Geoscience Reference
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For example, it is unclear how to define “natural” fire regimes, while account-
ing for thousands of years of Native American burning practices in many parts
of the Americas. Many ecosystems have clearly co-evolved with continual native
burning, though to what degree this burning has changed ecosystems is controver-
sial. Nevertheless, the concept of historical fire regimes remains a useful tool to
understand whether fires are changing over time, as well as differentiating types of
wildland fires and the systems through which they burn (Swetnam et al.
1999
).
To further illustrate the concept of fire regimes, it is helpful to consider two very
different sorts of wildland fires and the areas in which they might occur. One wild-
land fire might sweep through 1000 acres of homogeneous grassland in the late fall,
while another might burn through 1000 acres of mountainous forest at the height
of summer. While our grassland may be flat with a continuous mix of vegetation,
our forest consists of rough topography with 500-800 year old mixed conifers with
patches of deciduous aspen and interspersed with small wetlands and meadows.
In the case of the grassland, fires might have occurred in the area every 2-6 years
resulting each time in almost complete consumption of all the above ground biomass
within the entire perimeter of each fire. However, within a year or two it might be
difficult to see much evidence of each fire. In the forest example, a fire might be the
first to occur in the area in centuries. The fire might burn in a patchy mosaic with
many areas seeing only light surface fire, leaving the entire overstory undisturbed in
those patches. Other areas might burn hot enough for the fire to get into the crowns
and kill patches of trees for dozens of acres at a time. The evidence of this mixed
conifer fire will endure on the landscape for centuries as many surviving trees will
carry fire scars, dead snags will persist in other areas providing valuable habitat,
and successional cycles will be reset in many forest patches but not others. A few
years after the fire, new aspen groves might expand because of their competitive
advantages in some of the new openings in the conifer forest, while many of the
meadows might expand as small trees in the surrounding areas were killed by the
light fire burning through the meadows' grass.
Each of these two hypothetical fires could be traditionally mapped with a single
1000 acre outline on a regional map and most news media would report that each
fire “destroyed 1000 acres.” However, the vastly different patterns on the landscape
illustrate the different elements of fire regimes that would be of interest to anyone
looking to better understand the processes or patterns involved with these fires. For
example, understanding the subtleties of the historical local fire regimes is critical
to understanding whether a given fire falls outside the historical range of variability.
If a wildland fire is well outside the historical fire regime of the area, it may indicate
other changes are occurring to the systems involved. Fuels may be accumulating
because of decades of fire suppression, or climate change might be leading to dryer
conditions. The linkages between these sorts of patterns and processes can help
analysts understand the processes that are occurring.
Fire regimes are sometimes simplified to aid in mapping. For example, Fig.
3.1
shows a map of simplified computer simulated historical fire regimes. As detailed
field data on historical fire frequencies can be difficult to produce, computer simu-
lations can provide a method to map these processes over broad scales. This map
