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dispersed in space. With the origin of postfire seeders, characteristics associated
with enhanced flammability ( Fig. 9.5 ) were likely to be an important driver not
only for the seeders but for providing suitable habitat for obligate resprouters as
well. One of the striking differences between MTC regions is the disproportio-
nately high representation of obligate resprouters in northern hemisphere systems
and the depauperate representation in South Africa and Australia. This may
derive from different origins for fire-prone sclerophyll shrublands in these regions
and in particular the early origin of highly flammable sclerophylls over large
expanses of nutrient-poor soils in these southern hemisphere MTC regions (see
Chapter 10).
One of the potential genetic effects of the persister life history is that resprouting
genets may be extremely old and the potential exists for greater accumulation of
deleterious alleles from somatic mutations (Wiens et al. 1987 ). This high genetic
load would potentially result in reduced seed set and enhanced selection for
resprouting (Lamont & Wiens 2003 ). This hypothesis could explain the anomal-
ously nearly non-existent seedling recruitment in some resprouters in Cape fynbos
( Retzia spp.), Banksia elegans in Western Australian woodlands (Bond & Midgley
2003 ), and in the monotypic genus Xylococcus in California chaparral (J.E. Keeley
personal observations).
Fire-dependent Recruitment
This describes postfire seeding that is restricted to a single pulse after fire, and is
well developed in many woody genera in MTC shrublands, as well as some MTV
crown fire shrublands distributed in other climatic regimes, such as Florida scrub
and southeastern Australian shrublands. It has been widely considered to be a
relatively recent Quaternary phenomenon (Raven & Axelrod 1978 ; Axelrod 1989 ;
Herrera 1992 ; Ackerly 2004a ; Valiente-Banuet et al. 2006 ); however, this designa-
tion is largely based on the absence of Tertiary fossils for these taxa. The depend-
ence in science on negative evidence is always weak but especially so when it comes
to the fossil record, which is highly biased against semi-arid fire-prone floras. As
discussed in more detail in Chapter 10 , despite the lack of a Tertiary record, many
of these presumed “Quaternary species” originated much earlier in the Tertiary;
for instance, molecular clock estimates place the origin of Fumana and Helianthe-
mum much earlier than the fossil record dictates (Guzma´ n & Vargas 2009a ). In
North America the highly disjunct chaparral pockets far outside the MTC in
Arizona and northeastern Mexico (see Fig. 5.1 ) containing postfire-seeding Arc-
tostaphylos pungens and Ceanothus greggii suggest early origins outside California,
prior to their widespread appearance in California Quaternary sediments. The
near total absence of Tertiary fossil floras from the southwestern United States
makes it difficult to test that idea, but it does illustrate the extraordinary bias
involved in using fossil records for pinpointing origins.
Although there is reason to believe the timing of the evolution of the postfire
seeding trait might have varied across different MTC regions, the primary factors
may have been very similar. The most important was the reliable creation of gaps
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