Agriculture Reference
In-Depth Information
Fig. 2.5
Global distribution of lightning during the period March to May 1987. (From Goodman
& Christian
1993
.)
crown fires as well as unburned patches. Stand averages of fire frequency
obscure some of the important ecosystem processes inherent in heterogeneous
burning patterns at fine scales. A further complication is that fire frequency may
change over time due to changes in climate and anthropogenic increases in
ignitions.
All four parameters of the fire diamond (
Fig. 2.1
) potentially constrain fire
frequency. However, given sufficient available fuel, ignitions will be a major
determinant. Lightning is the most abundant natural source of fires and its
frequency exhibits striking global differences (
Fig. 2.5
). With the noticeable
exception of central Chile (see
Figure 10.4
), all MTC regions experience frequent
lightning strikes, although on a global basis none of the MTC regions would
be considered lightning hot spots and of those that do occur in MTC regions,
a significant proportion occur during winter storms and seldom contribute
to fires.
The relative impact of human ignitions is largely a function of the background
lightning ignition frequency and the impact is geographically quite variable.
In many mountainous environments the natural lightning strike density is high
and human population density is low, and thus humans have had minimal
impact on fire frequency. MTC regions have been particularly vulnerable to
human ignitions due to the high population centers in coastal portions of all five
regions and relatively moderate lightning ignition frequency in these lowland
areas (
Fig. 2.6
).
In terms of understanding human impacts on fire regimes, the length of human
occupation, and thus period of time over which humans have altered fire fre-
quency may be an important factor. For example, the Mediterranean Basin has
potentially had fire regimes altered by human ignitions for more than 50 000 yrs
