Agriculture Reference
In-Depth Information
Box 8.1 (
cont.
)
communities containing dense C
4
grasses is insensitive to variations in fire
return interval (e.g. Williams
et al.
2002
; Wright & Clarke
2007
). This reflects
the relatively high frequencies of fire driven by these grasses.
In southeastern Australia the MTV existing under a summer rain climate
is suitable for warm-season C
4
grasses. Here the understories of sclerophyll
woodlands on non-sandy substrates have a significant C
4
component (Mott &
Groves
1994
; Prober
et al.
2002
). This contrasts with MTC sclerophyll wood-
lands in southwest Australia, which comprise very different ecosystems in
which C
4
grasses are poorly represented (Murphy & Bowman
2007
). In
the east edaphic factors come into play, leading to extensive temperate
C
4
-dominated grasslands on fertile, igneous substrates (Lunt & Morgan
2002
).
The striking exceptions are the infertile habitats (rock and sand) within the
old fire world
of MTV which lack C
4
cover. The exception is mallee shrublands
(semi-arid) that often contain
Triodia
on sandy substrates (Rice & Westoby
1999
; Bradstock & Cohn
2002a
). In these regions (
c
.3235
latitude), gradients
between heath and mallee-heath (
Triodia
) may be
steep and related to overall rainfall (e.g. Pausas & Bradstock
2007
). These
represent an interface between “new” and “old” fire worlds, where the balance
between space/competition, habitats and flammability could be finely poised.
The nature of this balance will be critical in these habitats, given their diversity
and sensitivity to changes in fire regimes. Would an alteration in seasonality
and amount of rainfall have minor, incremental effects or could boundaries be
poised in a manner that leads to a major realignment of these fire worlds (i.e. a
grass fire cycle beyond the resilience of woody taxa, see
Fig. B12.1.1
)? Alterna-
tively, will elevated levels of atmospheric carbon dioxide (CO
2
) bolster the
dominance of woody species in temperate vegetation, via enhanced water use
efficiency?
The temperate, infertile MTV habitats represent the last frontier remaining
unconquered by C
4
grasses in Australia. Candidate invaders include indigenous
(e.g.
Triodia
) and exotic grasses. Predictions of twenty-first-century climatic
change indicate increases in summer rainfall in southern MTV regions (CSIRO
2007
) along with a tangible chance of overall decline. Such shifts may synergis-
tically change ignitions, fire weather and habitat availability to favor these
invaders in MTV regions. While this is speculative, attention is warranted given
that diverse communities of MTV fringe the southern regions of the continent.
A realignment of
fire worlds
in this way could leave the diverse enclaves of
mainland MTV stranded in terms of available ecological space. Given evidence
that the composition of biomes is relatively inflexible over evolutionary and
ecological time (i.e. species migrations tend to be confined within similar
biomes, Crisp
et al.
2009
) the implications of rapid global change are manifestly
serious.
Triodia
) and mallee (
þ
