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prevailing westerly winds these records would not provide an adequate picture of
fire activity in the more interior regions. This is unfortunate because macrofossil
records suggest an early Miocene rise of chaparral elements in the southwestern
portion of the Great Basin. On the basis of climate models it seems likely that
semi-arid chaparral-like vegetation was more widely distributed at this time in the
southwestern portion of North America, but there is a lack of Miocene fossil sites
from the region to confirm this. Depending on the extent of fire-prone landscape,
fire activity could have started earlier in this southwestern chaparral than sug-
gested by Herring's (
1985
) data. Conceivably, much of the early evolutionary
history of MTV and fire may have occurred outside California and some fire-
adaptive traits may have been present at the time of late Miocene expansion of
chaparral into California.
Can we place the origin of fire-adaptive traits, such as resprouting, lignotubers,
flammability, fire-dependent recruitment and obligate seeding (see
Figure 9.5
)
within the context of the geological history for chaparral? The majority of genera
are known from Eocene fossils; all are vigorous postfire resprouters and some
have lignotubers (see
Chapter 9
), suggesting an early origin for resprouting and
perhaps lignotubers as well. Whether or not these traits arose in response to fire is
not known; however, sclerophyllous shrubs likely inhabited patches of fire-prone
landscape since their origin and thus there is little reason to rule out fire as a
potential selective factor.
Perhaps the trait most clearly associated with fire is postfire seeding, as the
delay in reproduction to a single postfire pulse of recruitment would require,
among other things, a highly predictable fire regime (see
Figure 9.5
). Several
genera with postfire seedling recruitment, and roots back to the Eocene, include
Ceanothus
,
Garrya
,
Fremontodendron
and
Malosma
. This is a highly conserved trait
as most if not all species in these genera have similar recruitment patterns, and this
trait may have an early origin as well.
One of the traits with the clearest dependence on a predictable fire regime is
the risky strategy of coupling loss of resprouting ability with postfire seeding;
the obligate seeding mode. This event likely took place early in some lineages.
For example, in the species-rich Californian shrub genus
Ceanothus
, the split
between
Ceanothus
and
Cerastes
subgenera is placed in the Oligocene (Jeong
et al.
1997
), and in the latter group obligate seeding is highly conserved as all
species in this subgenus are of this functional type. The other speciose genus in
chaparral with obligate seeding species is
Arctostaphylos
and they first show up in
the fossil record in the early Miocene (Brown
1935
; LaMotte
1952
), although for
reasons discussed above they potentially originated earlier in the southwestern
USA. An early origin for obligate seeding in this genus is suggested by the
presence of this strategy in species endemic to Tertiary substrates (e.g.
A. myrtifolia
,
Gankin & Major
1964
) and the presence in species that occur far outside Califor-
nia, including both winter drought, summer rain chaparral in northeastern
Mexico and bimodal rainfall sites in Arizona. Unlike the
Cerastes
lineage of
Ceanothus
, obligate seeding is not highly conserved and associated with habitat
