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Eco-physiological studies of factors determining the distribution of subalpine Eucalypts at Snug Plains, Southern Tasmania

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Davidson, NJ (1985) Eco-physiological studies of factors determining the distribution of subalpine Eucalypts at Snug Plains, Southern Tasmania. PhD thesis, University of Tasmania.

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Abstract

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°C 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°C/m) and a steep
gradient in minimum temperature (7 .3°C 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°C 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°C 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.

Item Type: Thesis (PhD)
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Date Deposited: 25 Nov 2013 05:16
Last Modified: 15 Sep 2017 01:06
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