Agriculture Reference
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variation (Raven & Axelrod
1972
). However, conclusions about equable Eocene
climates are in part an artifact of the general coarsening of temporal resolution
with increasing distance back in time, as climatic oscillations on timescales of
10-100 kyr are permanent features of earth history (Bennett
2004
).
The Eocene world does not appear to have been highly conducive to frequent
widespread burning; however, it was not completely lacking in seasonality condu-
cive to periodic fires. Decadal-scale oscillations can create droughts conducive to
fires, even in otherwise aseasonal climatic regimes. Also, the model of universal
equable conditions is disputed by modeling studies that show substantial annual
seasonality effects in continental interior landscapes (Sloan & Barro´ n
1990
; but
cf. Wing & Greenwood
1993
). For example, during this epoch seasonal aridity
was evident in central Australia (Greenwood
1994
) and the southwestern USA
(Peterson & Abbott
1979
). Consistent with regional variation in seasonality is the
middle Eocene appearance of shrubs and grasses in parts of southwestern North
America, interpreted as signs of decreased precipitation and increasing severity of
the dry season (Frederiksen
1991
). The region is thought to have had a winter
drought and summer rain climate and a very long dry season that was able to
sustain high-intensity forest fires. In coastal southern California late Eocene
climates appear to have had an annual rainfall of 500 to 600 mm that was
distinctly seasonal (Abbott
1981
; Frederiksen
1991
). With this level of precipita-
tion there could have been sufficient fuels to carry fire, and the seasonality would
have created burning conditions on a regular basis. Thus, there is reason to expect
that early in the Tertiary, fires were an ecosystem factor on some landscapes,
despite more equable conditions in other parts of the landscape. These fire-prone
pockets potentially spread fire to more mesic landscapes during longer-interval
cyclical droughts, expanding fire-prone vegetation, as observed today in tropical
forest types (Cochrane
2003
).
Oligocene
Cenozoic plate tectonics moved the continents closer to their current configur-
ation (Milne
2006
), contributing to global circulation patterns homologous to
contemporary patterns. Middle Oligocene glaciation of Antarctica, coupled with
the formation of Drakes Passage resulting from movement of South America
away from Antarctica, led to cold water flow into northern latitudes (Axelrod
et al.
1991
). This strengthened high-pressure systems so that drier climates spread
more widely than previously seen in the Cenozoic. These changes contributed to
marked climate change during the transition from Eocene to Oligocene and
seasonality increased over much broader portions of the globe (Eldrett
et al.
2009
). During the Oligocene, global temperatures dropped by more than 10
C,
climates became much more seasonal and arid landscapes expanded (Wolfe
1994
).
Potential fire-prone environments undoubtedly expanded during this time.
Several investigations have suggested late Oligocene as the origin of MTC
in some regions. On the basis of proposed circulation patterns, Beard (
1977
),
