Ecologists, biogeographers, and palaeobotanists have long viewed the distribution of ecosystems as controlled by climate and soils, with limited appreciation for the role of fire. In the July/August issue of BioScience, Dr. Juli Pausas from the Universitat d’Alacant, Spain, and USGS scientist Dr. Jon Keeley review the evidence from different disciplines demonstrating that wildfire appeared early in the origin of terrestrial plants, has played an important role throughout the history of life, and today poses major challenges to humans’ ability to adapt to this natural ecosystem process.

The authors present an historical perspective, from the origin of land plants to the present, that explores the multitude of roles fire has played on earth, and on how humans have affected those roles. They hypothesize that 1) the world cannot be understood without considering fire because it has had strong ecological and evolutionary consequences on biota, including human evolution; and in turn, 2) since the rise of humans, fire regimes have been heavily controlled by people, often in ways that greatly impact the sustainability of some ecosystems.

By the beginning of the Paleozoic Era (540 Ma), Earth’s oxygenated atmosphere was sufficient to carry fire, but the lack of terrestrial plant fuels limited the possibility of fire. However, with the earliest Silurian origins of land plants there is evidence of fire, and subsequently over geological time the importance of fire has waxed and waned in association with changes in atmospheric oxygen levels, a critical component of the fire process (Fig. 1a). The peak of ca. 31% atmospheric oxygen during the Carboniferous (compared with the current 21%) greatly facilitated combustion under a far greater range of moisture conditions.

Several lessons are evident from the geological history of fire on Earth. First, some of the same fire regimes evident today were present in early land plant communities. For example, understory surface fire regimes similar to those in many Northern Hemisphere coniferous forests are indicated by Devonian pre-gymnosperm forests, and infrequent crown fires evident in New World boreal forests were present in the Carboniferous (345 Ma) lepidodendron forests.

This analysis leads to a better understanding of the necessary ecological conditions for wildland fires, as they involve four specific parameters (Fig. 1b). In addition to biomass fuels to spread a fire there must be a dry season that converts potential fuels to available fuels, and the geological record shows such fire climates under both summer drought and winter drought climates as is the case today.

Humans initiated a new stage in ecosystem fire, utilizing it to remake the Earth in ways more suited to their lifestyle. However, as human populations have expanded their use of fire, their actions have come to dominate some ecosystems and change natural processes in ways that threaten the sustainability of some landscapes. Humans have had profound impacts on fire regimes, both by suppressing fires as well as greatly increasing fire frequency, both of which have contributed to destructive impacts on ecosystems. Although humans have adapted fire for their own use, there is a profound need for humans to become fire adapted, creating a better balance between land management needs and the sustainability of natural ecosystems.

Figure 1. Components of the fire process (a), and ecological conditions for wildland fires. Credit: Pausas and Keeley, 2009.

Management Implications

• Many plant adaptations to fire appear to be very ancient, occurring long before the evolution of contemporary floras. However, no species is “fire adapted”; rather species are adapted to a particular fire regime, and alterations from that regime can have devastating impacts on sustainability of many ecosystem components.
• The diversity of fire regimes present today are likely ancient as well and today require a regional approach to fire management.
• Climate change predictions of future fire regimes have focused entirely on temperature effects on fuel conditions and need to broaden the perspective to include interactions with type conversion and increased rates of biomass decomposition, as well as a host of other ecosystem processes.
• Key to human adaptation to fire is a balance between reduction of fire hazard threats to communities and sustainability of natural ecosystems. Managers need to recognize that while these goals may be compatible in some forest types, they are often in conflict in other ecosystems.

Pausas, J.G. and J.E. Keeley. 2009. A burning story: The role of fire in the history of life. BioScience 59:593–601.

[Complete article can be downloaded from web site listed above.]

Download this publication brief in pdf format