Author:
Hill Robert S.,Brodribb Tim J.
Abstract
The three southern conifer families, Araucariaceae, Cupressaceae and
Podocarpaceae, have a long history and continue to be an important part of the
vegetation today. The Araucariaceae have the most extensive fossil record,
occurring in both hemispheres, and with Araucaria in
particular having an ancient origin. In the Southern Hemisphere
Araucaria and Agathis have
substantial macrofossil records, especially in Australasia, and
Wollemia probably also has an important macrofossil
record. At least one extinct genus of Araucariaceae is present as a
macrofossil during the Cenozoic. Cupressaceae macrofossils are difficult to
identify in older sediments, but the southern genera begin their record in the
Cretaceous (Athrotaxis) and become more diverse and
extensive during the Cenozoic. Several extinct genera of Cupressaceae also
occur in Cretaceous and Cenozoic sediments in Australasia. The Podocarpaceae
probably begin their macrofossil record in the Triassic, although the early
history is still uncertain. Occasional Podocarpaceae macrofossils have been
recorded in the Northern Hemisphere, but they are essentially a southern
family. The Cenozoic macrofossil record of the Podocarpaceae is extensive,
especially in south-eastern Australia, where the majority of the extant genera
have been recorded. Some extinct genera have also been reported from across
high southern latitudes, confirming an extremely diverse and widespread suite
of Podocarpaceae during the Cenozoic in the region.
In the Southern Hemisphere today conifers achieve greatest abundance in wet
forests. Those which compete successfully with broad-leaved angiosperms in
warmer forests produce broad, flat photosynthetic shoots. In the Araucariaceae
this is achieved by the planation of multiveined leaves into large compound
shoots. In the other two families leaves are now limited to a single vein
(except Nageia), and to overcome this limitation many
genera have resorted to re-orientation of leaves and two-dimensional
flattening of shoots. The Podocarpaceae show greatest development of this
strategy with 11 of 19 genera producing shoots analogous to compound leaves.
The concentration of conifers in wet forest left them vulnerable to the
climate change which occurred in the Cenozoic, and decreases in diversity have
occurred since the Paleogene in all regions where fossil records are
available. Information about the history of the dry forest conifers is
extremely limited because of a lack of fossilisation in such environments. The
southern conifers, past and present, demonstrate an ability to compete
effectively with angiosperms in many habitats and should not be viewed as
remnants which are ineffectual against angiosperm competitors.
Subject
Plant Science,Ecology, Evolution, Behavior and Systematics
Cited by
178 articles.
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