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little evidence of this and earlier origins cannot be ruled out. It is widely believed
that speciation is relatively rapid in herbaceous taxa and there is empirical evi-
dence for this (Smith & Donoghue
2008
). The primary factor appears to be the
shorter life span in herbaceous plants that increases the number of generations
relative to woody plants. However, in the case of postfire annuals, this is not true
since they do not complete a life cycle annually but rather only after fire, much like
woody postfire seeders (C.J. Fotheringham personal communication).
In the Mediterranean Basin, diversity of annuals and other herbaceous species
increases after fire and some have germination that can be stimulated by heat and
California. Essentially all postfire species are widely distributed in other disturbed
habitats and few if any have deep seed dormancy that is only triggered by fire cues.
There are several possible hypotheses to explain this difference: (1) fire has played
a longer and more selective role in California chaparral (Pausas
et al.
2006b
); (2) a
longer human presence in the Mediterranean Basin (see
Chapter 13
) has disrupted
natural processes, causing the extinction of postfire endemic taxa; or (3) there was
a former pyro-endemic flora that has undergone selection during the Holocene for
occupying other anthropogenic disturbances.
Not all northern hemisphere sclerophyllous shrubs that comprise an important
part ofMTC shrublands have embraced fire bymodifying their reproductive cycle
to delay recruitment to postfire conditions. Many taxa persist in the face of
repeated fires by vigorous resprouting that rapidly recoups prefire canopies. All
have Tertiary origins and some authors have suggested these are just the lucky
survivors of random extinctions (Herrera
1992
; Valiente-Banuet
et al.
2006
).
Alternatively, a strategy of not concentrating reproduction to a single postfire
pulse could be very adaptive for these obligate resprouters as they invariably
occur on more mesic fertile sites where rapid resprouting limits gaps for postfire
seeding (see
Chapter 9
).
Southern Hemisphere Fire Responses
Charcoal levels indicate fire was present but infrequent in the Eocene of southern
Australia, it increased in the Oligocene, and was substantially higher in the
Miocene (
Fig. 10.9
) (Martin
1996
; Kershaw
et al.
2002
). A similar middle Miocene
spike in fire activity has also been reported for New Zealand (Pole
2003
). These
patterns suggest changes in fire regimes but they can be interpreted multiple ways.
Eocene fires could have been generally less frequent, as this record implies, or they
could have been much more frequent but occurring as localized fires that did not
frequently contribute evidence of fire to deposition sites (Martin
1996
). In other
words fires could have been concentrated on localized sites exposed to annual
cycles of soil aridity or to broader areas subjected to periodic droughts at a
decadal or longer scale. Due to the bias inherent in the fossil record, such sites
would be poorly represented or missed entirely. Evidence that this likely is the case
comes from molecular phylogeny studies of the Myrtaceae. Distantly related
