Agriculture Reference
In-Depth Information
Unlike California chaparral, postfire fynbos has a low cover of herbaceous fire
ephemerals and therefore lacks the dry flashy fuels that can cause young chaparral
stands to burn (Zedler
et al.
1983
; Keeley & Zedler
2009
). However, fynbos differs
from all MTC ecosystems in having a significant component of perennial
graminoid growth forms, primarily restios (Restionaceae) and sedges (Cypera-
ceae). In many fynbos communities, perennial graminoids produce a savanna-like
physiognomy with a more or less continuous grass layer interspersed with ericoid
shrubs and emergent proteoid shrubs. Kruger (
1977
) suggested that the presence
of a graminoid layer, together with fine-leaved ericoid shrubs, accounted for the
shorter fire interval in fynbos relative to California chaparral and Mediterranean
Basin matorral, and both lack a significant perennial herbaceous component (see
also van Wilgen
1982
; van Wilgen & van Hensbergen
1992
). However, Australian
heathlands have a higher fire frequency than fynbos and they generally lack a
dense restio cover, but like fynbos they do have a more or less continuous layer of
fine fuels and a discontinuous layer of taller emergent proteoid shrubs.
It is interesting to note that postfire fuel accumulation rates of fynbos are not
dissimilar to montane C
4
grasslands of South Africa, which also produce
2.5 Mg
ha
1
yr
1
with 1000 mm precipitation (O'Connor & Bredenkamp
1997
) yet typic-
ally burn at intervals of 1-2 yrs. It is not the biomass alone, but the type of biomass
that influences the fire regime. C
4
grassy fuels are not only more continuous than a
young postfire fynbos stand but also cure in the winter dry season, producing highly
flammable fuel. Fynbos graminoids are evergreen and have high moisture content
in the initial postfire recovery phases. The implication is that the capacity to carry
fire is dependent on dead fuel accumulation rates after the first few postfire years.
Given sufficient biomass to burn, weather conditions strongly influence flam-
mability by altering the moisture content of fuels. Van Wilgen & Burgan (
1984
)
used a burning index based on the U.S. National Fire Danger Rating System, to
analyze climate and weather effects on the occurrence of fynbos fires. The burning
index peaked in the summer months, November to March, for most of their
weather stations but not for the coastal ranges in the eastern regions where the
burning index was relatively consistent across the year, with bimodal peaks in
winter and summer. The winter peak is associated with dry, hot bergwind condi-
tions. The number of fires, and the area burned, in different months closely
matched the climate-derived burning index peaking in summer in most stations
but occurring all year round in the coastal mountains. Thus, fire season in Cape
fynbos seems largely determined by weather. Moreover, van Wilgen & Burgan
(
1984
) also found a strong relationship between area burned (large fires) and
extreme values of the burning index, suggesting that weather conditions are an
important contributor to the high fuel continuity conditions that trigger large fires
(see also Forsyth & van Wilgen
2008
; Wilson
et al.
2010
). We do not yet know the
extent to which the frequency of such weather conditions determines fire frequen-
cies in the Cape region. To do that, we would need to know both the frequency of
the extreme weather events and how often they were associated with large fires,
and this analysis has not been done.
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