Fire and the Fire Regime Framework - Fire in Mediterranean Ecosystems - page 50

20-100 Mg ha 1 . On more oligotrophic soils typical of Gondwana landscapes
of the Western Cape of South Africa or Western Australia, shrubland fuel
loads are markedly lower. South African fynbos typically burns with return
intervals around 15 yrs and at this frequency may have fuel loads around
10-20 Mg ha 1 , but in rare instances when left unburned for much longer
periods may approach fuel

loads more typical of California chaparral (van

Wilgen et al. 2010 ).
These biomass levels don't tell the whole story as not all aboveground stems are
equally vulnerable to igniting and to spreading fire. Typically, the available fuels
are considered to be dead stems and live stems < 6 mm in diameter plus foliage.
However, severe fire weather can increase the range of stem sizes that are available
fuels. Since a greater proportion of early seral stages of shrub succession com-
prises smaller diameter stems, flammability may be relatively high (see Fig. 5.3 ).
This contradicts the dogma that fire hazard increases with stand age, which
seems like an overly simplistic view (e.g. Paysen & Cohen 1990 ; M.J. Baeza et al.
2011).
Many factors affect the extent to which fuels contribute to fire behavior. Live
fuels and dead fuels have very different characteristics. Climate plays a key role by
affecting fuel moisture, and this is particularly important in determining flamma-
bility of live fuels (see next subsection). Dead fuels are key because they ignite
most readily and in shrublands the ratio of dead to live fuels affects the extent to
which live fuels will ignite and this ratio varies by species and age. The size of dead
fuels is also very critical because smaller diameter stems will ignite faster than
larger diameter stems, due in part to the fact that they dry faster and heat
penetrates faster. Thus, vegetation types such as heathland on poor soils that
generate very fine fuels may be much more fire-prone than coarser vegetation of
more fertile soils such as chaparral (Ojeda et al. 2010 ). Even within a plant
community there are marked structural differences that make some species much
more fire-prone than others, for example retention of dead branches as opposed to
shedding them (see Chapter 9 ). These generalizations apply to crown fire regimes
and very different patterns are to be expected in vegetation types where fires are
driven by surface fuels (e.g. Scarff and Westoby 2006 ).
Total aboveground biomass in most forests is substantially greater than in
shrublands but for many forests with a history of surface fire regimes, it is the
dead surface fuels that are considered available fuels, although large logs may
not be included. In such forests the surface fuels can range from 10 Mg ha 1 to
morethan200Mgha 1 ( Table 2.1 ) , dependent on site productivity and time
since fire. In some forests the fuel load also includes live and dead standing
understory shrubs and tree saplings that can more than double the fuel load
(Keifer 1998 ;Sackett&Haase 1998 ), but are not always included in fuel
inventories.
Structural characteristics also may affect surface fuel arrangement as in the
greater packing density of small fir needles, compared with the larger fascicles of
pine needles that do not compact as densely on the forest floor. This packing