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

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 stress. 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 tissues. 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.