Fact Sheet 4.9. Fire Management in Grasslands
Fire management entails changing burning regimes to alter carbon pools in the
Use and Potential
Fire often is an essential tool in pastoral lands for controlling woody weeds,
removing dead biomass, stimulating regrowth, hunting, controlling pests, and
clearing land. In many areas, fire regimes are strongly influenced by human
actions such as controlled burning, back-burning, firebreaks, and rapid response.
Reduced fire frequency or fire prevention tends to increase mean soil, biomass,
and litter carbon levels (Jones et al., 1990). In particular, fire management
increases the density of woody species in many landscapes (Archer, 1994; Archer
et al., 1995; Scholes and Archer, 1997).
The magnitude of carbon storage associated with woody growth can be large.
In the Orinoco Llanos, for example, protection from fires for 25 years increased
the total carbon pool by a mean of 1.4 t C ha-1 yr-1; carbon stocks increased
by 5.6 and 29 t C ha-1 in the vegetation and soil pools, respectively. If open
forest is allowed to form, up to 5.69 Gt C may accumulate over 51 years, if
extrapolated over the full area of the Orinoco Plains (28 Mha) (San Jose et
al., 1998). Similarly, in 60 Mha of savanna lands in northeastern Australia,
Burrows et al. (1998, 2000) report that management and environmental changes
(predominantly decreased fire frequency) are increasing carbon pools by 30 Mt
yr-1 in aboveground woody biomass and 10 Mt C yr-1 in below-ground woody biomass;
aboveground biomass pools could increase 2-5 t C ha-1 (open grassland) to 15-75
t C ha-1 (closed woodland), and similar changes in below-ground biomass stocks
can be expected (Burrows et al., 1998). Similar changes have been found
in Africa (northern Guinea savanna): Protection from burning for 26 years resulted
in large increases in tree density and basal area compared with burned plots,
as well as increases in soil carbon concentrations (Brookman-Amissah et al.,
1980). Scholes and Hall (1996) suggest that increased tree cover in savannas
could be contributing a worldwide sink of 2 Gt C yr-1. This potential could
be limited in parts of Africa and South America, however, because of population
pressures on land use and in other regions because of changes toward more sustainable
stocking rates that will increase burning opportunities for woody vegetation
management (Hall et al., 1998).
Optimizing fire timing may increase biomass in some systems while increasing
productivity. Conventional spring and autumn burning in some perennial grasslands,
for example, have a long-term negative effect on live biomass and standing crop;
midsummer burns facilitate more effective recovery of the grasses (Cox and Morton,
1986). In some ecosystems, burning has little effect on aboveground or below-ground
biomass (e.g., Senthilkumar et al., 1998).
Charcoal generated by fires can constitute 8 g C kg-1 soil and represent up
to 30 percent of the soil carbon content of some Australian soils (Skjemstad
et al., 1996); it probably constitutes a significant part of the inert
or passive soil carbon pool. Reduction or removal of fire will result over long
periods (several centuries) in reduction of this pool.
Direct, repeated measurements of basal area increase in woody species can be
made cost-effectively to high levels of accuracy (e.g., Back et al.,
1997; Brown, 1997; Vine et al., 1999); these measurements can be combined
with allometric equations for the species involved (e.g., Burrows et al.,
2000) to calculate aboveground and below-ground biomass carbon change. Associated
soil carbon sampling (see Chapter 2) can be carried out
to calculate total system carbon change. The results can be scaled up to regional
levels by using statistical sampling methods (Austin and Heyligers, 1989) or
via remote sensing (Danaher et al., 1998). For areas where woody components
are not a large part of the carbon fluxes, sampling regimes described in Fact
Sheet 4.6 can be used.
Sampling for charcoal pools is a unique feature related to this activity. The
slow rate of change of these relatively inert pools create uncertainty about
whether including this pool is appropriate to the short time frame of the Kyoto
Protocol. Furthermore, analysis is likely to be expensive. The size of the pool
and the relatively poorly known dynamics, however, suggest that research is
needed to determine the significance of this pool under reduced-fire regimes.
