1 Spatial Modelling of the Terrestrial Environment - Georeferencial

176 Spatial Modelling of the Terrestrial Environment
Fires have a range of environmental effects across multiple scales in time and space.
For example, fires at the local scale significantly reduce the vegetation cover, and more
intense fires may completely remove all above-ground vegetation. At the appropriate time
and place such processes maybe advantageous, for example, in rapidly returning nutrients
to the soils after grassland senescence, or by keeping forest surface litter levels low so as
to reduce the likelihood of larger, more intense fires that may kill normally fire-tolerant
vegetation. Furthermore, the reproductive biology of many species in fire-prone systems is
closely related to fire activity (Whelan, 1995), with, for example, heat or chemicals in the
smoke emitted by the fire triggering flowering and/or seed release (Enright et al., 1997).
Additionally, the nutrient-rich post-fire ash bed may be important for the establishment of
seedlings in certain systems. However, at the local scale, fires can also alter the species
makeup and ecology of an area and render the underlying soils more prone to erosion
(Johansen, 2001). In extreme cases dramatically altered fire regimes may cause the local
extinction of some species (Keith, 1996), whilst at the regional scale fire may also have
significant ecological effects, resulting in altered flows of energy and matter through the
landscape and the generation of spatially complex mosaic patterns (Turner et al., 1994,
1997). At these regional scales, the particulates and gases released during burning can
concentrate at levels far beyond recognized safe human health limits, leading to hazardous
visibility and poor air quality. A high-profile example of regional-scale fire-related pollution
occurred during the 1997–1998 fires in SE Asia, when an El Niño-related drought allowed
fires to become much more intense and widespread than is normally the case, and which
also significantly reduced the incidence of ‘cleaning’ of the atmosphere by rainfall (Wooster
and Strub, 2002). The widespread haze blanketed areas of SE Asia for weeks and resulted in
regional economic losses of more than $1.4 billion, exposing more than 20 million people to
potentially dangerous levels of air pollution (Brown, 1998; Schweithelm, 1998). Such is the
magnitude of fire activity on Earth, particularly in the tropics, that pyrogenic gaseous and
particulate emissions are now recognized to be highly significant at the global scale, where
Earth’s atmospheric chemistry, cloud and rainfall characteristics, and radiative budget are
all influenced by the combustion products (Rosenfeld, 1999; Andreae and Merlet, 2001;
Chou et al., 2002).
Clearly, for the above reasons, biomass burning and pyrogenic emissions must be rigorously
considered when analysing the effect of wildfires on Earth’s surface and atmosphere
(Beniston et al., 2000). As such, the quantification of biomass burning activity is now a
major focus in the global change community and the large-scale, repetitive measurements
required to quantify terrestrial fire activity mean Earth Observation (EO) satellites have
been identified as a key technology for the study of pyrogenic activity and the assessment
of emissions (Robinson, 1991). However, whilst the emission of certain chemical species
and particulates can in theory be examined directly through the use of EO (Kaufman et al.,
1990; Goode et al., 2000; Kita et al., 2000; Kaufman et al., 2002), not all species are easily
inventoried in this way. Instead the majority of EO-based studies of fire emissions have
been limited to mapping the location of active fires or burnt areas (Scholes et al., 1996),
with little information provided on the fire physical characteristics such as fire intensity,
rate of spread, etc. The absence of this information has been highlighted as a potential
problem in certain studies of fire ecology (Whelan, 1995; Morrison, 2002), as has the reliance
on maps of burnt area to estimate the amount of biomass combusted in the fire event
(Andrea and Merlet, 2001). This is because the relationship between remotely measured