Agriculture Reference
In-Depth Information
Fire and Paleohistory of MTV
Northern Hemisphere Fire Responses
Throughout the Cenozoic, landscapes in the Mediterranean Basin and western
North America have had sufficient seasonality and fuels to generate fire-prone
conditions in marginal habitats with flammable sclerophyllous vegetation.
Although there are reports of Tertiary fires from both regions (e.g. Frederiksen
1991
; Cichocki
1998
; Figueiral
et al.
1999
,
2002
; Kolcon & Sachsenhofer
1999
),
how widespread fire was is unknown. Miocene and Pliocene macrofossil floras
from western North America are relatively silent on the issue of fire, and it is not
clear if this is because fires were rare or because of a lack of adequate searching, or
because the macrofossil record has an inherent bias against recording semi-arid
fire-prone landscapes. Considering the widespread presence of fire-adapted forest
and shrubland taxa in the region today, many of which appear to be little changed
since their Tertiary origin,
it seems unlikely fire was not a feature of some
ecosystems.
Evidence for Tertiary fire is substantially lower than that for Paleozoic and
Mesozoic fire (see
Chapter 9
). Part of this may be methodological in that
Mesozoic fire evidence is derived from microfossil charcoal (Scott
2000
) and
Tertiary studies in western North America are largely based on macrofossil leaf
impressions (e.g. Axelrod
1982
; Wolfe
et al.
1998
). Also, the Mesozoic fire evi-
dence is largely from mire peatlands and these habitats are relatively uncommon
in western North America. Nonetheless, a comparison of peatlands through time
suggests a marked decline in peatland fire activity from the Mesozoic to the
Tertiary (Diessel
2010
). Factors responsible for this fire pattern are unclear and
it is unknown whether or not it is a useful signal for fire activity on the broader
landscape. Additionally, it is unknown whether or not this change in fire activity is
tied to the Tertiary shift from gymnosperm and pteridophyte dominated to
angiosperm dominated landscapes, although this has been proposed (Bond &
Scott
2010
). The potential for Tertiary expansion into marginal semi-arid habitats
that the angiosperms encouraged could have resulted in altered global fire regimes
and a change in landscape distribution of fire so that peatlands no longer com-
prised the dominant fire-prone landscape.
One of the few northern hemisphere Tertiary studies of fire is based on
estimates of charcoal deposition in Pacific Basin marine sediments (Herring
1985
). This study supports the idea that fire has been present through the
last half of the Tertiary but the amount of highly flammable landscape increased
10-100 fold in the late Tertiary (
Fig. 10.9
). Although there are good reasons for
implicating late Tertiary climatic changes in this increased fire activity, equally
important were structural changes in plant fuels at both an individual as well
as the landscape scale. This late Tertiary increase in fire activity has been linked
to substantial ecosystem shifts in subtropical environments from woodlands
to C
4
-dominated grasslands (Keeley & Rundel
2005
). It is apparent that in
