Agriculture Reference
In-Depth Information
0
-10
-20
-30
-40
-50
0.0001
0.0010 0.0100 0.1000 1.0000
Charcoal flux (
m
g cm
-2
yr
-1
)
Fig. 10.9
Estimated charcoal flux from aeolian transport to a Pacific Ocean deep drilling core.
(Data from Herring 1985.)
subtropical latitudes pockets of these C
4
grasses persisted through much of the
Oligocene and Miocene but expanded rapidly at the end of the Miocene to form
extensive grasslands, and this appears to be fire driven. This is a useful model of
fire-adapted MTV, which likely persisted in isolated fire-prone pockets for a
significant part of the Tertiary and then greatly expanded at the end of the
Tertiary.
The Herring (
1985
) data cannot reflect much about fire activity in the Mediter-
ranean Basin but samples from the eastern Pacific may reflect fire activity
along the Pacific Coast of North America. The timing of increase in fire activity
(
Fig. 10.9
) was coeval with the late Tertiary expansion of chaparral in coastal
California. In this case early Miocene islands of chaparral on drought-prone
substrates, embedded in a landscape of less flammable woodlands, expanded as
the intensification of the MTC extended the drought-prone landscape. As the
sclerophyllous MTV expanded its range it would have increased the predictability
of fire activity, which in turn would have fed back into an even greater expansion
of chaparral. An empirical model of how this process might have worked is
illustrated by the demonstration on oceanic islands of increasing fire activity with
increasing island size due to the greater probability of lightning-ignited fires on
larger islands (Wardle
et al.
1997
). Additionally, in the case of chaparral, feedback
processes would potentially have affected selection for flammability and other fire
traits. As chaparral flammability increased it would have further eroded mesic
woodlands already losing ground to climate changes, as outlined by Jackson
(
1968
) and Bowman (
2000
) for the replacement of tropical forests by Australian
sclerophyll forests. The paleoclimate data indicate an increase in summer drought
in the late Tertiary, which further contributed to greater expanses of fire-prone
landscape (e.g.
Fig. 10.1
).
The marine charcoal deposition record (
Fig. 10.9
) undoubtedly represents a
localized record of fire activity on the west coast of North America. Due to the
