Fire shapes global vegetation
A world that never burned would contain twice as much forest.
An area's temperature and rainfall have always been thought of as key to which plants grow there, to whether it becomes grassland, savanna or forest. But fire may have the biggest influence on the global distribution of vegetation, report researchers from South Africa and Britain.
In a fire-free world, forest cover would double at the expense of grasslands and savannas say William Bond of the University of Cape Town and his colleagues in the journal New Phytologist1. The team used a computer model to predict how plant patterns shift over time with changing climates.
For example, a type of grassland consisting of warmth-loving plants appeared in the tropics between 6 million and 8 million years ago, and quickly spread around the globe. This study suggests that the grassland's spread was primarily influenced by fire, says Bond, although it is not known why there were more fires at that time.
"Fire is more than an unnecessary evil," says team member Ian Woodward, an ecologist at the University of Sheffield, UK. Without fire, forests would leap from forming 26% of the world's vegetation to 56%, reports the team. Tropical grasslands and savannas, such as those in South America and Africa would shrink to half their current extent; temperate grassland and Mediterranean shrubland would be reduced by nearly two-thirds.
"Fire is grossly under-evaluated in terms of its global impact on ecosystems," says Jon Keeley, an ecologist with the US Geological Survey's Western Ecological Research Center in California. "Anybody who looks at the global picture will be very surprised," he adds.
For years, US policy was to extinguish every fire on public lands. But in August 2000, government agencies developed the National Fire Plan, which recognizes that wildfires play an important role in maintaining certain ecosystems.
Fire control in the United States is fraught with controversy, however, because communities are still being built on fire-prone lands. Spectacular and destructive wildfires are a regular feature of the American summer. In 2004, for example, Alaska experienced its worst fire season on record: more than 6 million acres of land burned in 700 fires across the state.
But, says Bond, "If we try and switch fire off, we would be losing ecosystems that have been around for millions of years. These natural ecosystems are dependent on burning, and have to be actively managed."
- Bond W. J., Woodward F. I. & Midgley G. F. New Phytologist 165, 525-538 (2005). doi:10.1111/j.1469-8137.2004.01252.x