The Influence of Hardening and Waterlogging on the Frost Resistance of Subalpine Eucalypts.

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Abstract
Marked interspecific differences in frost resistance were shown to exist between eucalypt seedlings grown in reciprocal field transplant trials conducted at a frost hollow on Snug Plains, south-eastern Tasmania. Populations of the six eucalypt species tested naturally inhabited the margins and base of this frost hollow. A growing season frost of - 5.5°C caused substantial damage to E. delegatensis, E. pulchella, E. urnigera, E. johnstonii and E. coccifera but only slight damage to E. gunnii. A winter frost of - 10.5°C caused markedly less damage than the growing season frost, suggesting that appreciable hardening occurs in winter and that growing season frosts may play a significant role in determining the distribution of subalpine eucalypts. E. pulchella appeared more resistant to growing season frosts than E. delegatensis while the reverse occurred in all but one of the experimental gardens during winter frosts. The reasons for this reversal and its significance are discussed. Frost chamber experiments confirmed the major differences between species in frost resistance and the importance of hardening. The reciprocal transplant trials suggested that seedlings from subgenus Symphyomyrtus were more frost resistant than those from subgenus Monocalyptus in sites prone to waterlogging. This was sup- ported by frost chamber trials in which a significant interaction between frost resistance and waterlog- ging was demonstrated. Species from subgenus Monocalyptus showed a reduced ability to harden under waterlogged conditions compared with species from subgenus Symphyomyrtus. This interaction may be of ecological importance since species from subgenus Monocalyptus do not occur in cold, waterlogged sites at Snug Plains. The present and previous experiments suggest that the distribution of subalpine eucalypts is dependent upon the interaction of a range of factors including the severity of winter frosts, resistance to growing season frosts, the suitability of the site for frost hardening and the degree of waterlogging.