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concluded that summer rains were still persistent in the region. Condit (
1938
)
offered an alternative interpretation and pointed out it is to be expected that as
summer rains declined during the Miocene one would not see the immediate loss
of summer-rain-dependent species as they would persist for a considerable time in
riparian zones or other wetlands. Consistent with this interpretation, Piru Gorge,
like many macrofossil floras, was a wetland site; the majority of the 22 species
were wetland taxa including
Typha
,
Equisetum
, three species of
Salix
, one of
Platanus
, and six species of
Populus
(Axelrod
1982
). Thus, the persistence of a
few summer-rain-allied species may not be an indicator of summer rains. Condit's
(
1938
) insight is consistent with contemporary patterns in that some taxa widely
distributed in summer-rain regions, such as
Amorpha
, still persist throughout
MTC California largely within riparian zones. In an analogous manner, MTC
chaparral sclerophylls such as
Malosma laurina
and
Hesperocyparis
(
Cupressus
)
forbesii
persist much further south into very arid parts of Baja California by
restricting their distributions along riparian watercourses.
Washingtonia
palms
currently restricted to riparian sites in MTC California may stand as another
example. In southern Australia, the restriction of rainforest species to riparian
corridors similarly has been interpreted as a response to declining summer rains
(Kemp
1981
).
Although the MTC appears to have established in coastal California somewhat
later, several lines of evidence point toward an early Miocene origin for the MTC in
more interior portions of western North America. At 17 Ma, macrofossil floras in
southwestern Nevada were already lacking subtropical and summer-rain taxa,
indicative of marked climate change and increasing summer aridity (Wolfe
1964
).
Axelrod and Schorn (
1994
) also reported a major floristic change at 15 Ma in
westernNevada, marked by the rapid disappearance of deciduous hardwoods allied
to those now in summer-rain southeastern USA, and this was attributed to increas-
ing summer drought. Based on the minimum summer rain currently tolerated by
contemporary summer-rain taxa, it was estimated that summer precipitation was
about a third of the annual total at 15 Ma. However, this estimate assumed that the
few remaining summer-rain taxa were indicative of the regional climate and were
not relicts persisting along riparian corridors or other wetland refugia. By late
Miocene (7 Ma) a distinctly MTC was in place in southern California (
Fig. 10.2
),
although Axelrod (
1982
) contended there was still about 15% summer rain, a value
he considered necessary to account for relictual summer-rain taxa.
Consistent with an early origin for the MTC in North America is the Miocene
origin of
Aesculus californica
(Hippocrastinaceae). This shrubby tree is endemic to
the MTC region of California. It exhibits an undeniable adaptation to summer
drought in its summer deciduous habit (
Fig. 10.3a
), which is quite distinct from
the mostly winter deciduous genus. Molecular data indicate an Eocene or earlier
origin for
Aesculus
and the split of
A. californica
sometime in the Miocene (Xiang
et al.
1998
), which suggests significant summer drought present at that time. One
could argue that this taxon originated under summer-rain conditions and the
summer-deciduous habit developed later; however, this seems unlikely because
