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genera exhibit a seemingly unique capacity to regenerate after fire from an
anatomical anomaly comprising strips of meristematic tissue buried beneath the
full thickness of the bark (Burrows
2010
). This trait appears to be rather ancient in
the group and suggests an important presence of fire in the environment through-
out much of the Tertiary (Crisp
et al.
2011).
Kemp (
1981
) suggested that the Tertiary trend of replacement of rainforest by
open sclerophyll vegetation may have been either a response to climate change or
to fire activity opening up closed-canopy forests and drying out understory fuels.
As suggested in
Fig. 1.4
, this may be an artificial distinction as climates and
geology interact with fire in driving plant assembly. Kemp (
1981
; see also Jackson
1968
; Bowman
2000
) suggested that for fire to produce such landscape changes it
would not require frequent fires, just a regime with fire cycles shorter than the life
span of the tree. Support for this model is the fact that there was widespread
rainforest disappearance during the Miocene from inland regions in association
with regular seasonal burning (Martin
1990
).
In southeastern Australia, as sclerophyllous vegetation expanded, charcoal
levels increased in the late Oligocene and remained high into the Miocene
(Kershaw
et al.
2002
; Holdgate
et al.
2007
). A key question is which is cause and
which is effect. Fire would favor sclerophyllous woodlands and shrublands, but
these taxa would also promote fire spread. Tertiary fossil floras show a clear
correlation between vegetation type and amount of pyrofusinite (fossil charcoal)
in a successional sequence where the driest vegetation was also the most fire-prone,
according to Blackburn & Sluiter (
1994
). These authors contended that burning
occurred after a change to drier conditions and development of a more sclerophyl-
lous vegetation; that is, fire was a response to, rather than a cause of, vegetation
changes. However, there is a rich body of literature that would argue the opposite
(e.g. Bond & Midgley
1995
; Schwilk
2003
; see also
Chapter 3
). Hill (
1990
)
contends that the fossil flora provides excellent evidence for a Tertiary increase
in abundance of
Eucalyptus
associated with increased charcoal levels; when one
considers the range of apparent fire adaptations in this taxon it is no surprise (e.g.
Burrows
2002
; see also
Figure 3.3c
,
d
).
Phylogenetic studies reveal that the mallee habit with lignotubers has arisen
independently several times in
Eucalyptus
in several geographic regions, suggesting
widespread influence of fire in the Tertiary (Hill
1990
). In this regard it is
important to recognize that resprouting and lignotubers are two distinct traits;
the former is widespread in woody dicots and the later is restricted to mostly MTV
(see
Chapter 3
). Evidence for a fire origin of lignotubers in
Eucalyptus
is implied
in the apparent loss of the lignotuber in taxa that have radiated into more mesic
fire-free sites.
In southwestern Australia Tertiary fire has potentially played a rather extensive
role in the evolution of this unique flora. Vast expanses of nutrient-deficient soils
have persisted since the Cretaceous and this is hypothesized to have played such
an intensive selective role that these landscapes were buffered from Tertiary
climatic fluctuations (Hopper
2009
). These sclerophyllous shrublands have
