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Frost tolerance and water relations of Banksia species in Tasmania past and present


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Blake, Julieanne 1997 , 'Frost tolerance and water relations of Banksia species in Tasmania past and present', PhD thesis, University of Tasmania.

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Over the past several million years the diversity of Banksia species in
Tasmania has declined. On the Tasmanian mainland today there are only
two species of Banksia . These are the widespread and morphologically
diverse Banksia marginata and Banksia serrata which has a very restricted to
the State's northwest.
It is not clear when the decline in Banksia began in Tasmania, but Pleistocene
sediments in western Tasmania demonstrate a relatively recent extinction for
at least some Banksia species. The most recent recorded Banksia
extinctions from Tasmania are B. kingii and B. strahanensis. These extinct
fossil species are closely related to the extant mainland species B. saxicola/B.
canei and B. spinulosa/B. spinulosa var. cunninghamii respectively.
Based on this information it is interesting that B. marginata managed to
survive the climatic upheavals of the Quaternary in Tasmania while most
other Banksia species were unable to.
This thesis investigates whether the extinction of B. kingii and B. strahanensis
from Tasmania during the Pleistocene could be linked to a physiological
incapacity to tolerate the climatic stresses of the Pleistocene glaciations, i.e.
drought and cold. A physiological examination of the drought and cold tolerance of the closest
living relatives of the fossil species implies that the frost and drought tolerance
of B. kingii and B. strahanensis were no less remarkable than that displayed
by a sample of Tasmanian populations of the very successful Banksia
marginata. It is therefore concluded that the Pleistocene extinction of B. kingii
and B. strahanensis from Tasmania is unlikely to be due to a physiological
weakness of drought and cold.
This study highlights the physiological mechanisms used Banksia marginata,
B. sax/cola, B. canei and B. spinulosa var. cunninghamii to survive climatic
stress. All species were able to significantly increase their bulk elastic
modulus in response to drought stress. Banksia marginata and B. spinulosa
var. cunninghamii were also able increase their apoplastic water content in
response to drought stress. The capacity for osmotic adjustment was not characteristic of any of the species examined. It is interesting that all of the
species examined underwent osmotic de-adjustment in response to drought
Frost experiments indicate that all of the Banksia examined have reasonable
frost tolerance. Although B. spinulosa var. cunninghamii was generally less
frost tolerant than the other species, the results were usually significant. The
frost results iimply that sensitivity of the fossil species to frost would have
unlikely to cause their extinction from Tasmania during the Pleistocene
glaciations. Indeed many of the Banksia marginata populations and B. canei
and B. saxicola were able to significantly improve their frost tolerance when
exposed to drought stress. The drought and frost experiments in this study
successful highlight the similarities of drought and cold stress in their effect on
plant physiology. Banksia marginate was examined in more detail than the closest living
relatives of the fossil species. This was done to help determine whether its
current success in Tasmania is the result of its capacity for physiological
plasticity and the genetic diversity of the species. The physiology results
indicate that this species is very desiccation tolerant, thus giving it an
excellent capacity to deal with drought and cold stress. The physiology
results in general also indicated significant differences the drought and frost
tolerance among populations of B. marginata. A small isozyme study was
performed which suggested there could be a genetic basis to the
physiological differences observed among two populations studied along an
altitudinal cline.
An attempt was also made to determine whether the high degree of
morphological variation evident in Tasmanian populations of B. marginate
could be positively correlated with physiological variation, which it couldn't.
Further work would need to done for this result to be accepted as conclusive.
It is likely that the success of Banksia marginata in Tasmania today is related
to a combination of genetic diversity of the species, its physiological plasticity
and its general hardy nature as evident from its extremely drought tolerant
The extinction of B. kingii and B. strahanensis from Tasmania during the
Pleistocene may have resulted during the interglacial periods of the
Pleistocene. Glacials are of a much longer duration than interglacials.
Interglacials may have been very climatically stressful for the Banksia. Other
hypotheses are considered in the thesis.
It is also likely that the genetic diversity of B. kingii and B. strahanensis was
less than B. marginata during the Pleistocene. This theory is based on the
current day morphological variation in the closest living relatives of the fossil
species which is considerably less than displayed by B. marginata. In
addition, the closest living relatives today have much narrower distributions
than B. marginata, hence perhaps having less chance of surviving the climatic
stress of the Pleistocene than the more widely dispersed and genetically
diverse B. marginata which may have had greater genetic reserves for natural
selection to act on.

Item Type: Thesis - PhD
Authors/Creators:Blake, Julieanne
Keywords: Plant-water relationships, Banksia
Copyright Holders: The Author
Copyright Information:

Copyright 1997 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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