Agriculture Reference
In-Depth Information
In all five regions many MTC plant lineages can be traced to sclerophyllous taxa
with origins in the early Tertiary under climates bearing little resemblance to the
contemporary winter rains and summer drought. Sclerophylly was of selective
value on sites where coarse-grained lower-fertility substrates produced soil-drought
stress under some regimen of seasonality, on an annual or longer frequency basis.
The MTC in most regions developed gradually through the Miocene with increas-
ing intensity of summer drought, and this coupling of drought with high summer
temperatures greatly expanded the drought-prone landscape. Concomitant with
this was an increase in fuel continuity and thus fire spread and predictability of fire.
Two features of the MTC have been of immense selective importance: the cool
growing season and seasonal soil drought. Both of these predate the MTC and
thus much of the present flora persists under conditions with some significant
similarities to environments present during their pre-MTC origins. The primary
impact of the MTC was a major expansion of fire-prone sclerophyll vegetation.
Over the last 10 million years, as the current climate unfolded, and a highly
predictable crown fire regime developed, species reassembled, and many fine-
tuned their adaptations to this regimen. Some lineages exploited newly generated
resource combinations and novel fire regimes and have speciated rapidly since the
end of the Tertiary. Other lineages remain ensconced in ecological niches little
changed from pre-MTC environments.
An emergent property of this model is that it provides a clearer picture of the
relationship of MTV that today persists outside the MTC regions. In many
respects the proximal selective factors of soil drought and fire are the same.
This is most evident in highly fire-prone evergreen sclerophyllous shrublands
and woodlands that persist in winter drought, summer rain climates as well as
aseasonal climates with decadal-scale droughts. Noteworthy examples include
outcroppings of fynbos in eastern South Africa, the evergreen scrub in the south-
eastern USA, and sclerophyllous shrublands and woodlands in southeastern
Australia. Although these landscapes have similar life forms and functional types
adapted to periodic high-intensity crown fires, these are not MTC ecosystems as
they typically differ markedly in composition, growth forms and some ecosystem
processes.
Likewise, each of the five MTC regions has ecosystems with marked floristic
differences and somewhat subtle structural and functional differences. What ties
these together is that the landscapes are dominated by sclerophyllous-leaved
MTV, and this continuity of fire-prone vegetation increases predictability of fire
and regimes dominated by crown fires. Not only does the combination of soil
drought and high temperatures greatly enhance flammability, but the expanse of
contiguous fuelbeds generated by the mild winters increases the probability of fire
spread through communities, and thus predictability of fire for individuals.
Despite the evident structural convergence of all five MTC regions, there are
really three different stories with respect to fire. These are the two northern
hemisphere systems, the two low soil fertility southern hemisphere systems and
the Chilean system.
