Agriculture Reference
In-Depth Information
shifts in
Arctostaphylos
, and there have been switches between this and facultative
seeding in several subspecific
Arctostaphylos
taxa (see
Chapter 9
).
Postfire seeding is also widespread in the Mediterranean Basin and includes
numerous shrubs:
Erica
spp. (Ericaceae), most Cistaceae, and many Fabaceae
and Lamiaceae, the majority of which recruit from dormant soil-stored seedbanks
are mostly postfire seeders and all have origins in theMiocene or earlier (Guzma´ n&
Vargas
2005
,
2009a
).
Cistus
is of interest because it is one of the few truly woody
obligate seeding genera in the Mediterranean Basin, and this trait is highly con-
served (see
Table 3.4
).
Cistus
appears to have had an early origin in the Oligocene
outside its present range but radiated sometime after late Miocene within the
Mediterranean Basin. Noteworthy is the fact that the
Helianthemum
has one dis-
junct species that is a postfire obligate seeder in California chaparral, although
whether this represents a vicariance event from an early origin or a recent
long-distance dispersal event is unknown.
Postfire seeding from serotinous cones characterizes many gymnosperms associ-
ated with crown fire shrublands in the Mediterranean Basin and California:
Pinus
spp. and Cupressaceae taxa (
Cupressus
,
Hesperocyparis
(NewWorld
Cupressus
) and
Tetraclinis
). There are numerous life history traits that are correlated with serotiny
that enhance postfire recruitment success (Keeley & Zedler
1998
) and patterns
of trait evolution support the conclusion that fire has played a major role in
the evolution of serotiny (Schwilk & Ackerly
2001
). California serotinous pines
in subsection Attenuatae (
Pinus attenuata
,
P. muricata
and
P. radiata
) originated in
Mexico or Central America during the middle Tertiary (Millar
1998
), suggesting an
early association with predictable fire regimes. Axelrod (
1988
; Axelrod & Deme´ re´
1984
) contended that by late Miocene they dominated a substantial portion of the
southern California landscape, but with the Quaternary intensification of the
summer drought were restricted to islands within a chaparral-dominated land-
scape. Alternatively, Millar (
1999
) has suggested the current disjunct populations
are not the result of contraction of a broader distribution but a long-standing
metapopulation pattern due to periodically fluctuating climates.
Very little is known about the timing of fire-adaptive traits in herbaceous taxa.
The precise timing of recruitment to postfire conditions in the highly diverse
California chaparral herbaceous flora indicates a highly evolved association with
fire. This association is so extraordinarily linked with fire these taxa are often
referred to as pyro-endemics (see
Chapter 5
). Soil-stored seedbanks can survive
more than a century and dormancy is broken by heat shock in some species but in
the majority of lineages it is broken by chemicals from biomass combustion.
Families with this “smoke”-stimulated germination include Hydrophyllaceae,
Scrophulariaceae, Boraginaceae and Asteraceae to name just a few. The origin
of smoke-stimulated germination is unknown but is potentially very ancient
(Pausas & Keeley
2009
). The postfire annuals in California chaparral are very
diverse but little is known about the timing of diversification. Raven & Axelrod
(
1978
) implied that this was a recent Quaternary phenomenon; however, there is
