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Eco-physiological studies of factors determining the distribution of subalpine Eucalypts at Snug Plains, Southern Tasmania
thesisposted on 2023-05-26, 03:26 authored by Neil Davidson
An examination was made of the ecological factors causing the changes in dominance between six eucalypt species; E. delegatensis, E. coccifera and E. pulchella (subgenus Monocalyptus); and E. gunnii, E. unigera and E. johnstonii (subgenus Symphyomyrtus) which formed a mosaic within and surrounding a shallow depression on a subalpine plateau (approximately 600 m in altitude) at Snug Plains, southern Tasmania. The populations of the six species at the 1 square kilometre study area were in most cases lower altitude ecotypes of subalpine species. However, morphometric studies of foliar samples from field trees and glasshouse grown seedlings of the one lowland species, E. pulchella, suggested that the Snug Plains E. pulchella population was of hybrid origin (\phantom hybrid\") with E. coccifera and E. pulchella as the putative parents. Reciprocal transplant trials established in six locations in the field at Snug Plains demonstrated that the key factors influencing seedling survival in the low lying sites were: severity of frost waterlogging and phytophagous insect attack. Differences in soil nutrition and texture had little influence on seedling growth and seedlings in elevated well drained sites grew rapidly. There was also a strong tendency in the field transplant trials for the species naturally dominant at the site to become dominant or show greatest growth on a relative basis at its site of origin. A record frost with an absolute minimum temperature of -22¬¨‚àûC at the radiating surface was recorded at Snug Plains in June 1982. Temperature measurements made in a transect across the study area indicated a marked vertical stratification of the air body occurred (up to 9¬¨‚àûC/m) and a steep gradient in minimum temperature (7 .3¬¨‚àûC over 200 m) was established between the base of the depression and the ridge top. The natural distributions of the six species studied were closely related to the minimum temperature recorded. Frost damage incurred by mixed pole stands on the margins of the. depressions was in places severe. Interspecific differences in frost hardiness were in the order (from greatest to least): E. gunnii > E. coccifera > E. johnstonii > f.: d~legatensis > E. pulchella. The severe frosts caused marked changes in dominance in mixed stands even though they resulted in few deaths. Exceptional frosts like those of June 1982 may have an important effect on the distributions of subalpine eucalypts. In the reciprocal transplant trials growing season frosts of -.5.5¬¨‚àûC inflicted severe damage on the unhardened seedlings of all species except ~nnii which exhibited outstanding frost resistance in the unhardened state. Winter frost of -10.5¬¨‚àûC caused markedly less damage which suggests that growing season frosts may play an important role in determining the distribution of subalpine eucalypts. Frost chamber experiments confirmed the importance of hardening pretreatment on relative frost resistance of the species studied and demonstrated a significant interactive effect of waterlogging on frost resistance. The Monocalyetus species were more susceptible to frost in waterlogged soils whereas no such relationship existed for the .sxmphyomyrtus species. Damage inflicted on seedlings in the transplant trials during a frost free period when waterlogged conditions prevailed indicated that the sxmphxomyrtus species E. gunnii E. urnigera and E. johnstonii were more waterlogging tolerant than the Monocalxptus species E. pulchella E. coccifera and E. delegatensis. This was confirmed. by glasshouse based trials which showed marked differences existed between the subgenera in tolerance to waterlogging. After 30 days of waterlogging the sxmehxomyrtus species had significantly higher midday stomatal conductances and water potentials than the Monocalxpt~? species. Further Sxmphxomyrtus species exhibited stem hypertrophy and developed extensive aerenchymatous adventitious root systems. The Monocalxptus species demonstrated no such morphological adaptations. The results suggest that the Monocalyetus species will be absent from waterlogged sites which agrees with the distributions of the species and measurements of soil moisture-status in the field. Phytophagous insect attack which affected 3 transplant trials caused severe damage to E. gunnii E. urnigera and E. johnstonii seedlings (up to 75% leaf' loss) but only slight damage to E. delegatensis and E. pulchella (up to 10% leaf loss). This suggests that selective insect grazing may have an important effect on the dominance patterns in regenerating mixed eucalypt stands at Snug Plains. Glasshouse based drought trials suggested differences in drought resistance existed between the subgenera. The Monocalyptus species maintained higher relative water contents (R.W.C.) at low water potentials than the Symphyomyrtus species. Therefore low tolerance to drought might b.e an important factor in explaining the absence of sxmphyomyrtus species from the ridge tops at Snug Plains. During the 1982-83 summer season south-eastern Tasmania was exposed to a drought of near record severity. As the drought developed dawn to dusk measurements of stomatal conductance and water potentials made in a mixed stand of the three Monocaly:ptus species on a ridge top at Snug Plains showed these species differed only slightly in water potential down to -4.3 MPa. The leaf water potentials of all species reflected the soil water potential. However measurements of R. W .C. demonstrated that E. 2_ulchella maintained a higher R.W.C. (61%) than either E. coccifera (55%) or E. delegatensis (48%) at these low water potentials. E. pulchella also exhibited less crown damage than the other two species during the drought and after the first effective rains demonstrated more rapid restoration of high R. W .C. and recommencement of shoot extension. Anatomical and morphological characters such as linear leaves and recovery from drought via epicormic buds near the branch tips also favoured E. pulchella in survival during and recovery after drought. An investigation of soil depths in the field and glasshouse trials including drought treatments confirmed that differences in drought tolerance were not due to interspecific differences in root pattern or depth. It was concluded that superior drought resistance was the primary reason for the presence of E. pulchella on the ridgetop sites. The relative drought resistance of the other two Monocalyptus species may also have influenced the positions they occupy on the rocky ridges at Snug Plains. The breadth of the ecological study conducted allowed the ecological niche occupied by each of the species studied to be defined in some detail and the main factors involved in the changes in dominance between species to be outlined. Furthermore the study provided strong evidence of morphological and physiological differences between the subgenera Monocalyptus and Symphyomyrtus which may help to explain differences in the ecological positions occupied by the subgenera in allopatry and the niche differences of the subgenera in sympatry."
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