used a split-period calibration and verification protocol to test the reliability of a multiple regression model using annual and antecedent precipitation (reconstructed from tree-ring width chronologies) to predict the percentage of fire-scar localities (i.e. sites, N=45) that recorded extensive fires within those sites (>25% of recorder trees scarred) each year between 1700 and 1899 ce. The model explains approximately 50% of the variation in annual fire activity. Applying the model to the entire precipitation reconstruction provides a proxy for annual area burned since 572 ce. There are no statistically significant differences between the period available for fire-scar study (1600 ce–present) and the Medieval Climate Anomaly (800–1300 ce) in terms of predicted annual area burned or the frequencies of regional fire years. Multidecadal and centennial variation in the frequencies of regional fire years, however, does indicate reduced surface fire frequencies from approximately 700–800 ce and 1360–1455ce. We hypothesize that these were periods when some forests were vulnerable to altered canopy structure, accumulated fuels, and increased fire severity.

Living on a flammable planet: interdisciplinary, cross-scalar and varied cultural lessons, prospects and challenges.

Living with fire is a challenge for human communities because they are influenced by socio-economic, political, ecological and climatic processes at various spatial and temporal scales. Over the course of 2 days, the authors discussed how communities could live with fire challenges at local, national and transnational scales. Exploiting our diverse, international and interdisciplinary expertise, we outline generalizable properties of fire-adaptive communities in varied settings where cultural knowledge of fire is rich and diverse. At the national scale, we discussed policy and management challenges for countries that have diminishing fire knowledge, but for whom global climate change will bring new fire problems. Finally, we assessed major fire challenges that transcend national political boundaries, including the health burden of smoke plumes and the climate consequences of wildfires. It is clear that to best address the broad range of fire problems, a holistic wildfire scholarship must develop common agreement in working terms and build across disciplines. We must also communicate our understanding of fire and its importance to the media, politicians and the general public. This article is part of the themed issue ‘The interaction of fire and mankind’.

The interaction of fire and mankind

This theme issue is a result of the Royal Society scientific discussion meeting organized by Andrew C. Scott, William G. Chaloner FRS, Claire M. Belcher and Christopher I. Roos at the Royal Society, London, 14–15 September 2015. Here, the complex interrelationships between fire and mankind that

The interaction of fire and mankind: Introduction

Fire has been an important part of the Earth system for over 350 Myr. Humans evolved in this fiery world and are the only animals to have used and controlled fire. The interaction of mankind with fire is a complex one, with both positive and negative aspects. Humans have long used fire for heating, cooking, landscape management and agriculture, as well as for pyrotechnologies and in industrial processes over more recent centuries. Many landscapes need fire but population expansion into wildland areas creates a tension between different interest groups. Extinguishing wildfires may not always be the correct solution. A combination of factors, including the problem of invasive plants, landscape change, climate change, population growth, human health, economic, social and cultural attitudes that may be transnational make a re-evaluation of fire and mankind necessary. The Royal Society meeting on Fire and mankind was held to address these issues and the results of these deliberations are published in this volume. This article is part of the themed issue ‘The interaction of fire and mankind’.

Global combustion: the connection between fossil fuel and biomass burning emissions (1997-2010)

Humans use combustion for heating and cooking, managing lands, and, more recently, for fuelling the industrial economy. As a shift to fossil-fuel-based energy occurs, we expect that anthropogenic biomass burning in open landscapes will decline as it becomes less fundamental to energy acquisition and livelihoods. Using global data on both fossil fuel and biomass burning emissions, we tested this relationship over a 14 year period (1997–2010). The global average annual carbon emissions from biomass burning during this time were 2.2 Pg C per year (±0.3 s.d.), approximately one-third of fossil fuel emissions over the same period (7.3 Pg C, ±0.8 s.d.). There was a significant inverse relationship between average annual fossil fuel and biomass burning emissions. Fossil fuel emissions explained 8% of the variation in biomass burning emissions at a global scale, but this varied substantially by land cover. For example, fossil fuel burning explained 31% of the variation in biomass burning in woody savannas, but was a non-significant predictor for evergreen needleleaf forests. In the land covers most dominated by human use, croplands and urban areas, fossil fuel emissions were more than 30- and 500-fold greater than biomass burning emissions. This relationship suggests that combustion practices may be shifting from open landscape burning to contained combustion for industrial purposes, and highlights the need to take into account how humans appropriate combustion in global modelling of contemporary fire. Industrialized combustion is not only an important driver of atmospheric change, but also an important driver of landscape change through companion declines in human-started fires. This article is part of the themed issue ‘The interaction of fire and mankind’.

Anthropogenic Burning, Agricultural Intensification, and Landscape Transformation in Post-Lapita Fiji

Slash-and-burn cultivation (swidden) is an important and extensive strategy among agriculturalists in Oceania. The length of the fallow period, in which non-cultivated vegetation is allowed to regrow, is critical to the sustainability of this strategy in tropical environments. Long fallow periods permit greater soil recovery and higher yields over the long term whereas shorter fallow periods drive cycles of soil degradation that ultimately result in a landscape that is too degraded for continued cultivation. Anthropologists recognize that decreasing swidden fallow times is a key form of agricultural intensification that may have shaped interpolity conflict and social complexity. Although it is easy to identify the degraded landscapes that are a legacy of this pattern today, it has been a challenge for archaeologists to identify the timing and rate at which such processes took place in the past. We use alluvial stratigraphic records of charcoal and stable carbon isotopes from a small drainage in Western Viti Levu, Fiji, to reconstruct the timing and rate of intensification of swidden agriculture from long-fallow clearing of native forest, to shorter fallow burning of secondary forest and grassland, to grassland conversion. Results suggest that swidden cultivation in the lower Sigatoka Valley did not commence until centuries after Lapita colonization (ca. 2950 cal BP). Early swiddening apparently used relatively short fallow periods coupled with residential mobility to sustain horticultural yields until mobility no longer became a viable option. Archaeological indicators of resource stress co-occur with persistent swiddening after 1450 cal BP, although these precede the collapse into degraded grassland conditions at 1000 cal BP. Archaeological evidence for conflict increase after landscape degradation, although emerging social inequalities only appear after centuries of degraded conditions and conflict.