Agriculture Reference
In-Depth Information
Northern Hemisphere Origins (Laurasia)
The evolution of contemporary sclerophyllous vegetation is largely a Tertiary
phenomenon, a period that opened the Cenozoic Era 65 Ma. Modern dicotyle-
donous angiosperm floras became widespread in the early Tertiary, a time when
the present-day continents of North America, Europe and Asia formed the
massive continent of Laurasia. The continents separated during the Eocene
(56-35 Ma), although plant migration routes between continents persisted further
into the Tertiary (Raven & Axelrod
1974
; Denk
et al.
2010
). At this time Laurasia
vegetation was dominated by mixed deciduous hardwood, evergreen coniferous
forests, mesic temperate woodlands and subtropical forests. We know almost
nothing about azonal communities of more restricted distribution.
As the continents separated, a number of Eocene genera (or ones of earlier
origin) were left in both western North America and Eurasia and these account
for the present-day occurrence of woody sclerophyll taxa such as
Arbutus
,
Cercis
,
Cupressus
(
sensu lato
),
Pinus
,
Prunus
,
Quercus
,
Rhus
and
Rhamnus
, and
non-sclerophylls
Aesculus
,
Juglans
,
Staphylea
and
Styrax
, in both regions (Axelrod
1975
; Wolfe
1975
; Fjellstrom & Parfitt
1995
; Milne
2006
). Although lacking a
macrofossil record, the distribution of a number of herbaceous taxa (Raven &
Axelrod
1978
) also would appear to have been part of this flora as today they are
represented in both North America and Eurasia; for example,
Antirrhinum
,
Datisca
,
Erodium
,
Galium
,
Helianthemum
,
Lotus
,
Salvia
,
Trifolium
and
Triodanis
.
The conifer genus
Tetraclinis
extended widely across western North America in the
Tertiary despite its restriction today to scattered parts of North Africa and the
western Mediterranean Basin (Kvacˇ ek
et al.
2000
). Other Eocene or older chapar-
ral taxa such as
Ceanothus
(Richardson
et al.
2000b
),
Heteromeles
(Phipps
1992
),
and
Malosma
(Miller
et al.
2001
) remained restricted to North America.
Nerium
,
Olea
,
Punica
and
Pyracantha
are Eocene taxa that apparently remained restricted
to Eurasia (Palamarev
1989
).
There is a tendency to see contemporary Mediterranean Basin floras as prod-
ucts of recent Quaternary events (Suc
1984
), in part because of our greater
familiarity with the climatic oscillations of that period. However, there is genetic
evidence indicating that the modern geographical structure of Mediterranean
plant populations may be traced back to the Tertiary history of taxa. Cork oak
(
Quercus suber
), an emblematic MTV sclerophyll tree, exhibits patterns of genetic
drift consistent with the Oligocene and Miocene breakup events of the continental
margin and lack of detectable chloroplast DNA changes for the last 15 Ma (Magri
et al.
2007
). Thus, climates conducive to MTV appear to have been present for a
significant portion of the Tertiary and may have led to much earlier origins than
commonly assumed.
Although Eocene climates were considered to have been warm, mesic and
equable, parts of southwestern North America were experiencing semi-arid
climates (Peterson & Abbott
1979
) conducive to sclerophyllous shrublands. In
the middle Eocene in southern California, and elsewhere in North America, pollen
