Agriculture Reference
In-Depth Information
approach is that one can theoretically predict climates for any given time and
location. However, this approach is fraught with difficulties due to the complexity
involved, as climate is a function of numerous factors: orogeny and its effect on
atmospheric circulation and rain shadows, plate tectonics and movement of plates
into new climatic zones, the thermohaline cell, ENSO events and Milankovitch
variations, to name just some of the parameters to be considered (Graham
1999
).
An indirect approach is to use fossil floras and deduce past climates based on
corresponding climates of related contemporary species (Wolfe
1995
). Although
widely used, this approach has a number of limitations, not the least of which is
that many areas of the world lack fossil floras for time periods of interest. Another
major problem is that these macrofossil floras are not an unbiased sample of the
landscape; rather, except for fossil sites generated by volcanic ash deposition,
they largely sample wetland vegetation and some unknown portion of the
surrounding landscape. This is a particular problem if one is concerned about
fire-prone landscapes since they tend to be on the arid end of the moisture
gradient. Landscapes with mosaics of mesic and arid patches will provide limited
information on the arid fire-prone portion of the flora until it reaches some
unknown level of landscape dominance. Those arid environments where fire has
likely been an important factor may not even have had suitable wetlands for fossil
deposits and thus lack a record entirely.
Since the goal of this discussion is to better understand the origins of fire-prone
sclerophyllous-dominated communities of MTV, which are widespread in con-
temporary MTC regions, we will begin with the history of climatic factors that
play a role in fire-prone systems. Climatic seasonality (at some scale; i.e. annual,
decadal or longer) is a necessary condition for any fire regime (see
Chapter 2
) and
such patterns have been present off and on since the mid-Paleozoic (see
Fig. 9.1
)
rise of land plants (Scott
2000
,
2010
; Pausas & Keeley
2009
). Throughout the
Mesozoic, large annual seasonal variations in rainfall were associated with mid-
latitude continental margins due to the alternation of high and low pressure
centers over continents and adjacent oceans (Parrish
et al.
1982
). Despite the
widespread characterization of global climates in the Mesozoic as “equable,” there
is evidence that some mid-latitude regions in the northern hemisphere experienced
seasonally dry conditions, even climates analogous with contemporary MTCs,
comprising wet winters and hot dry summers (Francis
1984
; Allen
1998
).
Early Cenozoic Climates
Most lineages contributing to our contemporary sclerophyllous MTV originated
sometime in the Late Cretaceous or early Cenozoic, and were shaped by climates
that fluctuated markedly during different epochs. Paleoclimatic inferences point
to the earliest Tertiary epoch, the Paleocene, as showing marked seasonality
over the more equable climate of the Late Cretaceous and with a steep north-
south temperature gradient (Davies-Vollum
1997
). This was followed by the
Eocene, which globally was a time of high temperatures and limited seasonal
