1 Fire as a natural disturbance

Fire is a natural agent of disturbance. The origin of fire is tied to the origin of plants, which are responsible for two of the three elements essential for fire to exist: oxygen and fuel. The third element – a heat source – has probably been available throughout the history of the planet. Before the appearance of photosynthetic organisms, the atmosphere lacked sufficient oxygen to support burning and before the existence of terrestrial plants, it lacked fuel. Evidence of low-temperature surface fires dates back 440 million years and fire appears to have been continuous since plants invaded the land.

The first evidence that fire actually altered, and had major impacts on ecosystem functioning dates back to the late Tertiary, about 10 million years ago. The spread of C₄ grasses during this period was due to increased fire activity which opened up woodlands and created environments more favourable to C₄ grasslands. The high flammability of such grasslands produced a feedback process that further increased fire activity.

Hominids have used lightning-ignited fire for perhaps as long as 1.5 million years, but first began to ignite fires between 200 000 and 400 000 years ago. Over recent millennia the occurrence of fire has been increasingly influenced by human activity and is currently a major cause of global atmospheric pollution and contributes significantly to the rise in greenhouse gases: CO₂ (carbon dioxide), CH₄ (methane) and N₂O (nitrous oxide). Fire and fire management are becoming increasingly important issues for ecosystem management.