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Abstract

An examination of the morphology and photosynthetic physiology of the two
dominant families of conifers in the southern hemisphere, the Podocarpaceae and
Cupressaceae, was undertaken here in order to describe the physiological limitations
which constrain their distribution. A species specific index of drought tolerance was
devised which quantified the maximum attainable water-use efficiency of photosynthesis
during imposed drought. This index, (ci/c a)min, measured the potential of leaves to
photosynthesize under water stress, comparing the maximum draw-down of internal CO2
concentration (ci) below ambient (ca). When operating at (ci/c a)min, the leaf was at the
maximum water stress before leaf damage (indicated by an irreversible loss of variable
fluorescence and photosynthetic rate) occurred. (ci/c a)min was found to be well correlated
with the minimum rainfall within the natural range of the 12 species investigated, and
this is strong evidence that the distribution of these species is largely determined by their
drought tolerance. Further support for the drought limitation hypothesis came from a
comparison of photosynthetic rates (as measured by chlorophyll fluorescence) of conifers in
the field. The apparent photochemical electron transport rate (ETR) was found to be
significantly reduced in a dry site when compared with a wet site. Most Podocarpaceae,
and many Cupressaceae were found to be highly drought sensitive, and probably became
increasingly susceptible to extinction during the substantial increase in aridity which
occurred in Australia during the Tertiary. The influence of features considered as possible
morphological drought adaptations on water loss were also examined. Wax plugs and
imbricacy were found to substantially decrease maximum stomatal conductance, with the
combination of wax plugs, imbricacy and epistomy reducing conductance to 17% of the value
expected on leaves with exposed, unplugged stomata. Competition for light is believed to
be another area where conifers suffer due to their lack of broad-leaves. Many podocarp
genera appear to have converged with angiosperms, producing discrete, bilaterally
flattened short shoots which seem to function as broad-leaves. The degree of shoot
flattening, as measured by shoot width, was correlated with the leaf saturation light
requirement, broad shoots requiring substantially lower light intensities for saturation.
From this it was inferred that the bilaterally compressed short shoot was an adaptation
to light competition with angiosperms.
Conifers in the southern hemisphere form a surprisingly distinct group which has
retained its character since the earliest Tertiary. The late Cretaceous- early Tertiary also
saw the radiation of angiosperms, and it has been suggested that this event resulted in the
global decline of the Coniferales. By determining how distributions of these taxa are
limited today, it was possible to shed some light on whether conifers in the southern
hemisphere were simply overrun and replaced by the more competitive angiosperms, or if
other factors such as climate change are likely to have played a part. A brief summary of
the evolutionary history of conifers in the southern hemisphere leads to the conclusion
that decreasing rainfall and fire have been the main influences on conifer extinction.

Item Type:	Thesis - PhD
Authors/Creators:	Brodribb, Tim
Keywords:	Conifers
Copyright Holders:	The Author
Copyright Information:	Copyright 1996 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references
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