An examination was made of the effect of natural frosts on pole stands of eucalypts growing within and surrounding a shallow depression at Snug Plains (alt. approx. 600 m) in south-eastern Tasmania. Marked differences in microclimate occurred between the slopes surrounding the depression and the base of the depression. The most severe frosts were experienced by the site at the base of the depression, and during a cold spell in June 1983 a record minimum temperature of -22°C was recorded just above the radiating surface at this site. Pronounced vertical stratification of the air occurred (up to 9'C per m) and a difference in minimum temperature of 7.3"C was recorded over a distance of 200 m between a ridge-top site and the site at the base of the depression. Cooling rates of up to 6 . 5 T per h were recorded during these severe frosts.The effect of frost in determining the distribution of six local species of Eucalyptus was examined by comparing the damage caused to mixed pole stands of five of the six species by the cold spell in June 1983. The order of frost sensitivity for fully hardened pole stands from the most resistant to the most susceptible was E. gunnii > E. coccifera > E. johnstonii 2 E. delegatensis > E. pulchella. The natural distribution of these species was closely related to the minimum temperature recorded at the various sites. Intense frosts during June 1983 caused marked changes in the dominance of mixed stands even though relatively few individuals were completely killed. These results suggest that rare, exceptionally severe frosts may play an important role in determining the distribution of subalpine eucalypts.

The response of three eucalypt species (Eucalyptus pulchella. Eucalyptus coccifera and Eucalyptus delegatensisj to a severe drought in the summer of 1982/83 was examined at Snug Plains, south-eastern Tasmania. Few large differences in leaf water potential (\j/i) or stomatal conductance (gs) were apparent even at the height of the drought when both ii/i and soil water potentials (y/s) reachedca. -4.5 MPa. However. E. pulchella maintained a higher relative water content (RWC) in its leaves than E. coccifera and E. delegatensis, and showed less severe crown damage. After the first light rains substantial interspecific differences in i^/ and gs occurred. Eucalyptus pulchella restored normal \j/i, gs and RIVC more rapidly than the other two species and. even for severely droughted trees, crown growth commenced via epicormic buds near the ends of its branches while for E. delegatensis and E. coccifera crown regeneration was via epicormic buds arising from stems and larger branches. This resulted in a change in dominance in certain stands and showed that E. pulchella was more drought-resistant than E. coccifera. which was in turn more resistant than E. delegatensis. This conclusion was confirmed during competition experiments using potted seedlings. However, potted seedlings differed from mature field trees by maintaining moderate gs at high vapour pressure deficits and closing stomata at fl below ca. -2.0 MPa. Substantial variation in the severity of drought symptoms was observed over short distances. This variation appeared to be determined by the moisture-holding capacity of the .soil and the biomass of the stand. Although differences in the rooting patterns of seedlings were evident, field measurements of i/// and ij/s suggested that all three species were exploiting the same water resource.In contrast to previous studies, the results suggest that large interspecific differences in tissue hydration and crown damage may be present, even though differences in i///, gs and characteristics of the root system may appear small.

The early growth characteristics of six Tasmanian species from each of the Eucalyptus subgenera Monocalyptus and Symphyomyrtus were examined. The species chosen from each subgenus came from a range of habitats extending from low altitude dry sclerophyll sites to subalpine sites. Marked differences in the rate of germination, percentage of viable seeds, total dry weight, and the root to shoot ratio were observed after 3 and 7 months' growth, as well as different responses to the level of nutrient provided. These subgeneric differences were generally larger than the differences observed between species chosen from one subgenus but from different ecological sites. The most striking differences were possibly the higher root to shoot ratio found in the subgenus Symphyomyrtus and the generally faster establishment of species from this subgenus. The results show that the two subgenera possess different early growth patterns. This may suggest that the subgenera exploit their environment in slightly different ways and may, at least partly, explain the frequent occurrence of mixed stands of eucalypts in south-eastern Australia. where stands frequently contain one species from each subgenus.

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.

The objective of this study was to identify attributes of the understorey vegetation, soil root biomass, soil chemistry and microbial community that may be associated with tree decline in high altitude eucalypt forests in Tasmania.The sites studied were in healthy eucalypt forest, forest in decline and forest containing dead eucalypts dominated by rainforest, in north-east (Eucalyptus delegatensis forest) and in north-west (Eucalyptus coccifera forest) Tasmania. In both regions bare ground, rock and shrubby species were associated with healthy sites whereas decline sites were associated with moss and a tall understorey with a high percentage cover of rainforest species. Healthy sites had low root biomass in the top 10 cm of the soil profile relative to decline and rainforest sites. Seedlings of high altitude species were grown in rainforest soil (0.314% N and 0.060% P) and healthy eucalypt soil (0.253% N and 0.018% P). The four eucalypt species studied had similar root to shoot ratio in the two soils, but the rainforest species, Nothofagus cunninghamii and Leptospermum lanigerum, had higher root to shoot ratio in the healthy eucalypt than in the rainforest soil. We produced three soil filtrates: (i) fungi and bacteria present; (ii) bacteria only present and; and (iii) sterile, from healthy, decline and rainforest sites in north-east and in north-west Tasmania and used linseed as a germination bioassay. Filtrates from the north-east decline and rainforest sites induced a significantly greater dysplastic germination response than healthy sites in (i) and (ii) filtrates, but this was not found in filtrates from sites in the north-west. We conclude that while the development of a rainforest understorey and elevated soil root biomass in the long absence of fire is generally associated with high altitude eucalypt decline, altered bacterial and/or chemical attributes of soil are not always associated with high altitude eucalypt decline.