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Impacts of increased water and nitrogen availability on photosynthesis of the continental Antarctic lichen Usnea sphacelata


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O'Brien, Marcelle Maree 2004 , 'Impacts of increased water and nitrogen availability on photosynthesis of the continental Antarctic lichen Usnea sphacelata', PhD thesis, University of Tasmania.

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Water availability and low temperatures are believed to be the main ecological
limits for continental Antarctic lichens. Lichen growth may also be limited by
nitrogen availability. Thallus nitrogen content of two dominant Antarctic
macro lichens decreases significantly at the onset of the summer snow melt,
but the causes are unknown.
Over the past 40 years precipitation rates have increased in the Windmill
Islands region of continental Antarctica. An increase in snow fall may result
in lichens being wet for longer periods during the summer, which may result
in an increase in photosynthetic activity. Should lichen growth be limited by
nitrogen availability, however, increases in growth due to increased
precipitation may increase the demand for nitrogen beyond supply.
Consequently, this may affect long term lichen survival.
Usnea sphacelata is the dominant terrestrial macrolichen of continental
Antarctica. In order to examine the impact of increased precipitation, in situ
stands of U sphacelata at two sites were subjected to a series of enhanced
water and nutrient treatments over two consecutive summers, and
photosynthesis, respiration and nitrogen status were investigated.
Increases in water availability did not affect gross photosynthesis or dark
respiration in Year 1, but there was a significant effect of watering volume on
net assimilation. In Year 2 neither gross photosynthetic or dark respiration rates
were affected and consequently, there was no impact of increased water
availability on the net carbon assimilation of U. sphacelata. There was,
however, a strong seasonal trend in the rates of dark respiration and net
photosynthesis, with dark respiration rates declining over the summer and net
carbon assimilation rate generally increasing.
Increasing nutrient availability had no effect on any of the gas exchange
variables at either site, however very significant seasonal effects were again
evident in dark respiration and net assimilation. Thallus nitrogen content was
not affected by either water or nutrient availability but was very strongly
affected by site and season. Lichens at the higher nutrient site had higher
thallus nitrogen content and higher carbon assimilation rates than those at the
lower nutrient site, but they had poorer nitrogen use efficiency. There was
also a significant positive relationship between thallus nitrogen and dark
respiration rate.
It therefore appears that photosynthetic capacity of U. sphacelata is
independent of water and nutrient availability over the summer. Should
climate change lead to increased snow fall to the terrestrial ecosystem in the
Windmill Islands, it is unlikely to impact on the nitrogen content or
photosynthetic capacity of U sphacelata, despite inherent variations amongst

Item Type: Thesis - PhD
Authors/Creators:O'Brien, Marcelle Maree
Keywords: Photosynthesis, Lichens
Copyright Holders: The Author
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Copyright 2004 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
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Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2004. Includes bibliographical references

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