PARENT SESSION
Posters P1D Photosynthesis and global change. Abstracts (699-710)

Climate dependent variations in photosynthesis: can they be used to predict variations in ecosystem respiration? Owen Atkin^*,1, Iain Hartley¹, Anna Armstrong¹, Greg Barron-Gafford², Ramesh Murthy², ¹ Department of Biology, York, North Yorkshire, UK² Department of Ecology and Evolutionary Biology, Tucson, Arizona, USA

ABSTRACT- Variations in ecosystem respiration are likely to be critical in determining future atmospheric CO₂ levels. In this study, we integrated CO₂ flux measurements done on individual leaves, soils and whole ecosystems to establish the extent to which respiratory flux can be predicted from measurements of daytime photosynthetic CO₂ uptake. We also assessed whether increases in photosynthesis result concomitant increases in the temperature sensitivity of above- and below-ground respiration. The experiment was undertaken in enclosed cottonwood (Populus deltoides) stands at the Biosphere 2 Research Centre in Tucson, Arizona. Entire stands of trees were subjected to rapid changes in atmospheric CO₂ concentration to assess the impact of altered rates of canopy level photosynthesis on ecosystem respiration and its underlying above and below ground components. The impact of sustained higher and lower canopy temperatures on respiratory fluxes was also assessed. Regardless of whether measurements were made using individual leaves, intact soils or whole ecosystems, rates of nighttime respiration were tightly coupled to the preceding daytime rates of photosynthesis. Importantly, the speed with which nighttime respiration responded to variations in canopy level photosynthesis differed between the above and below components of CO₂ efflux; whereas nighttime leaf respiration responded almost immediately to variations in the rate of photosynthesis during the preceding daytime period, it took several days for soil and ecosystem level respiration to fully respond. The sensitivity of leaf respiration to nighttime fluctuations in temperature was greatest in trees that exhibited a high rate of photosynthesis during the daytime. Similarly, the response of soil respiration to a sustained change in temperature over several days was greatest in ecosystems where daytime photosynthesis was high. Taken together, our results highlight the importance of photosynthesis in determining not only respiratory flux in whole ecosystems, but also the temperature dependence of above and below ground respiration over different time scales.

KEY WORDS: photosynthesis, temperature, ecosystem respiration, Poplar