In a
grassland or
chapparal biome (as well as some others),
periodic burning occurs as a
natural and necessary
catastrophe. The
fire spreads all across the
ecosystem, killing almost everything in its path (
animals usually escape by burrowing underground or crossing over and edge). As a result, the
growth of
shrubbery is
curtailed, and further
evolution of the ecosystem is prevented;
trees will not crowd out the
grasses and bushes (and thus change the ecosystem into a
forest). In addition, the burning
rejuvenates the fertility of the
soil.
Since the dawn of ecological engineering, humans have regulated this periodic burning. On a wildlife reserve or national park composed of grassland territory, humans torch the land every few years by a very complex and somewhat dangerous process.
First, two concentric ditches are dug around the territory to be burned. There is to be no life whatsoever in these ditches; when the fire reaches a ditch, there must be nothing left to burn, or it will rage on out of control. The patch of land between the two ditches is then burned as a safety precaution; the dead, unburnable land here will further enforce the purpose of the dead land in the ditches. Anything within the burned territory that should not be burned (such as a windmill) is surrounded by a similar ditch
When the weather becomes favorable, the ecologists will begin the actual burning. Once the fire is set, it will be carried by the wind across all the land marked for destruction. As another safety precaution, a backfire is set before the main burning actually begins. The backfire is a wave of flame at the end of the burning range that travels against the wind. The headfire will stop upon impact with the backfire; thus the backfire is a method of control.
Once the headfire is set, it is quickly boosted by the wind, and the whole area goes up in flames. By the time it hits the backfire, the land is completely scorched, and the burn is complete.
The burns are much easier to conduct safely in the winter than in the summer; the summer heat and dryness contribute to the strength of the fire, making it more difficult to control. However, summer burns are more effective than winter burns for the same reason.