Agriculture Reference
In-Depth Information
also partly represent areas of
core habitat
where rainforest species have an advan-
tage over sclerophyll competitors. Often, such habitat effects may be confounded
with fire-refuge effects (e.g. moister gullies may provide a competitive advantage
for mesic taxa, along a barrier to spread of intense fires). The disappearance of
rainforest in the Pleistocene from southwestern Australia is consistent with this
model. The subdued terrain in this region translates into a scarcity of incised
canyon refugia and the annual summer drought of this MTC region potentially
increases annual fire hazard.
By contrast, under the cool, high rainfall climate of western Tasmania, the
nature of these interactions is altered sufficiently through both “top-down”' (very
infrequent severe fire weather, low ignition rates) and “bottom-up” (large
expanses of moist habitat, impeded drainage, organic soils, etc.) effects on habitats
shaped by predominant infertile soils. Thus, the resilience and competitive status
of sclerophyll vegetation is partly constrained, allowing a higher probability for
rainforest to become more widespread. This may then allow more scope for
variation in fire regimes to determine vegetation distributions (Jackson
1968
;
Bowman
2000
; King
2004
). Differences in habitat availability and fire weather
under this climate result in different fire regime-vegetation interactions, compared
with the mainland. This may represent the climate-fire margin of MTV domin-
ance, where the inherent tendency of sclerophyll vegetation to dominate at broad
scales (landscapes, regions and biomes) is diminished and unstable: that is, the
zone of transition between alternative stable states dominated by these differing
entities (Warman & Moles
2009
).
The development of such a favorable coupling of habitats, functional
types and emergent fire regimes can be invoked to explain the development
and expansion of MTV through the Tertiary (
Table 8.2
). Such mechanisms
can be inferred obliquely but not fully validated through pollen or macrofossils.
Nonetheless, key elements of these mechanisms are described in contemporary
research, such as: fire weather/ignition effects in determining boundaries
of high-latitude rainforest (Kitzberger & Veblen
1999
;Veblen
et al.
1999
,
2008
); lack of resilience in Gondwanan rainforest lineages (Hill & Read
1984
;
Kirkpatrick & Dickinson
1984
); and differential flammability (Kirkpatrick &
Dickinson
1984
).
Fire and climate are likely to have had an interactive role in the rise of major
lineages that typify Australian MTV. Such a role may be extended into the
Quaternary, after sclerophyll vegetation came to predominate, albeit with signifi-
cant fluctuations. Quaternary shifts in the balance of rainforest and sclerophyll
(and within-sclerophyll composition - casuarinas and
Callitris
vs. eucalypts) may
therefore be seen as the outcome of coupled shifts in habitat availability and fire
regimes, rather than simply the unilateral outcome of either climate or fire (e.g.
Kershaw
et al.
2002
). A more refined appraisal of the relative contributions of fire
weather and habitat availability as determinants of fire is required to understand
the contemporary nature of MTV and the effect of humans and their changing
populations in the late Pleistocene and Holocene.
