Agriculture Reference
In-Depth Information
0.50
35
0.40
30
0.30
25
0.20
20
0.10
0.0
-400
15
-300
-200
Time (Ma)
-100
0
Fig. 9.3
Variations in atmospheric CO
2
(solid line) and O
2
(dashed line) over the past 400 million
years predicted by the geochemical mass balance model (from Keeley & Rundel 2005 based on
data by Beerling et al. 1998).These atmospheric changes likely had profound impacts on fire
regimes as increased CO
2
availability would enhance fire activity by increasing fuel production
and declines in O
2
would potentially restrict fire activity (see text).
Different Mesozoic fire regimes were actively selecting plant traits that in some
instances are still evident in today's flora, for example in two California endemics,
coast redwood (
Sequoia sempervirens
) and the interior giant sequoia (
Sequoiaden-
dron giganteum
). Today redwoods occupy moist coastal forests with very long fire
return intervals that generate high-intensity crown fires. As with many woody taxa
in crown fire regimes, redwoods are vigorous resprouters from basal burls. The
giant sequoia of interior mountains is associated with high lightning ignition
landscapes that historically have burned in frequent surface fires, a fire regime
that has not selected for resprouting capacity, but rather self-pruning and thick
bark to withstand understory burning and reduce the potential for crown fires.
Earth history included periods of high and low fire activity, which Scott and
Glasspool (
2006
) have suggested were tied to changes in atmospheric oxygen levels
(
Fig. 9.3
). Oxygen, however, never dropped sufficiently to exclude fire, as evident
by the paleofires described for the late Permian at both high (Glasspool
2000
) and
low latitudes (Uhl
et al.
2007
) and for the late Triassic (Jones
et al.
2002
).
Presumably changes in climatic seasonality likewise could have affected the
waxing and waning of fire activity, perhaps through changes in plant (fuel)
structure (Belcher
et al.
2010
). The capacity to occupy drier seasonal habitats
due to the origin of seeds may also have been a factor promoting the Paleozoic
spread of fires (Rowe & Jones
2000
; Uhl & Kerp
2003
). Biotic changes in plant
structure (e.g. Bond &Midgley
1995
) likewise would have affected the incidence of
paleofires.
Even prior to the Cenozoic radiation of angiosperms that led to our contem-
porary MTV, fire-adapted sclerophyll shrubland, analogous to modern MTV
shrublands, dominated some Cretaceous landscapes (Batten
2002
). For example,
southern and eastern England during the Early Cretaceous had a moderate winter
