Agriculture Reference
In-Depth Information
pine distribution to either high latitudes or high elevations, in both Europe and
North America (Millar
1998
).
It is thought that an important time of radiation in
Pinus
was the Oligocene
as the marked cooling and drying conditions would have greatly expanded poten-
tial habitats, in both Europe and North America (Klaus
1989
; Millar
1998
).
At this time in North America, pines were successful along the southern part
of the Rocky Mountain Cordillera.
Pinus ponderosa
or allied taxa expanded
throughout this range down into southern Mexico.
Pinus ponderosa
moved into
California in early Miocene, before an intense summer drought climate had
developed, and even today its widespread distribution indicates no specific rela-
tionship to summer drought climates (see
Chapter 5
). Based on molecular studies
it appears that early to middle Miocene was the time of greatest radiation in
Pinus
(Willyard
et al.
2007
).
Southern Hemisphere Origins (Gondwana)
Sclerophyllous-leaved MTV, characteristic of dominants in MTC climates today,
has been present on Australian landscapes since at least the Eocene, and some
lineages have persisted to the present (Hill & Brodribb
2001
). Throughout the
warm and humid conditions of the Eocene, much of Australia was dominated by
rainforest trees, podocarp gymnosperms and pteridophytes. At this time the
region of contemporary MTC, southwestern Australia, had
Nothofagus
(Fagaceae)
dominants, with Proteaceae and Casuarinaceae subdominants. However, even at
this very early stage of the Tertiary, sclerophyllous taxa such as
Banksia
and
Dryandra
(Proteaceae) were present (Beard
1977
) and by late Eocene in central
Australia, rainforest was displaced by sclerophyll vegetation on slopes and ridges
(Martin
2006
). Sclerophylls may also have played a successional role following dry
periods that are indicated by sedimentological evidence in the form of silcretes
(Kemp
1981
). Prominent sclerophylls such as
Banksia
were already quite diverse
(Hill & Christophel
1988
). They were pinnately lobed taxa that are today mostly
restricted to the southwest.
In southwestern Australia vast expanses of landscape have nutrient-deficient
soils that have been continually weathered since the end of the Cretaceous and
after greatly accelerated lateritization in the Oligocene and Miocene (Johnstone
et al.
1973
). These oligotrophic soils favored the spread of large contiguous
populations of xeromorphic heathland and other sclerophylls (Hopper
1979
).
The present-day sclerophyllous flora is thought to have evolved from old lineages
that originated during the Late Cretaceous or early Tertiary in isolated pockets of
highly weathered nutrient-deficient soils on landscapes dominated by rainforests.
Fossil records show that half the families and genera in kwongan heathland were
present in the Eocene (Lamont
1982
).
This sclerophyll vegetation developed in response to nutrient-poor lateritic soils
and has persisted through a remarkable range of climatic changes (Hopper
1979
,
2009
), although it has been argued that fire played a more important role in
