Agriculture Reference
In-Depth Information
The boundary with MTV (
old fire world
) is demarcated within the region where
fire responds with a monotonic increase to declining moisture. What other com-
binations exist in continental Australia and how do these relate to MTV and its
environments?
In marked contrast to MTV are the tropical grasslands and savannas domin-
ated by grasses with the C
4
photosynthetic pathway. Although their distribution is
strongly controlled by climate, the origins and rapid expansion of C
4
grasses in the
late Tertiary in conjunction with development of the tropical monsoonal climates
appear to have been strongly influenced by fire (Bond & Keeley
2005
; Keeley &
Rundel
2005
). Indeed, their very maintenance today is dependent on regular
fires to prevent encroachment by sclerophyll woodland (see
Chapter 7
). This
model implicitly involves the interaction of climate, life history and flammability
as discussed above. Specifically, the monsoonal climate, selecting for high-
productivity warm-season grasses (C
4
plants) capable of producing heavy fuel
loads of highly combustible herbaceous foliage, and unlimited lightning ignitions
set the stage for a highly fire-prone landscape. The capacity of C
4
grasses to
rapidly regrow leads to an almost annual fire frequency, a fire regime incompat-
ible with most woody growth forms. All of this derives since the end of the
Tertiary and thus represents a
new fire world
in contrast to the older origins of
the MTV
old fire world
(
Box 8.1
).
Box 8.1 Old vs. New Fire Worlds
C
4
grasses in Australia encompass both perennial (e.g.
Themeda
) and annual
(e.g.
Sorghum
) life-history patterns in conventional tussock growth forms
(Mott & Groves
1994
). However, the endemic Australian genus
Triodia
exhibits a remarkable hemispherical growth habit derived from needle-like
tillers (Rice
et al.
1994
). Hummock grasses form a prominent component of
arid and semi-arid communities, dominated by shrubs or trees throughout
arid Australia (Mott & Groves
1994
), but also extending into the wet
tropics (Russell-Smith & Stanton
2002
). The hummock growth habit of
interlocking tillers results in an ideal flammable arrangement of live and dead
material (Bradstock & Gill
1993
). Hummock grasses are sclerophyllous and
adapted to the infertile habitats (Rice & Westoby
1999
) that predominate
across the continent. By contrast, C
4
tussock grasses predominate in
deeper, heavier textured soils, with or without woody dominants (Mott &
Groves
1994
).
C
4
grass-driven fire regimes are linked to available moisture, with high
frequencies (1-5 yrs) in the monsoonal tropics and decadal frequency in arid
landscapes where anomalous pluvial events periodically provide sufficient
connectivity of biomass for major fires (Allan & Southgate
2002
; Southgate &
Carthew
2007
). Relative to MTV, the composition of arid and tropical
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