Agriculture Reference
In-Depth Information
western Argentina, which is an area that still regularly has lightning-ignited fires
and weak MTC (see
Chapter 6
). In addition to well-developed lignotubers there is
evidence of a somewhat diverse postfire annual flora (see
Fig. 6.7
).
Conclusions
MTC began forming at least as early as mid-Miocene and strengthened through
the later Tertiary, often in regions outside of contemporary MTC regions. This
pattern of winter rains and summer drought was well developed in all regions by
the end of the Tertiary, and since then its intensity has waxed and waned with
various climatic cycles. Associated with the MTC are shrublands, woodlands and
forests dominated by sclerophyllous-leaved taxa. Much of this MTV has its
origins prior to the fully developed MTC and the factors primarily responsible
for its development are seasonal droughts, low or moderate fertility soils and fire.
Sites of limited resources have selected for the sclerophyll leaf functional type, and
coupled with periodic droughts these conditions have led to fire-prone vegetation,
and feedback processes on trait evolution have likely contributed to enhanced
flammability.
Many recent studies have addressed issues of origin of MTC species, and almost
all have related these patterns to changing climates and soil and have largely
ignored fire as an important evolutionary driver. However, since at least the
middle Tertiary fire has been a potential ecosystem process on landscapes associ-
ated with contemporary MTC regions. Fire regimes varied in predictability and
frequency in association with the extent of sclerophyllous vegetation and scale of
seasonal drought and these have been influenced by both climate and geology.
Seasonality is a prerequisite for fires and has been present at annual, decadal and
longer cycles throughout much of the evolutionary history of MTV. The seasonal
timing of drought is not a determinant of fire-prone landscapes but it potentially
affects the amount of landscape conducive to fire. The primary effect of the MTC
was to couple drought with high temperatures, which did not qualitatively affect
niche characteristics as much as it quantitatively expanded the extent of fire-prone
landscape. Synergism associated with this expansion of fire-prone landscape
affected the predictability component of the fire regime and had profound impacts
on success of fire-dependent species. This model explains the widespread distribu-
tion of highly fire-prone sclerophyllous vegetation in MTC regions and more
localized occurrences of MTV in non-MTC regions such as in southeastern
Australia and North America.
All five MTC regions are dominated by sclerophyllous shrublands and there are
three separate stories of fire, plant evolution and community assembly. In the two
northern hemisphere systems the origins of most sclerophyll shrubs began in the
early Tertiary amidst a landscape of mesic forests and woodlands. Their origins
are hypothesized to have been on marginal sites more susceptible to drought
stress, such as ridgelines and course-grained substrates that would have inhibited
