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A study on the biology of four Tasmanian cushion species


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Gibson, Neil 1988 , 'A study on the biology of four Tasmanian cushion species', PhD thesis, University of Tasmania.

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Donatia novae-zelandiae, Abrotanella forsteroides, Dracophyllum minimum and
Phyllachne colensoi are four superficially similar bolster species found in the
Tasmanian alpine vegetation. The ecological differentiation of the ranges of these
species, the dynamics of the formation and persistence of complex mosaics of
two or more of these species and the functional significance of the bolster form
were areas in which there were many hypothesis but few data.

Growth studies undertaken in Tasmania showed significant seasonal differences
in the competitive ranking of cushion species. Annual shoot production ranged
from 426-709 g/m2 for A. forsteroides; 359-477 g/m2 for D. minimum;
322-572 g/m2 for D. novae-zelandiae; and 282-285 g/m2 for P. colensoi.
These figures are greater than those reported from cushion communities in New
Zealand and from physiognomically similar communities in the northern
hemisphere, probably reflecting the maritime climate and the longer growing
season of the Tasmanian alpine zone.

Seed production was highly variable between sites with D. novae-zelandiae
producing between 2,370-38,900/m2 and D. minimum between 0-
17,140/m2 over the 1982/83 summer. Estimates of the lateral growth rate of
A. forsteroides in open situations on peat substrates ranged from 6.7-14.7 mm
diameter increase/year. D. minimum seedlings on rocky and well drained
mineral soils only achieved lateral growth rates of between 0-7 mm/year.

Data from the growth studies showed that cushion distribution at Mt. Field could
not be explained by either soil moisture or soil nutrient preferences. Growth
was found to be highly variable from year to year and season to season both
between species and within species over their altitudinal ranges. It was
concluded that interspecific competition between bolster species plays little
role in determining their overall distribution patterns. Climatic modelling
suggests that distribution of the species is primarily controlled by the
temperature (this is presumed to relate to competition from taller growing
shrubs and graminoids) and past climatic history. Donatia ilovae-zelandiae,
Abrotanella forsteroidel, and Dracophyllum minimum may still be expanding
their ranges following the climatic amelioration since the height of the Last

Investigations into the dynamics of a bolster heath modified and improved the
model proposed by Jackson (1981). Pathways of succession due to changes in
water table appear to result from complex interactions between the water table
level, propagule source and species already present on the site. Data from a peat
core indicate that once bolster communities have become established, they can
persist for very long periods. In the succession from bare ground to closed
bolster communities there is a general lack of pioneering species. Results from
the growth study, the patterns of reinvasion of drained tarns and studies of
permanent photopoints are consistent with the view that succession in bolster
communities is best described by inhibition or tolerance models (Connell and
Slatyer 1977). Competition appears to play an important role at the
establishment phase in mature bolster communities but once establishment has
occurred it is much less important. Competition also appears to be of little
importance in the building phase of bolster Communities in highly stressed

Attempts to elucidate dynamics by studying pattern in mosaic bolster
communities using spectral analysis failed due to the inability of this technique
to separate patch size and inter-patch distance.
The roots of all cushion species freeze at temperatures between -1° and -5° C.
Nonetheless the insulating properties of cushion peals are so effective that it is
unlikely that root freezing ever occurs with adult plants in the field, at least
under the present climatic conditions. Similarly the thermal characteristics of
peat soils are such that seedling death due to root freezing would also be an
extremely rare event.

Short interval temperature measurements showed A. forsteroides capable of
achieving cushion surface temperatures 10° C above ambient temperatures. It
is suggested that this feature of cushion plants may allow them to undertake
photosynthesis under otherwise limiting conditions and may significantly extend
their growing season.

Anatomical investigation of the four cushion species showed a highly variable
internal structure. This diversity is difficult to interpret. The scleromorphic
nature of most of the species may be related to nutrition. The xeromorphic
features exhibited by the four species and the thermal characteristics of the
cushion form are inconsistent with the hypothesis that these species suffer
significant periods of water deficit.

Item Type: Thesis - PhD
Authors/Creators:Gibson, Neil
Keywords: Mountain plants
Copyright Holders: The Author
Copyright Information:

Copyright 1988 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1989. 8 pts in pocket at back of vol. Includes bibliographies

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