Agriculture Reference
In-Depth Information
10
Fire and the Origins of
Mediterranean-type Vegetation
The mediterranean-type climate (MTC) is widely agreed to have been in place in
all five MTC regions since at least the late Pliocene (see
Fig. 9.1
),
2 Ma, with
much of the contemporary mediterranean-type vegetation (MTV) present and
contributing to a highly fire-prone environment. There is far less agreement on:
(1) the timing of the origin of the MTC, (2) the timing of and factors responsible
for the origins of MTV, and (3) the paleohistory of fire and extent to which it has
played a role in the origins of MTV. Ample evidence exists to suggest a much
earlier origin of MTC and MTV.
A widely held paradigm is that many of the woody sclerophylls that
comprise MTV are much older than the Pliocene and thus have not adapted
to contemporary fire-prone MTC conditions (Axelrod
1989
;Herrera
1992
;
Verdu´
et al.
2003
; Ackerly
2004a
). Most of these have origins in the Tertiary
Period of the early Cenozoic and are viewed as relictual taxa that represent
evolutionary inertia and are present today merely by chance avoidance of random
extinctions.
This needs further examination for several reasons. The MTC appears to
be older than commonly accepted and it developed gradually over a period of
10-20 million years and thus its date of origin is a matter of how one defines
MTC. Also, the critical environmental features that have played a selective role
in MTV species may not have changed since their origin in the Tertiary; for
example, soil drought
per se
may be the key selective factor rather than summer
soil drought, or predictability of winter rains may be a key selective factor as it
has shaped numerous life history characteristics. Both soil drought and winter
rains preceded the MTC yet persist under a MTC. Under this model a species
could exist through multiple climatic changes and be tracking the same prox-
imal environmental conditions. Lastly, regardless of the seasonal distribution
of rainfall, MTV likely has had a long association with stressful substrates
experiencing seasonal soil drought, and thus has been fire-prone throughout
much of its evolutionary history. Under this model the primary change to the
environmental template brought about by the MTC was an expansion of
drought-prone landscape (
Fig. 10.1
) and the potential this had for fire spread.
This scenario though does not preclude more recent innovation of some fire-
adaptive traits or species radiation in some groups in response to the expansion
of drought-prone landscapes in MTC regions.
∼
