Agriculture Reference
In-Depth Information
Other than a few wetland species, herbaceous plants are absent from most
macrofossil records, which is unfortunate because postfire floras in northern
hemisphere MTC shrublands are dominated by a diverse assemblage of annuals
and we know relatively little about their origins. Raven and Axelrod (
1978
)
considered much of the diversification of herbaceous groups in California to be
concentrated in the Pleistocene because this was the time of greatest intensification
of the summer drought climate. Although this may be true, it says little about the
origin of this postfire flora, which is clearly not dependent on summer drought as
there is a diverse postfire winter annual flora in the bimodal winter and summer
rainfall climate of Arizona chaparral, including many of the same taxa as in
California chaparral (Fotheringham
2009
). This postfire ephemeral flora appears
to be highly dependent on winter rainfall and not on the duration or timing of
drought, although California's MTC may have played a role in speciation of these
taxa. Based on what is known about the Tertiary history of winter rain there is
every possibility many of these lineages had very early origins.
The Mediterranean Basin history is broadly similar to western North America.
Being part of Laurasia, the basin shares a lot of early history with North America
and the early Tertiary sclerophyll forest vegetation was quite similar to that in
western North America (Axelrod
1975
). The two regions have a number of genera
in common due to vicariance of taxa originating in the Eocene or earlier. As in
North America, sclerophyllous vegetation arose in the Tertiary on edaphically
generated drought-prone sites on a landscape dominated by more mesic broadleaf
forests, and then expanded in the mid to late Miocene (Palamarev
1989
; Barro´ n
et al.
2010
).
One landscape difference between these two regions is the broad latitudinal
gradient over which plants could shift their distribution in response to repeated
climatic changes in North America and more limited opportunities in the
Mediterranean. Plant migration may have been inhibited at times due to the
Mediterranean Sea, although it most definitely was not a complete barrier to
long-distance dispersal (Guzma´ n & Vargas
2009b
).
In both Europe and North America,
Pinus
is in some respects the
Eucalyptus
of the northern hemisphere in that it is a fire-adapted sclerophyllous-leaved tree
that dominates MTC ecosystems. Also, like the eucalypts, the pines predate
the MTC and today are widely distributed far outside the MTC of both Europe
and North America, as well as in Central America and Asia (Richardson
1998
).
The origin of
Pinus
dates to the Mesozoic (Willyard
et al.
2007
), but its radiation
is a Cenozoic phenomenon (Millar
1998
). Sometime in the Cretaceous the
Diploxylon subgenus separated from the Haploxylon subgenus. Many of the
latter taxa have radiated into climatically stressful habitats such as deserts
or subalpine environments, whereas most of the Diploxylon taxa are tied to fire-
prone habitats (Keeley & Zedler
1998
; Schwilk & Ackerly
2001
). Faced with
intense competition from the rapidly radiating angiosperms, pines and other
gymnosperms were at a decided disadvantage in warm and mesic conditions
(Bond
1989
). As a consequence, climates through much of the Eocene limited