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

• Main Author: O'Brien, MM
• Format: Thesis
• Language: English
• Published: 2004
• Subjects: Photosynthesis; Lichens; Antarctic; Windmill Islands; Antarc*; Antarctica
• Online Access: https://eprints.utas.edu.au/21072/; https://eprints.utas.edu.au/21072/1/whole_O%27BrienMarcelleMaree2004_thesis.pdf

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 ...