Current Knowledge and Scientific Uncertainties
There is copious documentation of the increase in woody density in savannas
and other woodlands (e.g., Archer, 1994; Archer et al., 1995), supported
by analyses such as soil C13/C12 profiles (e.g., Boutton et al.,
1998; Burrows et al., 1998). Management appears to be a more significant
factor in these changes than environmental factors such as CO2, climate, and
increased nitrogen deposition (Archer et al., 1995). There is difficulty
in definitively attributing the proportion of change from each factor, which
will vary by location. There is significant variation in rates of accumulation
of woody biomass by location (e.g., 0.25-2.5 t ha-1 yr-1 for South African savannas)
(Scholes and van der Merwe, 1996), and differences in potential pool sizes are
likely. This situation requires some spatial disaggregation to meet uncertainty
levels specified for reporting.
Monitoring, Verifiability, and Transparency
Monitoring to detect change can be carried out by repeat sampling procedures
(Back et al., 1997; Critchley and Poulton, 1998). Detailed guidelines
for establishing a monitoring network and ensuring its representativeness are
given by Brown (1997), MacDicken (1997), and Vine et al. (1999). At the
individual plot level, the combination of allometry plus measured stem growth
increment (including the use of dendrometers) is a powerful and accurate indicator
of aboveground biomass flux. A verification and auditing team could evaluate
satellite imagery to confirm that the integrity of registered sites was maintained,
and auditing could be undertaken of a subset of these sites.
Biomass carbon increases are likely to continue for at least 50 years, though
at reduced rates with time (Scholes and van der Merwe, 1996; Burrows et al.,
1998). In some systems, the period of accumulation may be 100 years or longer
Burning transfers a large proportion (up to 90 percent) of the aboveground carbon
and nitrogen pools in some grasslands, and 3 percent of the total nitrogen pool
to 10 cm in the soil (e.g., Kauffman et al., 1994), into the atmosphere
as CO2, CO, CH4, N2O, NOX, and particulates. The mix of the gases depends on
the material burned and the conditions of burning. Burning results in greater
soil temperature, which increases soil CO2 fluxes (Knapp et al., 1998).
Thus, burning results in a short-term loss of carbon from ecosystems. Replacement
of this carbon, however, generally occurs within 1 to 3 years for grasslands
(somewhat longer for woody plants). Where demands for fuel wood or agricultural
products dictate or where population density is high (e.g., West Africa), increase
in woody plant density will most likely be kept in check by ongoing management.
GHG emissions directly from burning (CO2, CO, CH4, N2O, NOX) and associated
grazing activities (CH4, N2O) cause most productive savannas to be net sources
of greenhouse emissions (0.06-0.2 t CO2-eq ha-1 yr-1 for semi-arid grasslands
in Australia; Moore et al., 1997). Elimination of burning along with subsequent
increases in woody plants and reductions in grazing livestock numbers can cause
these systems to become net sinks for GHGs (about 1 t CO2-eq ha-1 yr-1; Moore
et al., 1997).
In the absence of human intervention, catastrophic fire is the major threat
to carbon storage in savannas. This situation may be impossible to prevent in
the long term (50-100 years) (Scholes and van der Merwe, 1996), but because
this vegetation type evolved under a regular burning regime, recovery after
fire to the pre-fire structure is usually rapid. Extended droughts can also
cause mortality (Fensham, 1998). Where mature woody plants die, they are likely
to be replaced, provided that fire frequency remains low. Dead trees may remain
standing, undergoing slow decomposition and providing a continuing but decreasing
carbon store for periods of up to decades. Tree clearing and thinning may occur
on dense stands to improve agricultural productivity.
For much of the world's broad-leaved savannas, as woody plant density increases,
potential livestock carrying capacity declines (e.g., Scanla, 1992)-as does
production for other purposes (e.g., Brookman-Amissah et al., 1980).
In many ecosystems, fauna and flora species are fire-dependent, so removing
or reducing fires may result in localized extinction or decline (e.g., Edroma,
1986). Increasing woody biomass may also reduce environmental services such
as catchment water yield. Reducing burning will reduce atmospheric loads of
particulates and various other forms of pollution.
Relationship to IPCC Guidelines
The Guidelines deal with savanna burning only in terms of non-CO2 gases; it
is assumed that there are no net losses of carbon, and the system is assumed
to be in balance on average.